Pr. Stéphane Godet

• CHIM-H-534 Materials selection
• CHIM-H-2001 Chimie physique, matériaux et fabrication, y compris les visites d'usine
• CHIM-H-2002 Sciences des matériaux
• CHIM-H-412 Microstructural design and characterization of inorganic materials

Articles dans des revues avec comité de lecture

2024

Atomic scale mechanism of reorientation induced plasticity in titanium alloys - experimental and first principles investigation of the mobile {134‾1} interfaces

Recent experiments identified a new type of stress induced structural transformation allowing to combine high strength, great work hardening and good ductility in multiphase Ti alloys. These properties are achieved through reorientation of the α′ martensite plates being in specific self accommodating 〈54‾1‾3〉 type II twin relation, i.e. under applied load one martensite variant reconfigure to its twinned configuration with visible motion of the {134‾1} twin boundary. This mechanism of plastic deformation was never observed before thus, its current understanding is fragmentary. In this article we present the results of experimental observations and ab initio calculations of 〈54‾1‾3〉 type II twins determining the crystallography of twin formation, structure and energy of the {134‾1} interface as well as mobility of the corresponding twinning disconnections. It was found that the investigated boundary has one of the lowest energies among known twinning modes in hexagonal Ti. Moreover, the 〈54‾1‾3〉{134‾1} disconnections have the smallest Burgers vector and step height in comparison to other active twinning systems. As a result, these disconnections are highly mobile which rationalize migration of the {134‾1} twin boundaries at straining. Furthermore, energy of the twin boundary and mobility of disconnections can be adjusted by particular alloying elements enabling the conscious development of new alloys exhibiting reorientation induced plasticity.

Influence of Austenite Grain Size on the Variant Configurations of Martensite in a Fe-30.5Ni-0.155C Alloy

A Fe-30.5wt%Ni-0.155wt%C alloy was annealed at two different temperatures to produce two different austenite grain sizes. In the coarse-grained specimen, hierarchical configurations of variants are formed and carefully analyzed using EBSD. These typical patterns result from the alternate formation of two perpendicular plate groups of variants over several length scales, and two distinct types of mechanical couplings are shown to occur sequentially in the process of the transformation of an austenitic grain. In the fine-grained specimen, the martensite start temperature is depressed below liquid nitrogen temperature, and the martensitic transformation can only occur under stress assistance. Grain size reduction brings about a dramatic change in the morphology of martensite and its configurations. Martensite is fully twinned, and martensite variants arrange themselves into self-accommodating configurations involving all four variants of the same plate group. Those specific configurations share striking similarities with those usually encountered in conventional shape memory alloys. The reversion of such microstructures upon heating is believed to be at the origin of the observed shape memory effect.

Revisiting the Crystallography of {225}γ Martensite: How EBSD Can Help to Solve Long-Standing Controversy

Explaining the crystallography of iron alloys martensite with a {225}γ habit plane remains a challenging task within the phenomenological theory of martensite crystallography. The purpose of this study is to re-examine the martensite formed in a Fe-8Cr-1.1C alloy using EBSD, which has a better angular resolution than the conventional transmission electron diffraction techniques previously used. The results show that the single morphological plates, which hold a near {225}γ habit plane, are bivariant composites made up of two twin-related variants. It is shown that a {113}γ plane is systematically parallel to one of the three common (Formula presented.) planes between the two twin-related crystals. This observation suggests that the lattice invariant strain of transformation occurs through a dislocation glide on the {113}γ 〈110〉γ system, rather than through twinning as is commonly accepted. Based on this assumption, the predictions of Bowles and Mackenzie's original theory are in good agreement with the crystallographic features of {225}γ martensite. Unexpectedly, it is the high shear solution of the theory that gives the most accurate experimental predictions.

Design rules to develop solute lean α+β titanium alloys exhibiting high work-hardening by reorientation induced plasticity

While work-hardening is typically considered in Ti as a prerogative of the β-metastable alloys, this paper introduces a novel perspective, presenting a set of alloy design rules to develop solute lean α + β titanium alloys exhibiting increased work-hardening capabilities. More specifically, reaching this goal is made possible through the development of α + α′ microstructures exhibiting Reorientation Induced Plasticity (RIP) within the α′ martensitic phase. The microstructural requirements for activating RIP and maximizing mechanical properties (i.e., combining high work-hardening, yield strength and ductility levels) are derived from an analysis of the microstructures/mechanical property relationships of various α + α′ samples. A set of design rules is provided. Emphasis is laid on the pivotal role of the chemistry of the α′ martensitic phase in RIP activation and a Molybdenum equivalent chemical criterion is proposed. The α phase is here suggested as a mean to reduce the prior β grain size and the resulting size of the martensite plates. This approach reveals that the versatile thermal treatments leading to α+α' structures broaden the mechanical property landscape, achieving large work-hardening capabilities (typically over 500 MPa) that can be combined with high yield strength (over 800 MPa).

2023

Reorientation Induced Plasticity (RIP) in high-strength titanium alloys: An insight into the underlying mechanisms and resulting mechanical properties

The present paper aims at providing a fine-scale analysis of the Ti-4.5Al-2.5Fe-0.25Si α+α'+βretained microstructures to give insight into the link between the microstructural characteristics of the alloy (phase fraction and chemistry, grain size, etc.) and the deformation mechanisms at play. These microstructures were found to exhibit outstanding work-hardening capabilities that have the great potential to be obtained simultaneously with a high yield strength when the microstructural features are carefully optimized. Ex-situ analyses coupled with TEM revealed the simultaneous occurrence of Reorientation Induced Plasticity (RIP) into the self-accommodated Fe-enriched α' martensite, TRansformation Induced Plasticity (TRIP) of the βretained phase and TWinning Induced Plasticity (TWIP) of the α phase that add to dislocation glide. The Fe-enriched martensite has the remarkable capability to induce reorientation through two distinctive mechanisms: by the motion upon deformation of the intervariant boundary associated to the [45¯13¯]α′ Type II twin, a rather classical mechanism although not often reported into α'; but more surprisingly into such a fine phase, by the creation and growth upon deformation of {101¯2}1¯011α′ twins. A 3-scale mechanical contrast is proposed to explain the remarkable work-hardening rates achieved. Reorientation is shown to be a key microstructural feature for the development of Ti alloys with superior mechanical properties.

2021

Effect of pH and ammonia concentration on the shape and size of fluorapatite nanoparticles obtained by chemical reaction

Nanoparticles of fluorapatite (FAp Nps) were prepared by one step chemical reaction in aqueous solutions. The pH of the system was kept constant at 6 and 8. The FAp Nps were characterized with Scanning Electron Microscopy-Energy Dispersive X-Ray Spectroscopy (SEM-EDAX), X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR). The Ca/P calculated from EDAX analysis was 1.69. The results of XRD analysis and FTIR showed the presence of FAp phases. The DRX results confirm the formation of FAp with a hexagonal structure. TEM images show semispherical and elongated nanoparticles with mean diameter of 3.78 nm, 8.63 nm and 10.46 nm, when the pH and ammonia concentrations is varied. Ammonia concentration and pH influence the morphology and size of the FAp Nps.

Effect of the Concentration and the Type of Dispersant on the Synthesis of Copper Oxide Nanoparticles and Their Potential Antimicrobial Applications

The bactericidal properties of copper oxide nanoparticles have growing interest due to potential application in the medical area. The present research investigates the influence of sodium dodecyl sulfate (SDS) and poly(vinylpyrrolidone) (PVP) on the production of copper oxide nanoparticles prepared from copper sulfate (CuSO4) and sodium borohydride (NaBH4) solutions. Different analytical techniques were used to determine the crystal nature, mean size diameter, and surface morphology of the copper oxide nanoparticles. The X-ray diffraction (XRD) patterns showed formation of nanoparticles of cuprite (Cu2O) and tenorite (CuO) when PVP and SDS were added at the beginning of the reaction. In fact, when the Cu/PVP ratio was 1.62, Cu2O nanoparticles were obtained. In addition, nanoparticles of CuO were synthesized when the Cu/PVP ratios were 0.54 and 0.81. On the other hand, a mixture of copper oxides (CuO and Cu2O) and cuprite (Cu2O) was obtained when PVP (Cu/PVP = 0.81 and 1.62) and SDS (Cu/SDS = 0.90) were added 30 min after the beginning of the reaction. Transmission electron microscopy (TEM) images show agglomerated nanoparticles with a size distribution ranging from 2 to 60 nm, while individual particles have sizes between 4.1 ± 1.9 and 41.6 ± 12.8 nm. The Kirby-Bauer method for the determination of antibacterial activity shows that small CuO (4.1 ± 1.9 nm) and Cu2O (8.5 ± 5.3 nm) nanoparticles inhibit the growth of Escherichia coli, Staphylococcus aureus MRSA, S. aureus and Pseudomonas aeruginosa bacteria. The antibacterial test of cotton fabric impregnated with nanoparticles shows positive results. The determination of the optimal ratio of copper oxide nanoparticles per cm2 of fabric that are able to exhibit a good antibacterial activity is ongoing.

2020

Crystallography and reorientation mechanism upon deformation in the martensite of an α-α' Ti-6Al-4V dual-phase microstructure exhibiting high work-hardening rate

The present study provides a fundamental understanding of the crystallography and the microscale behavior of the V-enriched and Al-depleted α' martensite taking place during the microstructure formation and the deformation of the dual phase α + α' microstructure produced in Ti-6Al-4V. This particular microstructure exhibits much larger work-hardening capabilities than the conventional wrought product. The as-quenched structure of the martensite is analyzed using the Phenomenological Theory of the Martensite Crystallography (PTMC) coupled with EBSD analyses. This approach sheds new light on the microstructure configuration obtained during the dual-phase treatment and its fine-scale mechanical behavior. The martensite preferentially forms into parallel groups of 3 self-accommodating variants separated by a misorientation of 63.26∘/[10¯553¯]α'. TEM analyses additionally show that the variants of a same group are separated by a hitherto unobserved {134¯1}α' type twin plane. Postmortem analyses after tensile testing demonstrate that this twinning plane is mobile under deformation. This allows the martensitic microstructure to exhibit the remarkable property to reorient under uniaxial tension. This unique property is shown to be intimately related to the mobility of the {134¯1}α' twinning plane thereby evidencing for the first time that twin boundary motion is not uniquely associated to the orthorhombic α'' martensite but can also occur in hexagonal α' martensite. Quantification of the Interaction Energy (IE) appears relevant to rationalize and predict the reorientation of the martensite. The critical influence of the parent β grains texture on the reorientation is evidenced, while the impact of this deformation mechanism on the ductility of the martensite is debated.

Microstructure study of cold rolled Al0.32CoCrFeMnNi high-entropy alloy: interactions between recrystallization and precipitation

Fabrication additive de pièces en titane par fusion sélective de lit de poudre

50 shades of colour: how thickness, iron redox and manganese/antimony contents influence perceived and intrinsic colour in Roman glass

Roman glass is studied here by means of optical absorption spectroscopy, in order to provide an objective method to quantitatively evaluate colour. The dataset is composed of 165 soda-lime silicate glass samples from various western European sites, mainly dated from the first to fourth century AD, and containing variable amounts of iron, manganese and/or antimony. Iron redox ratios and colour coordinates (based on the CIELab colour system) are determined and put in relation with the thickness of samples and their manganese/antimony contents. Results reveal thickness as a crucial parameter when discussing glass hues, thus leading to a differentiation between the ‘intrinsic' and ‘perceived' colour of glass objects (i.e. the colour of the object with the thickness normalised to 1 mm, and that with its original thickness, respectively). Apart from HIMT and purple glass, the concentration of ferrous iron appears to be correlated with a* — a colourimetric parameter determining how green the glass is. Significant relations of antimony/manganese contents versus iron redox and glass colour are also considered, resulting in quantitative arguments to entitle antimony-decoloured glass as the most oxidised and colourless glass.

Correction to: Selective Laser Melting of Al0.3CoCrFeNi High Entropy Alloy: Printability, Microstructure, and Mechanical Properties

The authors regret that the following mistakes were made.

2019

In Situ Quantitative Assessment of the Role of Silicon During the Quenching and Partitioning of a 0.2C Steel

Silicon is an essential alloying element added in quenching and partitioning (Q&P) steels to delay and/or suppress carbide precipitation. However, there is a strong industrial interest to reduce the silicon content as it has detrimental effects on the metallurgical route. This work investigates by means of in situ high-energy XRD (HEXRD) the effect of silicon on the microstructural evolution during quenching and partitioning of a commercial 0.2C-2.3Mn grade. The results of this study highlight the role of the bainite transformation during the reheating and partitioning steps for effective austenite retention. Silicon influences the kinetics of austenite decomposition into bainite and finally promotes the stabilization of austenite. This is explained by the ability of silicon to suppress carbide precipitation (i) at the interface between bainite and austenite and (ii) in the martensite matrix. Carbide precipitation at the bainite/austenite interface decreases the amount of carbon that diffuses from bainite to austenite, subsequently accelerating the bainite transformation kinetics and preventing austenite stabilization. Carbide precipitation in martensite reduces the amount of carbon available for partitioning in austenite, further preventing its stabilization. Additions of elements such as Cr or Mo could be therefore considered in order to reduce the austenite decomposition in low-silicon steel grades.

Selective Laser Melting of Al0.3CoCrFeNi High-Entropy Alloy: Printability, Microstructure, and Mechanical Properties

Al0.3CoCrFeNi high-entropy alloy (HEA) was additively manufactured by powder-bed selective laser melting (SLM) with emphasis on its microstructure and tensile properties. Al0.3CoCrFeNi showed excellent printability, enabling fabrication of fully dense products. The microstructure of the SLM as-built HEA consisted of a single-phase disordered face-centered cubic solid solution with fine columnar grains elongated along the build direction. The characteristic features of the as-built microstructure were a <110> fiber texture aligned toward the build direction and a large dislocation density. As a consequence, printed Al0.3CoCrFeNi HEA exhibited superior tensile strength in comparison with as-cast or wrought counterparts.

Improving the atmospheric plasma deposition of crystalline inorganic coatings

The deposition of good quality crystalline inorganic coatings by atmospheric pressure dielectric barrier discharge remains a challenge. Thanks to an original coupling of a substrate heating device based on an inductive current loop and located under the dielectric and an atmospheric pressure dielectric barrier discharge, we show that one can deposit in one step crystalline vanadium oxide and titanium oxide, with grain sizes bigger than those achieved by post-deposition annealing.

To be purple or not to be purple? How different production parameters influence colour and redox in manganese containing glass

The purpose of this paper is to understand why manganese containing Roman glass could be purple or colourless in spite of having very similar chemical compositions. The strategy followed to tackle this question consists in the production of glass with the same chemical composition as Roman glass whereby various production parameters were controlled and systematically analysed. It is shown that redox and colour of glass is more likely to have been managed through internal control through the choice of raw materials and the addition of organic matter. The main difference between ancient and modern glass production relies on the lower melting temperature of Roman furnaces, so that sulphate would have played a less important role in the redox determination.

In situ characterization of a high work hardening Ti-6Al-4V prepared by electron beam melting

A multi-phase Ti-6Al-4V prepared by electron beam melting and thermal post treatments has been shown to exhibit increased strength and ductility over standard wrought or hot isostatic pressed Ti-6Al-4V. The mechanical improvements are due to a prolonged, continuous work hardening effect not commonly observed in Ti alloys. In situ x-ray diffraction and high resolution digital image correlation are used to examine the strain partitioning between the phases during tensile loading with post-mortem electron microscopy to characterize the deformation behavior in each phase. Specimens heat treated between 850 and 980 °C were tested and the effect of annealing temperature on the micromechanical response is discussed. It is shown that the work hardening is the result of composite load-sharing behavior between three mechanically distinct microstructures: large α lamellae and a martensitic region of fine acicular α' and a third phase not previously reported in this alloy.

Effect of Zn on the grain boundary precipitates and resulting alkaline etching of recycled Al-Mg-Si-Cu alloys

The Zinc level in many 6000 Al alloys (Al-Mg-Si-type) was set to a maximum of 0.03 wt% as common industrial practice. Concentrations above 0.03 wt% can modify the alkaline etching mechanism causing the surface to go from a desired smooth look associated with a grain boundary attack (GBA), into an undesired speckled appearance due to preferential grain etching (PGE), visible after anodizing. This significantly limits the ambition of reducing the carbon footprint of Aluminum by increasing the amount of recycled material in the production process. Because Cu has been reported to counteract this negative effect of Zn, the present study is dedicated to contribute to the understanding of this interaction and is focused on Zn and Cu in peak aged AlMgSi alloys. The chemical composition of the various precipitates formed in two Al6063 alloys was studied by means of TEM and EDX, whereby Zn was found in the Q phase (AlMgSiCu) grain boundary precipitates. Two alloys, with different content of Cu and Zn were studied. The Zn/Cu ratio in the bulk was similar for both alloys, but the level of Zn in the Cu containing Q particles in the grain boundaries was different. The increased Zn concentration in Q phase precipitates is believed to decrease the potential of the precipitates with respect to the surrounding matrix. Thus, the Cu/Zn ratio in these alloys is extremely important as it defines the potential differences that in turn cause either GBA or PGE.

On the making, mixing and trading of glass from the Roman military fort at Oudenburg (Belgium)

This paper presents the analysis of decoloured and naturally coloured glass from well-dated contexts in the southwest corner of the Roman fort at Oudenburg (Belgium) ranging from the late second to the early fifth century AD. The aim is three-fold. First, provide comparative material in the study of glass consumption from the northwestern provinces of the Roman Empire. Secondly, evaluate possible diachronic shifts in the applied decolourizing agent to produce colourless glass as to assess potential correlations between glass production recipes, provenance and chrono-typology. Finally, provide an added value to the research of glass recycling and mixing in the Roman imperial period. Nine subgroups are distinguished based on their chemical composition determined by LA-ICP-MS: Sb-only, two groups of Mn-only, four groups of mixed Mn-Sb, HIMT and one glass without any decolouring agent. The Sb-decoloured glass is used in the earliest phases and can be attributed to an Egyptian provenance. The two subgroups of Mn-glass likely come from different provenances: one from Egypt and the other later one from the Levant. Most of the glass shows high marks of mixing based on high trace elements concentrations and the simultaneous presence of antimony and manganese. Inhomogeneous mixing of manganese and antimony was also detected through μXRF. One Mn-Sb subgroup likely comes from mixing antimony glass with HIMT. The obtained results help better recognise the shifts in applied glass recipes throughout the Roman imperial period and improve our understanding about the mixing and recycling of glass to supply a Roman military camp.

Interface toughening in multilayered systems through compliant dissipative interlayers

The effect on an interlayer on the toughness of an interface between a ductile thin film and an elastic substrate is investigated by finite element modeling and assessed towards experimental measurements. The model is based on an asymptotic K -field formulation re- lying on cohesive zone elements to simulate the fracture process. A compliant interlayer tends to increase the interface toughness by promoting plastic dissipation in the thin layer. Additional toughening can result from the development of plastic strains in the inter- layer. The magnitude of these two toughening mechanisms depends on the film thick- ness, among other parameters. The model predictions are confirmed by comparison with wedge-opening test data performed on a multilayer composed of a thin Cu layer and a polymer interlayer embedded between two stainless steel substrates. These findings lay the foundation for the design of tougher multilayers and provide a critical assessment of experimental protocols for interface toughness measurements requiring the bonding of a dummy substrate, such as used for DCB or four point bending tests.

Into the quenching & partitioning of a 0.2C steel: An in-situ synchrotron study

Micromechanical behavior and thermal stability of a dual-phase α+α' titanium alloy produced by additive manufacturing

In order to improve the tensile properties of additively manufactured Ti-6Al-4V parts, specific heat treatments have been developed. Previous work demonstrated that a sub-transus thermal treatment at 920 °C followed by water quenching generates a dual-phase α+α′ microstructure with a high work-hardening capacity inducing a desirable increase in both strength and ductility. The present study investigates the micromechanical behavior of this α+α′ material as well as the thermal stability of the metastable α' martensite. To that end, annealing of the α+α′ microstructure is performed and the resulting microstructural evolution is analyzed, along with its impact on the tensile properties. A deeper understanding of the micromechanics of the multiphase microstructure both before and after annealing is achieved by performing in-situ tensile testing within a SEM, together with digital image correlation for full-field local strain measurements. This approach allows the strain partitioning to be quantified at a microscale and highlights a significant mechanical contrast between the two phases. In the α+α′ microstructure, the α′ phase is softer than the α phase, which is confirmed by nanoindentation measurements. Partial decomposition of the martensite during annealing induces a substantial hardening of the α′ phase, which is attributed to fine-scale precipitation and solution strengthening. A scale transition model based on the iso-work assumption and describing the macroscopic tensile behavior of the material depending on the individual mechanical behavior of each phase is also proposed. This model enables to provide insights into the underlying deformation and work-hardening mechanisms.

Titanium Alloy Repair with Wire-Feed Electron Beam Additive Manufacturing Technology

Wire feeding can be combined with different heat sources, for example, arc, laser, and electron beam, to enable additive manufacturing and repair of metallic materials. In the case of titanium alloys, the vacuum operational environment of electron beam systems prevents atmospheric contamination during high-temperature processing and ensures high performance and reliability of additively manufactured or repaired components. In the present work, the feasibility of developing a repair process that emulates refurbishing an "extensively eroded" fan blade leading edge using wire-feed electron beam additive manufacturing technology was examined. The integrity of the Ti6Al4V wall structure deposited on a 3 mm thick Ti6Al4V substrate was verified using X-ray microcomputed tomography with a three-dimensional reconstruction. To understand the geometrical distortion in the substrate, three-dimensional displacement mapping with digital image correlation was undertaken after refurbishment and postdeposition stress relief heat treatment. Other characteristics of the repair were examined by assessing the macro- and microstructure, residual stresses, microhardness, tensile and fatigue properties, and static and dynamic failure mechanisms.

2018

On the mode I toughness of adhesive bonds exhibiting strain-softening and re-hardening

The mode I fracture toughness of an interface between an adhesive material exhibiting strain-softening behavior and an elastic substrate is investigated by finite element modeling. The model is based on an asymptotic K -field formulation relying on cohesive zone elements to simulate the fracture process. A parametric phenomenological hardening law is used to cover the plastic response behavior of a wide range of amorphous glassy polymers, involving a first hardening peak followed by a softening and a re-hardening response. The effect of the peak hardening stress and of the softening stress on the joint toughness is investigated for different elastic mismatch and interface strength. The joint toughness is found to significantly increase with increasing amplitude of the softening, and, this, at constant yield stress and peak hardening stress. This is related to larger plastic zone sizes and larger plastic strains, hence to more energy dissipation. Increasing the interface work of fracture magnifies this effect. There is an interesting coupling between the elastic mismatch and softening, related to their effect on the fracture process zone length. One outcome of these findings is the potential impact of ageing which is known to modify the amplitude of the peak hardening and softening, hence leading to an evolution of the joint toughness with time.

Micro and nanoscale characterization of complex multilayer-structured white etching layer in rails

Micro-to nano-scale characterization of the microstructures in the white etching layer (WEL), observed in a Dutch R260 Mn grade rail steel, was performed via various techniques. Retained austenite in the WEL was identified via electron backscatter diffraction (EBSD), automatic crystallographic orientation mapping in transmission electron microscopy (ACOM-TEM), and X-ray diffraction (XRD). EBSD and ACOM-TEM methods were used to quantify grains (size range: 50 nm-4 µm) in the WEL. Transmission electron microscopy (TEM) was used to identify complex heterogeneous microstructural morphologies in the WEL: Nano-twinning substructure with high dislocation density in the WEL close to the rail surface and untransformed cementite and dislocations in the WEL close to the pearlite matrix. Furthermore, atom probe tomography (APT) revealed a heterogeneous through-thickness distribution of alloying elements in the WEL. Accordingly, the WEL is considered a multi-layered martensitic microstructure. These findings are supported by the temperature calculations from the shape analysis of the manganese profile from APT measurements, related to manganese diffusion. The deformation characteristics of the WEL and the pearlite beneath the WEL are discussed based on the EBSD measurements. The role of deformation in the martensitic phase transformation for WEL formation is discussed.

Highlights of the special issue on metal additive manufacturing

This editorial introduces a special issue on metal additive manufacturing (AM) discussing the main highlights and contributions to the field of each paper of the special issue classified by alloy type. These papers treat a range of additive manufacturing technologies.

On the effect of Q&P processing on the stretch-flange-formability of 0.2C ultra-high strength steel sheets

Quenching and Partitioning (Q&P) has been proposed as a novel heat treatment to produce cold rolled sheets with excellent strength and sufficient formability for cold stamping. The impact of Q&P processing on microstructure and tensile properties has been extensively studied in contrast to the lower attention devoted to its effect on stretch-flange-formability. In this study, the stretch-flange-formability of Q&P microstructures is investigated by means of hole expansion tests carried out on punched holes. The balance between tensile properties and hole expansion ratios (HER) is discussed and compared to three model microstructures: biphasic dual-phase (DP), single phase quenched & tempered (Q&T) and quenched & austempered (QAT). It is shown that Q&P microstructures exhibit a better combination of tensile ductility and stretch-flange-formability than fully martensitic (excellent HER but poor tensile ductility) and austempered microstructures (good ductility but poor HER). The study of the impact of the Q&P parameters demonstrates that stretch-flange-formability is further promoted by choosing low quench temperatures and long partitioning times. The hole expansion properties are linked to the hardness gradients in the microstructure, evaluated by nanohardness mapping. The narrower nanohardness distribution in the Q&P microstructure leads to better hole expansion ratios compared to bainitic microstructures obtained by austempering, where the presence of hard M/A blocks is unavoidable.

Synthesis and characterization of zinc oxide nanoparticles for application in the detection of fingerprints

Stable zinc oxide nanoparticles were prepared by chemical process. Zinc oxide nanoparticles were synthesized using a modified method with zinc nitrate and sodium hydroxide at 60° C. The synthesized nanopowders were characterized in terms of chemical composition (EDS), structure FTIR and XRD, particle size and morphology by TEM. The XRD results confirm that ZnO nanoparticles were obtained with hexagonal arrangement (Wurtzite). The nanoparticles showed sizes between 10 to 30 nm and semispherical forms. The luminescent properties of the synthesized nanoparticles were measured in a photoluminescence assay on a Raman instrument. The samples were irradiated with two laser beams of different wavelengths. The application of the fingerprints on different surfaces was done using deferments surfaces.

Effect of the concentration of NaBH4 and N2H4 as reductant agent on the synthesis of copper oxide nanoparticles and its potential antimicrobial applications

The antibacterial activity of various nanoparticles is gaining increasing interest due to its potential medical applications. In this work, we presented the synthesis of copper oxide nanoparticles prepared by chemical reduction from aqueous solutions of copper sulfate (CuSO4) with sodium borohydride (NaBH4) and hydrazine hydrate (N2H4) as reductant and polyvinylpyrrolidone (PVP) as stabilizer. The X-ray diffraction spectra showed the formation of tenorite (CuO) and cuprite (Cu2O) nanoparticles when different ratios of CuSO4/NaBH4 and CuSO4/N2H4 were used. Photographs obtained by transmission electron microscopy (TEM) showed agglomerates of grains with a narrow size distribution (from 20 to 70 nm), whereas the radii of the individual particles were between 2 and 20 nm. Smaller nanoparticles and narrower particle size distributions were obtained when NaBH4 was used. The results of antibacterial activity using the Kirby-Bauer method showed that nanoparticles obtained with NaBH4 presented a reasonable bactericidal activity. Pseudomonas aureginosa and Staphylococcus aureus were more susceptible to the particle size than Escherichia coli. In addition, with small amounts of Cu2O in samples of CuO nanoparticles, the antibacterial susceptibility against Pseudomonas aureginosa was improved. Finally, nanoparticles of CuO incorporated into cotton by applying ultrasound waves remained impregnated after five washes.

A XANES investigation of the network-modifier cations environment before and after the Na+/K+ ion-exchange in silicate glasses

μ-XANES is used to study the modifications in the alkali and alkaline-earth environments induced by the Na+/K+ ion-exchange process in various Na -silicate glasses. The results indicate that the ion-exchange process induces a shortening of the Na-O, Ca-O and Mg-O bond distances. The contraction of the Na-O, Ca-O and Mg-O coordination shell allows a better accommodation of the K+ cations in the glass network and thereby leads to partial relaxation of the stress developed by the Na+/K+ ion-exchange. Nevertheless, despite the stress relaxation process, the K+ environment in the ion-exchanged glass is not equivalent to the one in Na,K-silicate as-melted glasses. Hence, this study clearly shows that the ion swapping forced K+ cations to occupy smaller sites which are not achievable via the melt quench route for glasses with the same K amount.

2017

Na+/K+ ion exchange in silicate glasses: Results from 17O 3QMAS NMR

The effect of Na+/K+ ion exchange on the distribution of the modifier ions and on their interaction with the oxygen atoms in the network is investigated in ternary Na,Mg- and Na,Ca- silicate glasses using 17O triple-quantum magic-angle-spinning nuclear magnetic resonance spectroscopy. The results indicate that the K+ ions preferentially exchange with the Na+ ions present in {Na,Ca}-NBO and {Na,Mg}-NBO environments, while the Na-NBO environment is not significantly affected. Remelting of the ion-exchanged glasses leads to a homogenization of the modifier ion distribution around the NBO sites. The dynamical site preference for the Na+/K+ ion exchange has far-reaching implications in our understanding of the alkali ion transport and more specifically of the mixed-alkali effect in silicate glasses.

Structural modifications induced by Na+/K+ ion exchange in silicate glasses: A multinuclear NMR spectroscopic study

The structural mechanisms of stress relaxation during Na+/K+ ion exchange is studied in a variety of Na silicate glasses with and without alkaline-earth modifiers, using one and two-dimensional 23Na and 29Si nuclear magnetic resonance (NMR) spectroscopy. The results suggest that significant structural modifications accompany the Na+/K+ ion exchange process in the form of a shortening of the average Na[sbnd]O and Si[sbnd]NBO distances and an opening of the Si[sbnd]O[sbnd]Si angles without any detectable change in the Qn speciation. These trends are similar to those observed in analogous mixed Na,K glasses derived via the melt-quench route, with increasing K:Na ratio. Consequently, the ion-exchange-induced reorganization of the glass network is accompanied by a partial relaxation of the stress generated by the exchange of smaller Na+ by the larger K+ and better accommodation of the latter ion as a modifier.

Influence on surface characteristics of electron beam melting process (EBM) by varying the process parameters

The use of additive manufacturing processes keeps growing in aerospace and biomedical industry. Among the numerous existing technologies, the Electron Beam Melting process has advantages (good dimensional accuracy, fully dense parts) and disadvantages (powder handling, support structure, high surface roughness). Analyzes of the surface characteristics are interesting to get a better understanding of the EBM operations. But that kind of analyzes is not often found in the literature. The main goal of this study is to determine if it is possible to improve the surface roughness by modifying some parameters of the process (scan speed function, number of contours, order of contours, etc.) on samples with different thicknesses. The experimental work on the surface roughness leads to a statistical analysis of 586 measures of EBM simple geometry parts.

On the relationship between the multiphase microstructure and the mechanical properties of a 0.2C quenched and partitioned steel

In the present work, Quenching and Partitioning (Q&P) heat treatments were carried out in a quench dilatometer on a 0.2 wt% carbon steel. The microstructure evolution of the Q&P steels was characterized using dilatometry, SEM, EBSD and XRD. The martensitic transformation profile was analyzed in order to estimate the fraction of martensite formed at a given temperature below the martensite start temperature Ms. Q&P was shown to be an effective way to stabilize retained austenite at room temperature. However, the measured austenite fractions after Q&P treatments showed significant differences when compared to the calculated values considering ideal partitioning conditions. Indeed, the measured austenite fractions were found to be less sensitive to the quench temperature and were never larger than the ideal predicted maximum fraction. Competitive reactions such as austenite decomposition into bainite and carbide precipitation were found to occur in the present work. Furthermore, a broad range of mechanical properties was obtained when varying the quenching temperatures and partitioning times. The direct contributions between Q&P microstructural constituents -such as retained austenite as well as tempered/fresh martensite- and resulting mechanical properties were scrutinized. This was critically discussed and compared to quenching and austempering (QAT) which is a more conventional processing route of stabilizing retained austenite at room temperature. Finally, Q&P steels were shown to exhibit an interesting balance between strength and ductility. The achievement of this interesting combination of mechanical properties was reached for much shorter processing times compared to QAT steels.

A strategy to improve the work-hardening behavior of Ti-6Al-4V parts produced by additive manufacturing

To improve the mechanical properties of additively manufactured parts, specific heat treatments must be developed. Annealing of electron beam-melted Ti-6Al-4V was performed at sub-transus temperatures and followed by water quenching. Such treatments generate an α + α′ dual-phase microstructure. Microstructural and mechanical characterizations revealed that the heat-treated specimens show a broad range of tensile properties, depending on the fraction of martensite. The specimens treated between 850°C and 920°C exhibit an increase in strength and ductility, which is related to a remarkable hardening behavior. Work-hardening is attributed to kinematic hardening arising from the mechanical contrast between the α and α′ phases.

Effect of Al2O3 content on the mechanical and interdiffusional properties of ion-exchanged Na-aluminosilicate glasses

The effect of the variation in the Al2O3 content on select physical properties of pristine and ion-exchanged glasses in the systems: (75 − x)SiO2mol%-25Na2O-xAl2O3 and (75 − x)SiO2-15Na2O-10CaO-xAl2O3, are reported. The surface compressive stress, surface hardness and K+/Na+ exchange ratio increase, while the depth of the interdiffusion distance decreases, with increasing Al2O3 content. These trends are shown to be consistent with the compositional variation of the glass transition temperature and the molar volume, and their atomistic and thermodynamic bases are discussed.

A comparative study of the reduction of silver and gold salts in water by a cathodic microplasma electrode

A comparative study of the reduction of aqueous silver (Ag) and gold (Au) salts to colloidal Ag and Au nanoparticles, respectively, by a gaseous, cathodic, atmospheric-pressure microplasma electrode is presented. The resulting nanoparticles (NPs) were characterized by ultraviolet-visible (UV-vis) absorption spectroscopy and transmission electron microscopy (TEM), and the aqueous solution composition before and after experiments was determined by ionic conductivity, electrochemical potential, and/or UV-vis absorption measurements. TEM showed that Ag and Au NPs were spherical and non-agglomerated when synthesized in the presence of a stabilizer, polyvinyl alcohol. The charge injected by the plasma was correlated to the maximum intensity in the absorbance spectra which in turn depends on the nanoparticle concentration. Separately, the charge injected was correlated to the metal cation concentration. Ag and Au reduction rates were found to be directly proportional to the charge injected, independent of plasma current and process time. Differences in the mechanism for Ag and Au reduction were also observed, and solution species generated by the plasma and their role in the reduction process (e.g. H2O2, electrons) is discussed.

Local corrosion behavior of additive manufactured AlSiMg alloy assessed by SEM and SKPFM

The local corrosion behavior of additive manufactured AlSiMg specimens was studied using the SEM and SKPFM techniques. A morphological characterization of corroded areas revealed that crystallographic pitting developed in the aluminum grains inside the melt pool borders, from where corrosion spread to adjacent zones. The local Volta potential analysis showed that there is a close relation between the cellular grain size and the potential difference between the silicon and the aluminum phase. SKPFM measurements explained why the melt pool borders were more severely attacked by corrosion than other regions in the surface of the AM specimens. In regions with larger and coarser microstructures, greater potential difference between the phases was found, which represents a higher driving force for galvanic corrosion.

Galvanostatic anodizing of additive manufactured Al-Si10-Mg alloy

The galvanostatic anodizing behavior of additive manufactured (AM) Al-Si10-Mg alloy was studied in H2SO4 electrolyte. The analysis of the voltage vs time response was complemented with a systematic characterization of the anodic oxide layer using a variety of techniques. In addition, a cast alloy of approximately the same chemical composition as that of theAMspecimens was used as a reference in this study. Significant differences were found in the voltage-time characteristics of the samples analyzed. Besides, an anisotropic anodizing behavior was observed in the additive manufactured specimens. Due to the fine silicon microstructure present in the additive manufactured samples, the anodic oxide growth was much more obstructed than for the cast alloy. Nevertheless, even though the oxide layer was generally thinner in the AM samples for the same conditions and anodizing time, a much more continuous and uniform oxide layer was found in the additive manufactured specimens compared to the cast alloy. The porous structure was found to be greatly affected by the fine distribution of the silicon phase in the AM parts.

2016

Size dependent fracture strength and cracking mechanisms in freestanding polycrystalline silicon films with nanoscale thickness

The size dependent fracture strength and fracture mechanisms of polycrystalline silicon films are investigated by analyzing a wide range of specimen lengths and widths with thicknesses equal to 240 and 40 nm. An on-chip method is used to deform the films and to determine the strength. The strength increases with decreasing external surface area. The thinnest films exhibit the lowest strength and a weaker size effect. The crack path changes from transgranular to intergranular with decreasing thickness. These results are related to differences in microstructure and surface roughness characteristics of the films as controlled by the fabrication process and thickness.

Geometrical control of lattice structures produced by EBM through chemical etching: Investigations at the scale of individual struts

A major drawback of metal additive manufacturing is the surface roughness of the manufactured components. This is even more critical when complex lattice structures are considered. An octet-truss lattice structure was fabricated by Electron Beam Melting. A chemical post-treatment was applied in order to improve the surface quality. The morphology of individual struts was characterized experimentally by high resolution X-ray tomography after different chemical etching times. The chemical etching treatment was found to be beneficial as it decreases significantly the occurrence of surface defects. The evolution of the elastic mechanical properties with the etching time was determined by FFT computations directly applied to the 3D volume of the struts. A cellular automaton based model was also developed in order to predict the morphological evolution of the as-built strut during etching. The model enables to predict the kinetics of dissolution as well as the evolution of the surface defects and the elastic mechanical properties of the samples. It also enables to determine the required etching time to firstly remove powder particles stuck to the surface and secondly to reduce the plate-pile like defect occurrence.

Effect of HIPping (Hot Isostatic Pressing) on electron beam melting Ti6Al4V parts after machining

The fast growing of Additive Manufacturing (AM) leads us to study the functionality of parts built by these processes. Recently, the Electron Beam Melting process and the Direct Melting Laser Sintering process are used to produce parts in the biomedical and aeronautical fields. The Ti6Al4V is largely used in these fields. This paper present an experimental study of machining Ti6Al4V alloy produced by Electron Beam Melting (EBM) before and after HIPping (Hot Isostatic Pressing). The results shows that the hipping has no significant influence on specific cutting pressure.

Effect of the electrolyte pH on the corrosion mechanisms of Zn-Mg coated steel

The initial pH effect on the dissolution of zinc-magnesium coated steel is investigated with or without chlorides for long immersion periods. The coating is characterized by TEM, SEM and GDOES. With immersion time, the alloy electrochemical behaviour is investigated by EIS and the corrosion products are analysed using ICP-OES and XPS. In alkaline medium, the formation of zincite is inhibited and chlorides do not hinder a passive layer formation. In chloride electrolyte at natural pH, chlorides are not detected on the metal surface. Chemical composition of corrosion products and their protective effect depend on the initial pH and its evolution.

Improving the mechanical efficiency of electron beam melted titanium lattice structures by chemical etching

The microstructure and mechanical properties of lattice structures of various relative densities manufactured by Electron Beam Melting were analyzed. Their microstructure was compared to that of bulk parts. Special interest was given to the effect of surface roughness on their elastic behavior. An efficient chemical etching post-treatment was performed on these structures. The elastic mechanical behavior of both as-built and etched structures was analyzed. Compression testing revealed that the important decrease in roughness caused by chemical etching results in an increase in relative stiffness, in comparison with an as-built structure of the same relative density.

Dislocation/hydrogen interaction mechanisms in hydrided nanocrystalline palladium films

The nanoscale plasticity mechanisms activated during hydriding cycles in sputtered nanocrystalline Pd films have been investigated ex-situ using advanced transmission electron microscopy techniques. The internal stress developing within the films during hydriding has been monitored in-situ. Results showed that in Pd films hydrided to β-phase, local plasticity was mainly controlled by dislocation activity in spite of the small grain size. Changes of the grain size distribution and the crystallographic texture have not been observed. In contrast, significant microstructural changes were not observed in Pd films hydrided to α-phase. Moreover, the effect of hydrogen loading on the nature and density of dislocations has been investigated using aberration-corrected TEM. Surprisingly, a high density of shear type stacking faults has been observed after dehydriding, indicating a significant effect of hydrogen on the nucleation energy barriers of Shockley partial dislocations. Ab-initio calculations of the effect of hydrogen on the intrinsic stable and unstable stacking fault energies of palladium confirm the experimental observations.

Electron beam melted Ti-6Al-4V: Microstructure, texture and mechanical behavior of the as-built and heat-treated material

Electron Beam Melting (EBM), a powder bed additive layer manufacturing process, was used to produce Ti-6Al-4V specimens, whose microstructure, texture, and tensile properties were fully characterized. The microstructure, analyzed by optical microscopy, SEM/EBSD and X-ray diffraction, consists in fine α lamellae. Numerical reconstruction of the parent β phase highlighted the columnar morphology of the prior β grains, growing along the build direction upon solidification of the melt pool. The presence of grain boundary αGB along the boundaries of these prior β grains is indicative of the diffusive nature of the β→α phase transformation. Texture analysis of the reconstructed high temperature β phase revealed a strong <001> pole in the build direction. For mechanical characterization, tensile specimens were produced using two different build themes and along several build orientations, revealing that vertically built specimens exhibit a lower yield strength than those built horizontally. The effect of post processing, either mechanical or thermal, was extensively investigated. The influence of surface finish on tensile properties was clearly highlighted. Indeed, mechanical polishing induced an increase in ductility - due to the removal of critical surface defects - as well as a significant increase of the apparent yield strength - caused by the removal of a ~150μm rough surface layer that can be considered as mechanically inefficient and not supporting any tensile load. Thermal post-treatments were performed on electron beam melted specimens, revealing that subtransus treatments induce very moderate microstructural changes, whereas supertransus treatments generate a considerably different type of microstructure, due to the fast β grain growth occurring above the transus. The heat treatments investigated in this work had a relatively moderate impact on the mechanical properties of the parts.

2015

Beneficial influence of an intercritically rolled recovered ferritic matrix on the mechanical properties of TRIP-assisted multiphase steels

The present study deals with the microstructure and mechanical properties of intercritically rolled TRIP-assisted multiphase steels. It is shown that the occurrence of the TRIP effect in a recovered ferritic matrix brings about an improved strength-ductility balance with respect to a fully recrystallised ferrite matrix. On the other hand, the intercritical deformation does not influence the austenite transformation rate during straining at room temperature. The improvement of the mechanical properties results from the interactions between the transformation strain and the recovered ferrite.

On the relation between orientation relationships predicted by the phenomenological theory and internal twins in plate martensite

The phenomenological theory of martensite crystallography predicts two equivalent solutions for a particular habit plane in the case of a Fe-Ni-C alloy. Those two solutions differ in the magnitude of the inhomogeneous shear and in the orientation relationship (OR) they hold with austenite. Only the OR associated to the low shear solution has been observed experimentally so far. In the present study, the orientation relationship associated to the high shear solution is assessed experimentally using TEM measurements.

On the relation between orientation relationships predicted by the phenomenological theory and internal twins in plate martensite

The phenomenological theory of martensite crystallography predicts two equivalent solutions for a particular habit plane in the case of a Fe-Ni-C alloy. Those two solutions differ in the magnitude of the inhomogeneous shear and in the orientation relationship (OR) they hold with austenite. Only the OR associated to the low shear solution has been observed experimentally so far. In the present study, the orientation relationship associated to the high shear solution is assessed experimentally using TEM measurements.

Dislocation-mediated relaxation in nanograined columnar palladium films revealed by on-chip time-resolved HRTEM testing.

The high-rate sensitivity of nanostructured metallic materials demonstrated in the recent literature is related to the predominance of thermally activated deformation mechanisms favoured by a large density of internal interfaces. Here we report time-resolved high-resolution electron transmission microscopy creep tests on thin nanograined films using on-chip nanomechanical testing. Tests are performed on palladium, which exhibited unexpectedly large creep rates at room temperature. Despite the small 30-nm grain size, relaxation is found to be mediated by dislocation mechanisms. The dislocations interact with the growth nanotwins present in the grains, leading to a loss of coherency of twin boundaries. The density of stored dislocations first increases with applied deformation, and then decreases with time to drive additional deformation while no grain boundary mechanism is observed. This fast relaxation constitutes a key issue in the development of various micro- and nanotechnologies such as palladium membranes for hydrogen applications.

Silicone rubber encapsulation for an endoscopically implantable gastrostimulator.

Gastrointestinal stimulator implants have recently shown positive results in treating obesity. However, the implantation currently requires an invasive surgical procedure. Endoscopy could be used to place the gastric stimulator in the stomach, hence avoiding the riskier surgery. The implant then needs to go through the oesophagus and be located inside the stomach, which imposes new design constraints, such as miniaturization and protecting the electronic circuit against the highly acidic environment of the stomach. We propose to protect the implant by encapsulation with silicone rubber. This paper lists the advantages of this method compared to the more usual approach of a hermetic enclosure and then presents a method to evaluate the underwater adhesive stability of six adhesive/substrate couples, using repeated lap-shear tests and an elevated temperature to accelerate the ageing process. The results for different adhesive/substrate couples tested, presented on probability plots, show that FR4 and alumina substrates with MED4-4220 silicone rubber are suitable for a first implantable prototype. We then compare these with the predicted lifetimes of bonds between historical standard silicone rubber DC3140 and different substrates and describe the encapsulation of our gastrostimulator.

Multiscale modelling framework for the fracture of thin brittle polycrystalline films: Application to polysilicon

Micro-electro-mechanical systems (MEMS) made of polycrystalline silicon are widely used in several engineering fields. The fracture properties of polycrystalline silicon directly affect their reliability. The effect of the orientation of grains on the fracture behaviour of polycrystalline silicon is investigated out of the several factors. This is achieved, firstly, by identifying the statistical variation of the fracture strength and critical strain energy release rate, at the nanoscopic scale, over a thin freestanding polycrystalline silicon film having mesoscopic scale dimensions. The fracture stress and strain at the mesoscopic level are found to be closely matching with uniaxial tension experimental results. Secondly, the polycrystalline silicon film is considered at the continuum MEMSscale, and its fracture behaviour is studied by incorporating the nanoscopic scale effect of grain orientation. The entire modelling and simulation of the thin film is achieved by combining the discontinuous Galerkin method and extrinsic cohesive law describing the fracture process.

2014

Analysis of Grain Boundary Character in a Fine-Grained Nickel-Based Superalloy 718

In the current work, sheets of superalloy 718 were processed via thermomechanical route by hot and cold rolling, followed by annealing below the δ phase solvus temperature and precipitation hardening to optimum strength. Grain boundary character distribution throughout the processing was mapped via EBSD and its evolution discussed. The results show that it is possible to process the alloy to a fine grain size obtaining concomitantly a considerably high proportion of special boundaries Σ3, Σ9, and Σ27. The precipitation of δ phase presented a strong grain refining role, without significantly impairing the twinning mechanism and, consequently, the Σ3, Σ9, and Σ27 boundary formations.

Crystallographic Reconstruction Study of the Effects of Finish Rolling Temperature on the Variant Selection During Bainite Transformation in C-Mn High-Strength Steels

The effect of finish rolling temperature on the austenite-(γ) to-bainite (α) phase transformation is quantitatively investigated in high-strength C-Mn steels using an alternative crystallographic γ reconstruction procedure, which can be directly applied to experimental electron backscatter diffraction mappings. In particular, the current study aims to clarify the respective contributions of the γ conditioning during the hot rolling and the variant selection during the phase transformation to the inherited texture. The results confirm that the sample finish rolled at the lowest temperature [1102 K (829 °C)] exhibits the sharpest transformation texture. It is shown that this sharp texture is exclusively due to a strong variant selection from parent brass {110}〈112〉, S {213}〈364〉 and Goss {110}〈001〉 grains, whereas the variant selection from the copper {112}〈111〉 grains is insensitive to the finish rolling temperature. In addition, a statistical variant selection analysis proves that the habit planes of the selected variants do not systematically correspond to the predicted active γ slip planes using the Taylor model. In contrast, a correlation between the Bain group to which the selected variants belong and the finish rolling temperature is clearly revealed, regardless of the parent orientation. These results are discussed in terms of polygranular accommodation mechanisms, especially in view of the observed development in the hot-rolled samples of high-angle grain boundaries with misorientation axes between 〈111〉γ and 〈110〉γ.

Mechanical characterization of cruciate and collateral ligaments of human knee

Plasticity mechanisms in ultrafine grained freestanding aluminum thin films revealed by in-situ transmission electron microscopy nanomechanical testing

In-situ bright field transmission electron microscopy (TEM) nanomechanical tensile testing and in-situ automated crystallographic orientation mapping in TEM were combined to unravel the elementary mechanisms controlling the plasticity of ultrafine grained Aluminum freestanding thin films. The characterizations demonstrate that deformation proceeds with a transition from grain rotation to intragranular dislocation glide and starvation plasticity mechanism at about 1% deformation. The grain rotation is not affected by the character of the grain boundaries. No grain growth or twinning is detected. © 2014 AIP Publishing LLC.

An alternative to the crystallographic reconstruction of austenite in steels

An alternative crystallographic austenite reconstruction programme written in Matlab is developed by combining the best features of the existing models: the orientation relationship refinement, the local pixel-by-pixel analysis and the nuclei identification and spreading strategy. This programme can be directly applied to experimental electron backscatter diffraction mappings. Its applicability is demonstrated on both quenching and partitioning and as-quenched lath-martensite steels. © 2014 Elsevier Inc.

Critical assessments and thermodynamic modeling of BaO-SiO2 and SiO2-TiO2 systems and their extensions into liquid immiscibility in the BaO-SiO2-TiO2 system

This study discusses rational reproduction of liquid immiscibility in the BaO-SiO2-TiO2 system. While a ternary assessment requires sub-binary descriptions in the same thermodynamic model, the related sub-binary systems BaO-SiO2, BaO-TiO2 and SiO2-TiO 2 liquid and solid phases have been evaluated using different thermodynamic models in the literature. In this study, BaO-SiO2 and SiO2-TiO2 were assessed using the Ionic Two Sublattice model (I2SL) based on experimental data from the literature. BaO-TiO2 was already assessed using this model. Binary descriptions developed were then used for the assessment of liquid immiscibility in the BaO-SiO 2-TiO2 system. Ternary interaction parameters were found necessary for rational reproduction of the new ternary experimental data gathered in the present work. The model parameters for each system were evaluated using a CAPLHAD approach. A set of parameters is proposed. They show good agreement between the calculated and experimental equilibrium liquidus, liquid immiscibility and thermochemical properties in the BaO-SiO 2-TiO2 system. © 2014 Elsevier Ltd. All rights reserved.

Formation of Nanostructures in Severely Deformed High-Strength Steel Induced by High-Frequency Ultrasonic Impact Treatment

Surface modification by the generation of a nanostructured surface layer induced via ultrasonic impact treatment was performed at the weld toe of a welded high-strength quenched and tempered structural steel, S690QL1 (Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt pct)). Such high-frequency peening techniques are known to improve the fatigue life of welded components. The nanocrystallized structure as a function of depth from the top-treated surface was characterized via a recently developed automated crystal orientation mapping in transmission electron microscopy. Based on the experimental observations, a grain refinement mechanism induced by plastic deformation during the ultrasonic impact treatment is proposed. It involves the formation of low-angle misoriented lamellae displaying a high density of dislocations followed by the subdivision of microbands into blocks and the resulting formation of polygonal submicronic grains. These submicronic grains further breakdown into nano grains. The results show the presence of retained austenite even after severe surface plastic deformation. The average grain size of the retained austenite and martensite is (Formula presented).35 nm, respectively. The in-grain deformation mechanisms are different in larger and smaller grains. Larger grains show long-range lattice rotations, while smaller grains show plastic deformation through grain rotation. Also the smaller nano grains exhibit the presence of short-range disorder. Surface nanocrystallization also leads to an increased fraction of low angle and low energy coincident site lattice boundaries especially in the smaller grains ((Formula presented). nm).

2013

Influence of amorphous phase separation on the crystallization behavior of glass-ceramics in the BaO-TiO2-SiO2 system

The possible role of a prior amorphous phase separation on the subsequent crystallization has been the topic of vigorous debates over the last decades and has not yet been clarified, especially regarding the role of the interfaces created by the phase separation. This study proposes to focus on the interplay between a prior amorphous phase separation and the crystallization of fresnoite in the BaO-TiO2-SiO2 system. The crystallization behavior of a non-stoichiometric composition inside the miscibility gap (called APS) is compared with the stoichiometric composition (called FRES) and a non-stoichiometric composition outside the miscibility gap (called NoAPS). The crystallization mechanisms are compared using differential thermal analysis (DTA) by calculating the Avrami parameters and the activation energies as a function of the particle size. The DTA study shows that the two non-stoichiometric compositions exhibit a pronounced surface crystallization behavior whereas FRES undergoes bulk nucleation. This is supported by a multi-scale microstructure characterization. Furthermore, this study demonstrates that the amorphous phase separation and the associated interfaces do not play any significant role in the nucleation step. Moreover, transmission electron microscope (TEM) and local orientation measurements show that the growth of the dendrites in not hindered by the SiO2-rich droplets. The final stage of crystallization of APS is tentatively explained by two compositions effects that must be further investigated: the viscosity effect and the formation of a eutectic.

Structure and formation mechanism of rolled-in oxide areas on aluminum lithographic printing sheets

The subsurface area introduced during rolling on the 1100 aluminum alloy series alters its surface properties, which makes it more susceptible to corrosion. A combination of different transmission electron microscopy techniques is employed to observe the orientation of small grain structures and the distribution elements in the subsurface layer. This approach provided valuable insight into the formation mechanism of the layer and the phenomena taking place during rolling. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Nanoscale characterization of the evolution of the twin-matrix orientation in Fe-Mn-C twinning-induced plasticity steel by means of transmission electron microscopy orientation mapping

The evolution of the orientation relationship between mechanical twins and the surrounding matrix with the degree of plastic deformation has been characterized at the nanoscale in twinning-induced plasticity steel. The recently developed automated crystal orientation mapping in transmission electron microscopy revealed that the ideal twin relationship is retained up to large levels of strain, while large orientation gradients are built up within the matrix. This particular evolution undoubtedly plays a role in the large work hardening rate of these steels.© 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Effect of deposition rate on the microstructure of electron beam evaporated nanocrystalline palladium thin films

The influence of the deposition rate on the formation of growth twins in nanocrystalline Pd films deposited by electron beam evaporation is investigated using transmission electron microscopy. Statistical measurements prove that twin boundary (TB) density and volume fraction of grains containing twins increase with increasing deposition rate. A clear increase of the dislocation density was observed for the highest deposition rate of 5 Å/s, caused by the increase of the internal stress building up during deposition. Based on crystallographic orientation indexation using transmission electron microscopy, it can be concluded that a {111} crystallographic texture increases with increasing deposition rate even though the {101} crystallographic texture remains dominant. Most of the TBs are fully coherent without any residual dislocations. However, for the highest deposition rate (5 Å/s), the coherency of the TBs decreases significantly as a result of the interaction of lattice dislocations emitted during deposition with the growth TBs. The analysis of the grain boundary character of different Pd films shows that an increasing fraction of high angle grain boundaries with misorientation angles around 55-65 leads to a higher potential for twin formation. © 2013 Elsevier B.V. All rights reserved.

Automatic crystallographic characterization in a transmission electron microscope : Applications to twinning induced plasticity steels and al thin films

A new automated crystallographic orientation mapping tool in a transmission electron microscope technique, which is based on pattern matching between every acquired electron diffraction pattern and precalculated templates, has been used for the microstructural characterization of nondeformed and deformed aluminum thin films and twinning-induced plasticity steels. The increased spatial resolution and the use of electron diffraction patterns rather than Kikuchi lines make this tool very appropriate to characterize fine grained and deformed microstructures. Copyright © Microscopy Society of America 2013 Â.

Structure analysis of aluminium silicon manganese nitride precipitates formed in grain-oriented electrical steels

We report a detailed structural and chemical characterisation of aluminium silicon manganese nitrides that act as grain growth inhibitors in industrially processed grain-oriented (GO) electrical steels. The compounds are characterised using energy dispersive X-ray spectrometry (EDX) and energy filtered transmission electron microscopy (EFTEM), while their crystal structures are analysed using X-ray diffraction (XRD) and TEM in electron diffraction (ED), dark-field, high-resolution and automated crystallographic orientation mapping (ACOM) modes. The chemical bonding character is determined using electron energy loss spectroscopy (EELS). Despite the wide variation in composition, all the precipitates exhibit a hexagonal close-packed (h.c.p.) crystal structure and lattice parameters of aluminium nitride. The EDX measurement of ~ 900 stoichiometrically different precipitates indicates intermediate structures between pure aluminium nitride and pure silicon manganese nitride, with a constant Si/Mn atomic ratio of ~ 4. It is demonstrated that aluminium and silicon are interchangeably precipitated with the same local arrangement, while both Mn2 + and Mn3 + are incorporated in the h.c.p. silicon nitride interstitial sites. The oxidation of the silicon manganese nitrides most likely originates from the incorporation of oxygen during the decarburisation annealing process, thus creating extended planar defects such as stacking faults and inversion domain boundaries. The chemical composition of the inhibitors may be written as (AlN)x(SiMn0.25N yOz)1 - x with x ranging from 0 to 1. © 2013 Elsevier Inc.

2012

Strain-induced development of very fine ferrite-cementite structures in eutectoid steels

The development of duplex microstructures consisting of very fine ferrite grains and spheroidized cementite particles during concurrent hot deformation has been investigated by hot torsion in a eutectoid steel. It is shown that the hot deformation of the undercooled austenite at temperatures close to Ar 1 induces and accelerates significantly the γ-to-pearlite transformation. Further deformation of the transforming pearlite does not lead only to the instantaneous spheroidization of the cementite lamellae but also refines the ferrite orientations units from 15 to 3 μm. Microstructures consisting of very fine ferrite grains (3 μm) and submicron cementite particles are produced at strain levels larger than ε = 1.5. The initial grains undergo progressive fragmentation by the generation of low angle boundaries (LABs) by dynamic recovery that progressively transform into high angle boundaries (HABs). The progressive increase of LABs misorientation is attributed to the presence of cementite particles and is referred to as continuous dynamic recrystallization (cDRX). Fine prior austenite grain size and moderate strain rates are found to promote the occurrence of cDRX. The resulting torsion textures consist of a strong 〈111〉 fiber dominated by the D {112} 〈111〉 component. During the subsequent annealing step, cementite particles coarsen preferentially at the triple junctions whereas the very fine ferrite grain are stable due to the strong pinning effect exerted by the cementite particles. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Advanced TEM investigation of the plasticity mechanisms in nanocrystalline freestanding palladium films with nanoscale twins

Nanocrystalline palladium thin films deposited by electron-beam evaporation and deformed by on-chip tensile testing reveal a surprisingly large strain hardening capacity when considering the small ∼25 nm grain size. The as-grown films contain several coherent single and multifold twin boundaries. The coherency of the twin boundaries considerably decreases with deformation due to dislocation/twin boundary interactions. These reactions are described based on a detailed analysis of the number and the type of dislocations located at the twin boundaries using high-resolution TEM, including aberration corrected microscopy. Sessile Frank dislocations were observed at the twin/matrix interfaces, explaining the loss of the TB coherency due to the Burgers vector pointing out of the twinning plane. Grain boundary mediated processes were excluded as a mechanism dominating the plastic deformation based on the investigation of the grain size distribution as well as the crystallographic texture using Automated Crystallographic Orientation Indexation TEM. Other factors influencing the plastic deformation such as impurities and the presence of a native passivation oxide layer at the surface of the films were investigated using analytical TEM. The twin boundaries observed in the present work partly explain the high strain hardening capacity by providing both increasing resistance to dislocation motion with deformation and a source for dislocation multiplication. © 2012 Elsevier Ltd. All rights reserved.

Deformation induced pearlite transformation and spheroidization: Effect of alloying additions

The aim of this research is to investigate the effect of Cr and Al (strong ferrite formers) on the strain-induced γ-to-pearlite transformation in eutectoid steels. The microstructure evolution during the hot deformation of three eutectoid steel grades was investigated using hot torsion testing. More specifically, the steels were deformed to strain levels varying from ε = 0,5 to ε = 1,5 at their specific Ar1 temperature. Hot deformation of the undercooled austenite leads to strain-induced γ-topearlite transformation and to the almost instantaneous spheroidization of the formed carbides. The corresponding microstructures consist of submicronic cementite particles and ferritic grains that are 1-5 μm in size. It is shown that 1,5% Cr addition and 0,5% Al addition increase the equilibrium transformation temperature but slower significantly the kinetics of the strain-induced transformation and consequently reduce the kinetics of cementite spheroidization and of ferrite recrystallization. © (2012) Trans Tech Publications, Switzerland.

Synthesis and antibacterial activity of silver nanoparticles against gram-positive and gram-negative bacteria

Synthesis of nanosized particles with antibacterial properties is of great interest in the development of new pharmaceutical products. Silver nanoparticles (Ag NPs) are known to have inhibitory and bactericidal effects. In this article we present the synthesis of Ag NPs prepared by chemical reduction from aqueous solutions of silver nitrate, containing a mixture of hydrazine hydrate and sodium citrate as reductants and sodium dodecyl sulfate as a stabilizer. The results of the characterization of the Ag NPs show agglomerates of grains with a narrow size distribution (from 40 to 60 nm), whereas the radii of the individual particles are between 10 and 20 nm. Finally, the antibacterial activity was measured by the Kirby-Bauer method. The results showed reasonable bactericidal activity against Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus. The standard dilution micromethod, determining the minimum inhibitory concentration leading to inhibition of bacterial growth, is still under way. Preliminary results have been obtained. From the Clinical Editor: In this paper the synthesis of Ag NPs via chemical reduction from aqueous solutions is discussed. Reasonable bactericidal activity against Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus was demonstrated. © 2012 Elsevier Inc.

2011

Grain scale analysis of variant selection during the gamma-epsilon-alpha phase transformation in austentic steels

Grain scale analysis of variant selection during the gamma-epsilon-alpha' phase transformation in austenitic steels

Austenitic steels can exhibit a complex transformation sequence during deformation. Indeed, the austenitic phase transforms first into bands of ε (HCP) martensite. This transformation is then followed by the formation of α' (BCC) martensite. In this study, the crystallography of the transformation together with the occurrence of variant selection is studied at the scale of individual austenite grains. About ten prior austenite grains deformed at different strain levels in uniaxial tension were analysed by means of EBSD techniques. One of the classical approaches to predict the variant selection phenomenon is based on the calculation of the interaction energy between the macroscopic stress and the shape deformation associated with the formation of the product phase. The formation of the α' variants was observed to lead to a very strong variant selection that cannot be fully explained by energetic criterion. It is suggested that the crystallography of the transformation sequence can account for the unexpected variants. © (2011) Trans Tech Publications.

Direct spheroidization of high carbon steels: Effect of thermomechanical processing

The eutectoid transformation of austenite can occur cooperatively (pearlite transformation) or by means of a non-cooperative mode (Divorced Eutectoid Transformation). In the cooperative mode, ferrite and cementite grow together, leading to the typical lamellar microstructure of pearlite. In the non-cooperative mode, spheroidal cementite particles grow directly from the undissolved carbides in the austenite phase. The transformation product is a fully spheroidized microstructure. In this study, the parameters promoting the occurrence of DET in a hypereutectoid steel (austenitization temperature, cooling rate, presence of proeutectoid cementite in the initial microstructure) were investigated. It is shown that low undercooling levels and a homogenous distribution of fine carbides in the austenite promote the DET over the lamellar transformation mode. The spheroidized microstructures produced by DET lead to larger ductilities comparing to those obtained by the lamellar transformation mode. © (2011) Trans Tech Publications.

2010

Direct spheroidization of high carbon steels : effect of thermomechanical processing

Microstructural evolution during spheroidization annealing of eutectoid steel : effect of interlamellar spacing and cold working

Eutectoid steels present a wide range of interesting mechanical properties (high strength, wear resistance, ductility and toughness) and could be a cheaper alternative to high strength low-alloyed steels (HSLA) in applications where weldability is not a critical requirement. The mechanical properties of pearlite are mainly dictated by the interlamellar spacing and the spheroidization of cementite. In this work, the spheroidization kinetics during annealing of a fully pearlitic steel produced in an electric arc furnace (EAF) is investigated. More specifically, the influence of a prior cold deformation and of the interlamellar spacing is studied using image analysis and hardness tests. It is shown that spheroidization is faster in fine pearlite than in coarse pearlite. Prior cold deformation strongly accelerates the spheroidization kinetics in fine and coarse pearlite. The tensile properties corresponding to different pearlite microstructure were measured and are compared to the hardness evolution during annealing. © (2010) Trans Tech Publications.

Quantitative analysis of variant selection and orientation relationships during the γ-to-α phase transformation in hot-rolled TRIP steels

2009

Variant selection during the γ-to-αb phase transformation in hot-rolled bainitic TRIP-aided steels

The variant selection phenomenon during the austenite to bainite phase transformation in hot-rolled TRIP-aided steels was quantitatively characterized at the level of individual austenite grains. The reconstruction of the electron backscatter diffraction maps provided evidence that bainite grows by packets of laths sharing a common {1 1 1}γ plane in the austenite. The affect of hot deformation is to reduce the number of packets that form. It is suggested that slip activity is important in understanding this effect. © 2009 Acta Materialia Inc.

Synthesis of silver nanoparticles by chemical reduction method and their antibacterial activity

Silver nanoparticles were prepd. by chem. redn. method. Silver nitrate was taken as the metal precursor and hydrazine hydrate as a reducing agent. The formation of the silver nanoparticles was monitored using UV-Vis absorption spectroscopy. The UV-Vis spectroscopy revealed the formation of silver nanoparticles by exhibiting the typical surface plasmon absorption maxima at 418-420 nm from the UV-Vis spectrum. Comparison of theor. (Mie light scattering theory) and exptl. results showed that diam. of silver nanoparticles in colloidal soln. is about 60 nm. We have used energy-dispersive spectroscopy (EDX), X-ray diffraction (XRD), transmission electron microscopy (TEM) and, UV-Vis spectroscopy to characterize the nanoparticles obtained. The energy-dispersive spectroscopy (EDX) of the nanoparticles dispersion confirmed the presence of elemental silver signal no peaks of other impurity were detected. The av. size and morphol. of silver nanoparticles were detd. by transmission electron microscopy (TEM). TEM photographs indicate that the nanopowders consist of well dispersed agglomerates of grains with a narrow size distribution (40 and 60 nm), whereas the radius of the individual particles are between 10 and 20 nm. The synthesized nanoparticles have been structurally characterized by X-ray diffraction and transmission high-energy electron diffraction (HEED). The peaks in the XRD pattern are in good agreement with the std. values of the face-centered-cubic form of metallic silver and no peaks of other impurity cryst. phases were detected. Addnl., the antibacterial activity of the nanoparticles dispersion was measured by Kirby-Bauer method. The nanoparticles of silver showed high antimicrobial and bactericidal activity against gram pos. bacteria such as Escherichia Coli, Pseudimonas aeruginosa and Staphylococcus aureus which is a highly methicillin resistant strain.

Synthesis of silver nanoparticles by chemical reduction method and their antibacterial activity

Silver nanoparticles were prepd. by chem. redn. method. Silver nitrate was taken as the metal precursor and hydrazine hydrate as a reducing agent. The formation of the silver nanoparticles was monitored using UV-Vis absorption spectroscopy. The UV-Vis spectroscopy revealed the formation of silver nanoparticles by exhibiting the typical surface plasmon absorption maxima at 418-420 nm from the UV-Vis spectrum. Comparison of theor. (Mie light scattering theory) and exptl. results showed that diam. of silver nanoparticles in colloidal soln. is about 60 nm. We have used energy-dispersive spectroscopy (EDX), X-ray diffraction (XRD), transmission electron microscopy (TEM) and, UV-Vis spectroscopy to characterize the nanoparticles obtained. The energy-dispersive spectroscopy (EDX) of the nanoparticles dispersion confirmed the presence of elemental silver signal no peaks of other impurity were detected. The av. size and morphol. of silver nanoparticles were detd. by transmission electron microscopy (TEM). TEM photographs indicate that the nanopowders consist of well dispersed agglomerates of grains with a narrow size distribution (40 and 60 nm), whereas the radius of the individual particles are between 10 and 20 nm. The synthesized nanoparticles have been structurally characterized by X-ray diffraction and transmission high-energy electron diffraction (HEED). The peaks in the XRD pattern are in good agreement with the std. values of the face-centered-cubic form of metallic silver and no peaks of other impurity cryst. phases were detected. Addnl., the antibacterial activity of the nanoparticles dispersion was measured by Kirby-Bauer method. The nanoparticles of silver showed high antimicrobial and bactericidal activity against gram pos. bacteria such as Escherichia Coli, Pseudimonas aeruginosa and Staphylococcus aureus which is a highly methicillin resistant strain.

The combined effect of static recrystallization and twinning on texture in magnesium alloys AM30 and AZ31

The potential for decreasing the texture intensity generated during the bulk deformation of Mg alloys was investigated using a combination of contraction twinning, double (secondary) twinning and static recrystallization. A large number of twins was induced by tensile deformation at room temperature. Their volume fraction and the variants selected during straining were found to be largely responsible for the changes evident in the deformation texture. Recrystallization of the twins generated a fine-grained microstruc-ture, although no growth into the matrix grains was observed. In this way, annealing of the deformed samples did not lead to significant further texture changes. © Carl Hanser Verlag GmbH & Co. KG.

2008

Microstructure, texture and mechanical properties of AZ31 Mg alloy produced by equal channel angular extrusion

Mass transfer and current distribution on a metallic wire

The current and copper deposit distribution on a metallic fibre (stainless steel, diameter of 2 mm) was studied numerically and experimentally. The experimental copper deposit was measured with an optical microscope and the current distribution was deduced. The influence of electrolysis time on copper deposit distribution was also studied. A typical current tertiary distribution was observed. The experiment with a longer electrolysis time exhibited a larger current variation around the wire. A numerical study of this problem was also carried out. The simulation involved a laminar and turbulent flow solver together with a numerical model for the mass transfer of ionic species due to diffusion, migration and convection. A good correlation was found between simulated and experimental results for experiments with a short electrolysis time. This numerical model was then used to study the influence of the flow velocity and the diffusion coefficient on the current density and on the average mass transfer around a wire a few microns in diameter. The general relation: Shp = 0.41 Rep"Sh 0.44 for 0.02 < Rep < 14.22 and 1000 < Sc < 12,000 was obtained. Comparison with data available in the literature demonstrates good agreement between our model and previous results. (c) 2008 Elsevier Ltd. All rights reserved.

An EBSD study of the misorientations related to dynamic recrystallization in Mg AM30 deformed at high temperatures

2007

Twinning and texture development in two Mg alloys subjected to loading along three different strain paths

The evolution of twinning and texture in two Mg-based (+Al, Mn, Zn) alloys was investigated using uniaxial tension, uniaxial compression and ring hoop tension testing at temperatures from ambient to 250 °C and a strain rate of 0.1 s-1. The results indicate that the initial extrusion texture plays an important role in the formation of different types of twins and that the twinning behavior also depends on the strain path. Contraction and double twinning are the dominant twinning mechanisms in uniaxial tension, while extension twinning prevails in uniaxial compression and ring hoop tension testing. Schmid factor analysis indicates that only components that are favorably oriented (i.e., with the highest SF values) can undergo rapid and complete twinning. The different twinning behaviors are shown to be responsible for the sharply contrasting strain hardening characteristics of the experimental flow curves and dramatic texture changes. © 2007 Acta Materialia Inc.

Influence of {10-12} extension twinning on the flow behavior of AZ31 Mg alloy

Uniaxial compression tests were performed on samples cut along the extrusion direction from AZ31 Mg alloy tubes. A stage of increasing work hardening rate was observed on representative true sigma-epsilon curves. Specimens compressed to various strain levels were examined by optical microscopy and electron backscattered diffraction (EBSD) techniques. The results indicate that the widespread formation of intersecting {10-12} extension twins is responsible for the increased strain hardening rate. (c) 2006 Elsevier B.V. All rights reserved

2006

Transformation textures in As-hot rolled TRIP steels

The microstructures of TRIP steels finish-rolled above and below the recrystallization-stop temperature (T-nr) are compared. Here, the retained austenite grains are equiaxed or elongated, respectively, according to whether final rolling was carried out above or below the Tnr. The recrystallized austenite did not contain a sharp texture, the best defined component of which was the cube. The bainite that formed in this case was characterized by weak concentrations of the Goss and rotated Goss and a fairly strong concentration of the rotated cube. It also displayed the transformation products of a retained rolling fibre in the prior austenite. The deformed austenite contained the typical fcc rolling texture, where the copper is considerably more intense than the brass under these conditions. After transformation to bainite, the presence of a strong transformed copper component is evident, together with somewhat less intense contributions from the three transformed brass components. The data indicate that strong variant selection took place in the deformed austenite and that it was also present in the recrystallized material, but to a lesser extent. The latter displayed evidence of incomplete recrystallization in that the transformation texture included components obtained from both "recrystallized" and "deformed" austenite.

Grain refinement of TRIP-Assisted multiphase steels through strain-induced phase transformation

To develop high performance steels for automotive applications, enhanced strengthening mechanisms are required. This study aims at assessing the critical parameters leading to the refinement of the strain-induced ferrite matrix of thermomechanically processed multiphase steels. Hot rolling simulations allowed the definition of the temperature, strain and cooling rate conditions bringing about the formation of strain-induced ferrite with a reduced grain size. The relationship between the deformation and the concurrent or subsequent phase transformations is highlighted thanks to a thorough characterisation of the generated microstructures. It is also shown that the prior austenite grain size influences the distribution of the second phases within the finely grained ferrite matrix.

Observations of the Gibeon meteorite and the inverse Greninger-Troiano orientation relationship

All the possible parallelism conditions associated with low-index crystallographic planes and directions in face- and body-centred cubic crystals are enumerated in terms of their minimum angle-axis misorientations. These include the Kurdjumov-Sachs (K-S), Nishiyama-Wassermann (N-W) and Pitsch orientation relationships, which are expressed in terms of close-packed (and near-close-packed) planes, close-packed directions, or both in the two crystal systems. By classifying these into either coplanar or codirectional relationships, it is possible to specify intermediate relations with either high-index directions or planes. In such a way, the Greninger-Troiano (G-T) relationship is approximated as a coplanar relation lying between the K-S and N-W relations. Based on geometric symmetry, a further correspondence relationship, which falls into the codirectional category and is located between the K-S and Pitsch relations, is represented both in pole-figure form and in Rodrigues-Frank space. It is referred to here as the inverse G-T, or G-T'. Experimental evidence from the plessite regions of the Gibeon meteorite is presented to support the validity of this relationship. It is shown that, in plessite, all the well known models, as well as their intermediates, are applicable to the gamma-to-alpha transformation. By contrast, in the Widmanstatten portions, the misorientations are largely limited to the K-S, G-T and N-W relations.

Use of Schmid factors to select extension twin variants in extruded magnesium alloy tubes

By analogy with dislocation glide, a Schmid factor criterion is often used in the case of twinning. The deformation textures of extruded AM30 magnesium alloy tubes deformed along different directions show clear evidence that extension twinning is responsible for the texture changes and that its activation is dictated by the Schmid factors applicable to different twinning planes. The rules for the selection of the twin variants in the present case are particularly well defined because of the sharp texture of the starting material. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Micromechanical characterisation of TRIP-assisted multiphase steels by in situ neutron diffraction

The flow behaviour of the constitutive phases in multiphase steels, possibly exhibiting a mechanically-induced phase transformation ( TRIP effect), is investigated using neutron diffraction conducted during uniaxial tensile loading. The BCC and FCC lattice strains of several specimens containing different amounts ferrite, bainite, martensite and metastable retained austenite are measured along elastic and plastic deformation. The validity of the measurements, as well as the strengthening resulting from the TRIP effect, are evaluated on the basis of overall mechanical equilibrium.

Texture evolution during the recrystallization of a warm-rolled low-carbon steel

The texture changes taking place during the recrystallization of a warm-rolled low-carbon steel were examined using electron backscattered diffraction. The deformation textures of the warm-rolled material are similar in shape to those of cold-rolled materials, but are somewhat more intense. The recrystallization textures resemble the deformation textures but with a more extended alpha fibre that includes the {133} < 471 > orientation; the gamma fibre extends to the {554}< 225 > orientation. These two orientations are related to the {112}< 110 > deformed grains by near 260 rotations about selected < 110 > axes. Nevertheless, both orientations appear in the early stages of recrystallization, an observation that does not support the oriented growth theory. The {111}< hkl > orientations are the first to recrystallize while the alpha fibre is present until the end of recrystallization. It is finally consumed by all types of grains as well as by subgrain coalescence. The similarities in the growth rates for the {111}< hkl > and random orientations and the late disappearance of the a fibre suggest that recrystallization takes place according to the high stored energy oriented nucleation concept. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Crystallographic relations between face- and body-centred cubic crystals formed under near-equilibrium conditions: Observations from the Gibeon meteorite

The orientations of the kamacite lamellae formed from a single prior-taenite grain were measured by analysing the electron backscatter diffraction patterns obtained using scanning electron microscopy. These are shown to be close to the Kurdjumov-Sachs and Nishiyama-Wassermann relations and their intermediate, i.e., the Greninger-Troiano relation. The orientations of the alpha grains in the plessite regions were also measured and these were found to be continuously distributed around the Bain circles formed by the variants of the common correspondence relationships, including the Pitsch one in this case. The local rnisorientations between individual face- and body-centred cubic crystals along their common interfaces were measured. These can be characterized by the orientation relationships mentioned above as long as a certain amount of tolerance is allowed. Orientation variations within individual kamacite lamellae were also analysed. The crystallographic data support the view that somewhat different mechanisms are involved in the formation of Widmanstatten structures and of the plessite in meteorites. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Crystallographic features of the gamma-to-alpha transformation in a Nb-added transformation-induced plasticity steel

A Nb-bearing transformation-induced plasticity (TRIP) steel was control rolled and a certain amount of austenite was retained through appropriate heat treatment. Electron backscatter diffraction (EBSD) measurements were conducted on specimens deformed to various reductions, and the orientations of the a grains formed within individual prior-austenite grains were compared to those expected from the common correspondence relationships. While most of the bainite orientations cluster around the Bain circles formed by these relations, the polygonal (allotriomorphic) ferrite formed at the austenite grain boundaries does not obey any of these relations with respect to the neighboring austenite orientations. Grain-scale variant selection was observed in both the deformed and undeformed austenite grains. The crystallographic features of the deformation-induced ferrite transformation are also discussed

Twinning-induced softening in polycrystalline AM30 Mg alloy at moderate temperatures

The effect of deformation twinning oil the strain hardening behavior of a polycrystalline AM30 Mg alloy was investigated. At temperatures < 150 degrees C, somewhat unusually, the n value increases with increasing temperature and decreasing strain rate. The softening effect C! induced by double and contraction twinning is responsible for the abnormal strain hardening behavior. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

2005

Representation of misorientations in Rodrigues-Frank space. Application to the Bain, Kurdjumov-Sachs, Nishiyama-Wassermann, Pitsch and Greninger-Troiano orientation relationships

Orientation relationships between individual crystals can be readily represented in Rodrigues-Frank space because of the 1-to-1 correspondence between each misorientation and a vector in the fundamental zone of this space. This is done by integrating the rotation angle and axis into a 3-component vector. The 3 classical orientation relationships describing the γ-to-α transformation, namely the Bain, Kurdjumov-Sachs, and Nishiyama-Wassermann, are represented in Rodrigues-Frank space. Also considered are the somewhat less common Pitsch and Greninger-Troiano relationships. The misorientations between these types of transformation variants are displayed in R-F space based on alternative ref. systems to highlight the differences. Examples of the various crystallog. relationships between fcc. and bcc. crystals during the γ-to-α transformation are given to demonstrate the advantages of the use of this space.

Microscale characterization of deformed microstructures of TRIP-assisted and multiphase steels

The improvement of the mech. behavior of high performance steels brings about a renewed interest for the work hardening rate resulting from deformation-induced martensitic transformation or mech. twinning. Even if these mechanisms are known for quite a long time, the deformation - transformation interactions that they induce is not yet fully characterized and understood. This study aims at characterizing the microstructure evolution of a Fe-Mn steel grade during straining thanks to TEM and high resoln. OIM. Particular patterns of austenite - ε and α' martensite are found.

Crystallographic relationships between FCC and BCC crystals: A study using EBSD techniques

The mechanisms governing the formation of transformation textures during the austenite-to-ferrite transformation are the subject of major debate. In this study, two extreme cases were examined: those of undeformed and deformed austenite. The first involves the transformation of austenite into Widmanstatten ferrite under "equilibrium" conditions in the Gibeon iron-nickel meteorite. This meteorite passed through the transformation at the rate of a few degrees per million years. Such cooling rates cannot of course be reached under laboratory conditions. The second concerns the transformation of hot rolled austenite after a quench into the bainite temperature range. These two behaviors were investigated by means of optical microscopy and electron backscatter diffraction (EBSD) techniques. The orientations of both the parent and product phases were measured and the orientation relationships are represented in Rodrigues-Frank (R-F) space. From the orientation of a particular FCC crystal, the crystallographic orientations of the product BCC crystals can be predicted according to the Bain, Kurdjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W) correspondence relationships. Comparison of the predicted and measured orientations reveals that the Bain rotation is never observed; the K-S and N-W relationships are both observed and there is a continuous distribution of orientations between the exact K-S and N-W positions. The formation of preferred orientations under non-equilibrium conditions is scrutinized. These results are compared to recent models accounting for variant selection.

Multiscale characterisation of the transformation texture in a high performance steel

The orientation relationships operating during the austenite (FCC) to BCC phase transformation were investigated in a high performance steel using X-ray and electron diffraction techniques and employing several length scales. These steels contain some retained austenite that permits the direct comparison of the textures of the parent (austenite) and product (ferrite, bainite, martensite) phases. X-ray diffraction allowed the global texture of the rolled parent austenite phase to be determined as well as that of its transformation product. EBSD techniques permitted study of the orientation relationships on a local scale. The observed correspondence relations are expressed in Rodrigues-Frank space. The exact Kurdjumov-Sachs relation was never found. The local spread of orientation in the parent austenite (due to deformation) is seen to be inherited by the bainite. This is attributed to the displacive mode of transformation to bainite. The influence of prior deformation of the austenite on the occurrence of variant selection was also studied. It appears that a critical strain is necessary to produce a significant amount of variant selection.

Development of high performance Nb-added hot rolled bainitic steels presenting a TRIP effect

The current main challenge of the automotive industry is to reduce the fuel consumption of vehicles by increasing the efficiency of their structural parts. It requires the development of new steels that combine the necessary ductility for the forming operations and high strength levels. This unique combination of properties depends on specific microstructures and particular strengthening mechanisms like the TRIP effect. This study deals with the influence of the hot rolling parameters on the microstructure and mechanical properties of 0.4C-1.5Mn-1.5Si steels with or without Nb microaddition. Deformation under the T-nr temperature leads to a refinement of the microstructure consisting of retained austenite and bainite and to an enhancement of the mechanical properties. Furthermore, the deformation leads to a larger amount of retained austenite and to intragranularly nucleated bainitic ferrite. The effects of the Nb addition and of the deformation temperature are demonstrated.

Variant selection during the transformation of deformed austenite in a niobium bearing TRIP steel

The transformation behavior of a 0.22wt%C-1.50wt%Mn-1.56wt%Si-0.045wt%Nb steel was studied after controlled rolling. Deformation below the no-recrystallisation temperature was employed to deform the prior austenite. During bainitic holding, some of the austenite was retained due to the enrichment of carbon in this phase. Using automated EBSD techniques, both the retained austenite and bainite orientations were measured and the orientation relationships between these phases are represented in Rodrigues-Frank space. The specific orientations predicted by the Bain, Kurdjumov-Sachs and Nishiyama-Wassermann relationships are compared with the measurements and the occurrence of variant selection is discussed

Representation of misorientations in Rodrigues-Frank space: application to the Bain, Kurjumov-Sachs, Nishiyama-Wassermann and Pitsch orientation relationships in the Gibeon meteorite

The three classical orientation relationships describing the gamma-to-alpha transformation, namely the Bain, Kurdjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W), are represented in Rodrigues-Frank (R-F) space. Two alternative reference systems are used to highlight the differences between the three types of misorientation. Some observations obtained on the Gibeon meteorite are analyzed using the two classes of reference system to reveal features of the transformation under conditions of very slow cooling. It is shown that the Bain correspondence relations are never satisfied, while the measurements fall in the full range of direction parallel conditions extending from the K-S to the N-W. The crystallographic features of the Pitsch orientation relation are presented in R-F space in Appendix A. The experimental observations conform to this type of transformation to a considerably lesser extent than to the classical K-S and N-W relations. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A study of the gamma-to-alpha transformation using EBSD techniques

The crystallographic relationship between the gamma and alpha phases in samples of the Gibeon meteorite and a TRIP steel was investigated by means of EBSD techniques. The orientations of the two phases were measured and are represented in pole figures. The results are compared to predictions made on the basis of the Bain, Kurdjumov-Sachs (K-S), Nishiyama-Wassermann (NW), Greninger-Troiano (G-T) and Pitsch orientation relationships. The local misorientation between individual fcc and bcc crystals along their common interface was measured to demonstrate the way in which the exact orientation relationship varies along the boundary. The local orientations within lamellae and laths of kamacite and bainite are compared to that in recrystallised ferrite polygons. The occurrence of variant selection during the transformation of deformed austenite is analyzed using a recent dislocation-based model.

Particle-stimulated nucleation of dynamic recrystallization in AZ31 alloy at elevated temperatures

Particle-stimulated nucleation (PSN) was investigated in magnesium alloy AZ31 to study the effect of the evolution of second-phases during extrusion and other metal forming processes. Compression tests were carried out on samples taken from coarse-grained as-cast magnesium alloy billets containing a lamellar Mg17Al12 eutectic phase and (Al, Mn) particles. These revealed that particle-stimulated DRX nucleation (PSN) was taking place during hot deformation and that this is facilitated by the fragmentation of the Mg17Al12. When Mg17Al12 dissolves into the matrix at about 350 degrees C, the (Al, Mn) particles remain effective in producing PSN at temperatures up to at least 400 degrees C. This suggests that alloy design leading to a suitable distribution of second-phase particles can improve the properties and formability of wrought magnesium alloys

Effect of rolling temperature, reduction and alloying additions on the texture of warm rolled steels

The effect of warm and cold rolling parameters on the development of annealing textures was studied in two low carbon steels containing additions of chromium. Two warm rolling temperatures (640 and 700 degrees C) were employed together with a reduction of 65%. The effects of an additional cold rolling reduction of 40% and of decreasing the heating rate during annealing were also studied. The ND fiber, < 111 >//ND, of the recrystallization texture was strengthened as the warm rolling temperature was decreased. However, all the warm rolled steels contained a retained RD fiber, < 110 >//RD. A noticeable improvement in both the continuity and intensity of the ND fiber was obtained when the sample was submitted to an additional 40% cold rolling reduction. The ND fiber was even more continuous and intense when a low heating rate was utilized, yielding r-values of 1.1 and 1.3 for the warm rolled and warm plus cold rolled samples, respectively.

The possible role of partial dislocations in facilitating transformations of the Nishiyama-Wassermann type

By analogy with the role of perfect dislocations in facilitating Kurdjumov-Sachs type transformations, it is proposed that partial dislocations may be responsible for Nishiyama-Wassermann transformations. Such an interpretation can account for the presence of both "positive slip" and "negative slip" variants within individual grains, a hitherto unexplained phenomenon. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Effect of hot band grain size on the anisotropy of warm rolled low carbon steels

In warm rolled steels, the intensity of the < 111 >//ND annealing texture, which favours formability, has been related to the formation of shear bands during rolling. Coarse hot band grain sizes (HBGS's) facilitate flow localization, the mechanism associated with the formation of shear bands. In this work, the effect of grain size after hot rolling was studied in a low carbon steel containing small additions of Cr and Mn. The formation of shear bands and their subsequent influence on the normal anisotropy r(m) and planar anisotropy Delta r in the annealed steels were of particular interest. Two HBGS's (18 and 30 mu m) were employed and the specimens were warm rolled to reductions of 65 and 80% at various temperatures between 640 and 700 degrees C. The results show that the frequency of shear banding is slightly lower for the smaller grain size. The normal anisotropy was not affected by the HBGS; by contrast, much lower Delta r values were associated with the finer grained steel.

Microscale characterisation of deformed microstructures of TRIP-assisted and multiphase steels

The improvement of the mechanical behaviour of high performance steels brings about a renewed interest for the work hardening rate resulting from deformation-induced martensitic transformation or mechanical twinning. Even if these mechanisms are known for quite a long time, the deformation - transformation interactions that they induce is not yet fully characterised and understood. This study aims at characterising the microstructure evolution of a Fe-Mn steel grade during straining thanks to TEM and high resolution OIM. Particular patterns of austenite - ε and α' martensite are found.

2004

Grain-scale characterization of transformation textures

Orientation relationships during the austenite-to-ferrite (gamma-to-alpha) phase transformation were investigated using electron back-scattered diffraction (EBSD) on a bainitic steel containing retained austenite. The steel was hot rolled within the austenite phase field, but below the 'no-recrystallization' temperature, to two different strains. The observed orientation relationships between the bainite and retained austenite are expressed in Rodrigues - Frank space. The exact Kurdjumov - Sachs relation was never found. The local spread of orientation in the parent austenite ( owing to deformation) is seen to be inherited by the bainite. This is attributed to the displacive mode of transformation to bainite. The influence of austenite prior deformation on the occurrence of variant selection was also studied. It is shown that a critical strain is necessary in order to observe a significant amount of variant selection

Critical assessment of the micromechanical behaviour of dual phase and trip-assisted multiphase steels

The strength and formability properties of high performance steels have been recently improved thanks to the combination of several strengthening mechanisms such as dislocation strengthening and mechanically-induced martensitic transformation within complex microstructures. These finely grained metastable microstructures are generated during controlled multistage thermomechanical treatments involving several phase transformations. However, the way the different active mechanisms combine in order to improve the properties of these steels is not yet clearly established. The present study proposes an overview of an experimental program devoted to the micromechanical characterisation of Dual Phase and TRIP-assisted multiphase steels. Thanks to different techniques such as X-Ray and neutron diffraction, strain mapping and transmission electron microscopy (TEM), it was possible to characterize the flow behaviour of the different constitutive phases and to measure the critical parameters of the microstructures responsible for the work hardening capabilities of the TRIP-assisted multiphase steels. Comparison of the experimental stress and strain partitioning within the heterogeneous microstructures with the predictions of different modelling assumptions was also carried out.

2003

Microtextural study of variant selection by EBSD in a bainitic steel containing retained austenite

The phenomenon of variant selection in a bainitic steel containing retained austenite was investigated. The steel was hot-rolled below the Tnr to true strains of epsilon = 0.2 and epsilon = 0.8 in the austenite temperature range. The orientation relationships between bainite and austenite are expressed in Rodrigues space. A strain of 0.2 is shown to be insufficient to induce variant selection. A hot deformation of 0.8 leads to a much smaller number of variants being created, which are shown to be related to the slip systems active during austenite deformation

Simulation of the hot rolling of TRIP-assisted multiphase steels

The influence of intercritical rolling on the microstructural development during the processing of hot rolled multiphase steels is characterised. Intercritical rolling leads to dynamic recovery of ferrite and to strain-induced transformation of austenite into ferrite. The prior deformation of austenite considerably influences the bainite transformation. It is shown that a smaller amount of austenite transforms into bainite when bainite forms from deformed austenite. This is explained by the physics of the bainite transformation

Critical assessment of the bainite transformation of finely grained and deformed austenite

This study deals with the influence of the austenite grain size and deformation state on the mechanism and kinetics of bainite transformation in several low-alloy steels differing by their C content. Nucleation and growth conditions are thoroughly modified when the austenite grain size becomes of the same order as the length of the individual bainitic ferrite platelets. As a consequence, the kinetics show some peculiarities. The austenite prior deformation also influences in a large way the morphology of the bainite - retained austenitc, microstructure.

2002

Effect of hot-rolling conditions on the tensile properties of multiphase steels exhibiting a TRIP effect

TRIP-assisted multiphase steels have been thoroughly studied in the cold-rolled and annealed state. The effects of hot-rolling conditions on these steels are much less studied even though these are of major importance for industrial practice. This study was carried out in order to understand the effect of the hot deformation of austenite on the tensile properties of TRIP-assisted multiphase steels. Two different compositions and microstructures are investigated. The first one is a low-carbon steel (mass content of 0.15 %) with a microstructure consisting of an intercritical ferritic matrix, bainite and retained austenite. The second one is a medium-carbon steel (mass content of 0.4 %) that consists of bainite and retained austenite. Both steels were deformed to various strain levels below the non-recrystallisation temperature of austenite. The medium carbon steel was deformed in the fully austenitic temperature range whereas the low-carbon steel was deformed in the intercritical temperature range. In both cases, the prior hot deformation of austenite brings about a large enhancement of the work-hardening capabilities. In the case of the medium-carbon steel, this effect can be attributed to a much larger TRIP effect taking place during straining. In the case of the low-carbon steel, the improvement of the work-hardening behaviour was attributed to an interaction between the martensitic transformation and the dislocations already present within the surrounding ferrite matrix

2001

Effect of hot-rolling in the austenitic region on the formation of isothermal bainite in a 0.4C-1.5Si-1.4Mn steel

This paper aims to give further insights on the changes that are brought on bainite formation by prior hot-deformation. To this end, a thermomechanical treatment was performed on blocks of a 0.4C-1.5Si-1.4Mn steel, The chemistry of this alloy allowed hot-rolling in the full austenitic range up to large reductions without any significant recrystallisation, The deformed plates were then partially transformed to bainite and the resulting microstructures were subjected to investigation. The laths of bainitic ferrite appeared to be grouped in packets in which they exhibit the same crystallographic orientation. When no prior hot-deformation is applied, the bainite packets originating from the same austenite parent grain were randomly distributed. However, severely deformed austenite grains were found to give rise to typical orientations of the bainitic ferrite laths. These results were interpreted in terms of dislocation arrays interacting with a displacive transformation

Communications publiées lors de congrès ou colloques nationaux et internationaux

2020

Towards work-hardenability of Ti-6Al-4V through a quenching and partitioning approach

2016

Characterization of electron beam melting process (EBM)

2015

Influence of quenching and partitioning conditions on the microstructures and mechanical properties of a 0.2C steel

Direct pearlite spheroidization

The zig-zag pattern revisited in Fe-31Ni-0.155C

2013

The fracture studies of polycrystalline silicon based MEMS

The advantages of micro-electro-mechanical systems (MEMS), such as low power requirement, miniaturized sizes and costs reduction, have already made significant impact in many technological fields. MEMS are now widely used as accelerometers, pressure sensors, and resonators etc. However, the determination of the mechanical properties of MEMS devices with high accuracy is still a challenging task due to their small dimensions and often anisotropic behaviour. This paper focuses on the modelling and simulation of the fracture of a key MEMS component, which is a polycrystalline silicon beam, by discontinuous Galerkin (DG) formulation combined with an extrinsic cohesive law (ECL) to describe the fracture process. As the beam is modelled by plane-stress 2D elements, an analytical equation to compute the effective fracture strength and the effective critical strain energy release rate in terms of the through-the-thickness fracture mode and of the orientation of the facet with respect to the crystal is also developed. At the end, a model is simulated, and the results are verified as per the physics of the problem and experiments. © 2013 IEEE.

2012

Influence of phase separation on the crystallization behavior of glass-ceramics in the BaO-TiO2-SiO2 system

Glass ceramics are of growing interest due to their enhanced properties comparing to the base glasses. The control of microstructures is consequently a major challenge in those systems. Even if it has been the topic of vigorous debates over the last decades, the possible role of a prior amorphous phase separation (APS) on the subsequent crystallization has not yet been clarified. This study focuses on the interplay between amorphous phase separation and the crystallization of fresnoite in glasses pertaining to the BaO-TiO2-SiO2 system. These glasses are known to undergo subliquidus phase separation via binodal or spinodal mechanisms for specific composition ranges in the ternary phase diagram. This process eventually leads to a final microstructure consisting of silica-rich droplets within a glassy matrix. The prior amorphous phase separation may have a significant effect on the subsequent nucleation and growth of crystals and furthermore on their morphology. Indeed, glasses with compositions falling outside the immiscibility gap have been reported to crystallize as strongly oriented crystals whereas glasses undergoing a prior APS lead to finer and randomly oriented crystals. This could be considered as an elegant way to improve optical properties such as the blue photoluminescence of fresnoite, particularly interesting for plasma screen applications. The aim of this work is to compare the crystallization behavior of glasses (i) with and (ii) without prior amorphous phase separation and also (iii) with the stoichiometric composition of fresnoite. This particular glass composition is well-known to bulk crystallize and to show high photoluminescence. The prior amorphous phase separation is shown to have a major influence for the glass compositions investigated. The microstructures of bulk samples are investigated using Scanning and Transmission Electron Microscopy. Differential Scanning Calorimetry and X-Ray Diffraction are also used to characterize the crystallization process on powder samples. The possible influence of the various interfaces present within the microstructure is investigated. In particular, the role of free surfaces or of droplets/matrix interfaces is scrutinized. Their respective influence on the nucleation and growth of fresnoite crystals is discussed in detail.

Warm rolling of eutectoid steels: Scale-up from hot torsion testing to pilot hot rolling trials1

The goal of this research is the development of a new industrial thermomechanical schedule that leads directly to a fine spheroidized microstructure after hot rolling of a eutectoid steel. Hot torsion testing performed at temperatures close to Ar1 demonstrates that hot deformation accelerates the ?-topearlite transformation and leads to the spheroidisation of the cementite lamellae. Extended dynamic recovery occurs during warm deformation of ferrite and the subgrains are shown to transform progressively into high angle boundaries. This process can be referred to as continuous dynamic recrystallization (cDRX) and is responsible for the refinement of the ferrite grains down to 3 μm. To explore the industrial feasibility of this thermomechanical treatment, a similar schedule was performed on an instrumented pilot scale rolling mill. The resulting microstructures are fully spheroidized because warm deformation induces the fracture of the cementite lamellae. The formation of a well developed subgrain network by dynamic recovery is also observed. However, due to the higher strain rates (5-10 s-1) compared to the torsion testing (0,1 s-1), the evolution of low angle boundaries into high angle boundaries during rolling is not observed.

2009

Properties of copper coatings obtained by cold gas dynamic spraying using helium

Two copper powders with a different size distribution were sprayed onto an aluminium substrate using the cold gas dynamic spraying technique. Both powders were sprayed at a fixed helium pressure but two gas temperatures were investigated: ambient and 200°C The oxide content of both the powders and the coatings have been measured and compared. The influence of the oxide content of the powder is discussed based on metallurgical characterization (optical microscopy,...), microhardness measurement, powder speed measurements (using a DPV/CPS 2000 system), and deposition efficiency comparison. The dislocation density in the coatings has been measured using the modified HallWilliamson and Warren-Averbach calculation method. The influence of the oxide shell on the deformation behaviour of the powder during the coating formation leads to different dislocation densities in the coating. Copyright © 2009 ASM International® All rights reserved.

2007

Texture evolution in AM30 Mg alloy deformed along different strain paths

The evolution of texture in an AM30 Mg alloy was examd. after deformation by uniaxial tension, uniaxial compression and ring hoop tension testing (RHTT). Temps. from ambient to 200° were covered at a strain rate of 0.1/s. The samples were cut from extruded AM30 Mg alloy tubes. Specimens deformed to various strain levels were examd. by optical microscopy and electron backscattered diffraction (EBSD) techniques. The results indicate that the initial extrusion texture and strain path together play important roles in the formation of different types of twins, which in turn significantly influence the evolution of the deformation texture. Contraction and double twinning are the dominant twinning mechanisms in uniaxial tension, while extension twinning prevails in uniaxial compression and RHTT. In addn., the deformation textures indicate that the active twinning planes are the ones with the highest Schmid factors.

Grain scale assessment of variant selection in a thermomechanically processed TRIP steel

Orientations of both the α and γ phases in a multi-phase com. steel were measured by means of electron backscatter diffraction (EBSD) techniques. Using the av. orientation of each austenite grain as the ref. frame, the orientation relationships between the two lattices were compared with the common orientation relationships (i.e. the Kurdjumov-Sachs and Nishiyama-Wassermann) in Rodrigues-Frank space. The occurrence of variant selection in individual austenite grains was examd. using a recent dislocation-based model. This model considers the role of the slip systems that were active during prior deformation, as well as those of in-plane reactions, cross-slip and the partial dislocations that are linked to specific variants. It also unites the competing K-S and N-W relationships through the dissocn. of perfect dislocations. Reasonably good agreement was obsd. between the predictions and the observations. Possible explanations for some of the discrepancies are also presented.

An EBSD study of orientation relationships during phase transformations in ultra high performance steels

The orientation relationships that apply to phase transformations in high-performance TRIP and TWIP steels were characterised by orientation imaging and EBSD techniques. The results are presented in the fundamental zones of Rodrigues-Frank space that correspond to the specific phase transformation under consideration (cubic to cubic or cubic to hexagonal). The use of Rodrigues-Frank space enables straightforward comparison to be made with orientation relationships proposed in the literature. The observations indicate that the active slip systems in the parent phase play important roles in variant selection.

On the phase transformations in hot rolled TRIP-assisted multiphase steels

TRIP-assisted multiphase steels exhibit an excellent balance of strength and ductility, which makes them very attractive for the automotive industry. These remarkable mech. properties can be attributed mainly to their composite-like microstructures and to the transformation of retained austenite into martensite during straining (Transformation-Induced Plasticity). The aim of this study is to highlight the interactions between the hot rolling conditions, the transformation of austenite and formation of the microstructure, and the resulting mech. properties. Various rolling simulation techniques were employed to det. how the composite microstructure is formed during the various steps of multi-stage thermomech. processing.

2006

Crystal plasticity modeling of texture development and hardening of twip steels

This paper addresses the issue of incorporating mech. twinning in a crystal plasticity model originally designed for dislocation slip only. Iso-deformation in the twin and in the parent grain have been considered. Different hypotheses are formulated about the interaction of adjacent grains: (i) uniform deformation (Taylor FC); (ii) Lamel model and (iii) ALAMEL model. One evaluates the effect of such hypotheses on the prediction of hardening, vol. fraction of twins and texture evolution. Then, model predictions are compared to exptl. measurements of the rolling texture on a Fe-Mn-C alloys in which the occurrence of mech. twinning is controlled both by the chem. compn. and the working temp.

Twinning behavior of AM30 Mg alloy during uniaxial tension and compression

The evolution of twinning in an AM30 Mg alloy was examd. by uniaxial tension and compression testing at temps. from ambient to 250° and a strain rate of 0.1/s. The samples were cut along the extrusion direction from extruded AM30 Mg alloy tubes. Specimens deformed to various strain levels were examd. by optical microscopy and electron backscattered diffraction (EBSD) techniques. The results indicate that the initial extrusion texture plays an important role in the formation of different types of twins and that the twinning behavior also depends on the strain path. Contraction and double twinning are the dominant twinning mechanisms in uniaxial tension, while extension twinning prevails in uniaxial compression. The different twinning behaviors are shown to be responsible for the sharply contrasting strain hardening characteristics of the exptl. flow curves.

Microstructure and texture evolution during the uniaxial tensile testing of AM30 magnesium alloy

The evolution of microstructure and texture was examd. in an AM30 alloy during uniaxial tensile testing at temps. from ambient to 350°C and at strain rates from 0.001 to 0.1/s. Samples were cut from extruded AM30 tubes along the extrusion direction. DRX was obsd. even at temps. as low as 150°C. However, it did not go to completion under any set of exptl. conditions. The microstructures near the fracture surfaces indicate that cracks are readily formed next to twins at high strain rates; such twinning-induced cracking is evident at temps. up to 200°C. Partial recrystn. also exacerbates the tendency for cavitation. No significant change in the texture occurred during straining along the extrusion direction at elevated temps. This suggests that the end-texture for extension was already set up during extrusion, which is responsible for the low ductility of the present material.

2005

Orientation relationships and variant selection during the γ-to-αb transformation in a hot-rolled trip steel

The transformation behavior of a 0.22wt%C-1.50wt%Mn-1.56wt%Si-0.045wt%Nb TRIP steel was studied after controlled rolling. Deformation below the no-recrystn. temp. was employed to strain the prior austenite. Using automated EBSD techniques, both the retained austenite and bainite orientations were measured; the orientation relationships between these phases are represented in pole figure form. The obsd. orientations are compared to the predictions assocd. with various correspondence relationships reported in the literature. The bainite does not follow any of these predictions exactly and exhibits a continuous spread, mainly between two coplanar Kurdjumov-Sachs variants. Moreover, the initial spread of orientation in the parent austenite grains rendered the detn. of a precise correspondence relationship somewhat difficult. The occurrence of variant selection during the nucleation and growth of bainite within deformed austenite grains is illustrated and the observations are compared to the predictions obtained from a dislocation-based model.

Crystallographic relationships between taenite and kamacite in the Gibeon meteorite

The orientations of kamacite (α) lamellae transformed from a single prior-taenite (γ) grain were measured by analyzing the EBSD (electron backscatter diffraction) patterns; these were compared with the orientation of the retained taenite on pole figures. The orientation relations between the α and γ phases in the plessite (a two-phase mixt. of α and γ) regions were also characterized. All the variants corresponding to the Kurdjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W) relationships were obsd., as were the intermediate reflections located between the coplanar K-S and N-W variants. The local misorientations between individual fcc and bcc crystals along their common interfaces were computed and it was found that the exact orientation relation varies along the boundary, a significant proportion of which does not satisfy any of the relations exactly. Orientation variations of up to about 10 degrees were obsd. within individual kamacite lamellae; these usually covered the range from one K-S variant, through the N-W position, to the other coplanar K-S variant. Numerous Neumann bands (mech. twins) were also detected within the kamacite lamellae, which indicate that some sort of deformation occurred during or after the α-to-α transformation.

Applicability of the Bain, Kurdjumov-Sachs, Nishiyama-Wassermann, Greninger-Troiano and Pitsch relations to transformations in the Gibeon meteorite and in a TRIP steel

The pole figure representations of the reflections assocd. with five commonly considered correspondence relations are described. These are shown to be clustered around the three Bain {100} reflections assocd. with the three Bain variants. The locations of the (min. angle) rotation axes in Rodrigues-Frank space are also illustrated. Examples taken from both diffusive (the Gibeon meteorite) and displacive (bainite formation in a TRIP steel) transformations are used to illustrate the relative importance of the five sets of correspondence conditions. It is demonstrated that the Bain relation is never obsd. and that the Pitsch and "inverse G-T" (Greninger-Troiano) transformations are relatively rare in these materials. By contrast, the exptl. results are continuously distributed from the K-S (Kurdjumov-Sachs), through the G-T, to the N-W (Nishiyama-Wassermann) positions and do not seem to favor any single one of these three relations.

Elevated temperature property measurements for warm forming magnesium and aluminum alloy tubes

2004

Dynamic recrystallization in coarse-grained AZ31 alloy at elevated temperatures

This report investigated the dynamic recrystn. (DRX) phenomenon in Mg alloy AZ31 billets to understand the deformation mechanisms and microstructure evolution during extrusion and other metal forming processes. Compression tests of coarse-grained as-cast Mg alloy billets contg. lamellar Mg17Al12 eutectic phase and (Al, Mn) particles suggest that temps. higher than 300°, strains greater than 0.25 and strain rates above 0.1/s lead to the formation of fine DRX grains in the deformed samples. Microstructural observations also indicate particle-stimulated DRX nucleation (PSN) during hot deformation of the AZ31 alloy, facilitated by the presence of Mg17Al12 and (Al, Mn) particles. While the Mg17Al12 is dissolved into the matrix at about 350°, (Al, Mn) particles may remain effective for PSN at temps. up to at least 400°C. Alloy design with a proper distribution of second-phase particles can provide the desired microstructure for improved properties and formability in wrought Mg alloys.

Warm Forming of Aluminum Alloy Tubes

Thermomechanical Processing of TRIP-assisted multiphase steels

2003

On the austenite retention in low alloy steels

Recent advances in the development of high performance steels presenting higher properties of strength and ductility rely on the TRIP effect, i.e. on the mech.-induced martensitic transformation of retained austenite dispersed in a soft ferrite-based matrix. As a consequence, the stabilization and retention of austenite at room temp. have become of primary importance, leading to specifically designed steel grades and thermal or thermomech. treatments. Particularly, carbon enrichment of the austenite during intercrit. annealing and bainite transformation was effective in retaining austenite. This study deals with the influence of chem., geometrical and mech. parameters of the microstructure of low alloy steels on the retention of austenite at room temp. The influence of different alloying elements on the inhibition of cementite pptn. and on the resulting austenite stabilization during the bainite transformation is scrutinised. The influence of the redn. in the austenite grain size on the mechanisms of nucleation and growth of bainite is demonstrated. Finally, the change in bainite morphol. resulting from prior deformation of the austenite is clearly exemplified and explained

2002

The macro- and micromechanics of TRIP-assisted multiphase steels, experiments and modelling

2001

Analyse microtexturale du phénomène de sélection de variants au cours de la transformation bainitique

2000

Mechanical stabilization of retained austenite in TRIP-assisted multiphase steels

TRIP (transformation induced plasticity) - assisted multiphase steels exhibit strength and ductility properties that are unique in low-alloy steels. This behavior is attributed to the dynamic work-hardening provided by the transformation of metastable retained austenite into martensite during straining. Usually two main factors are thought to influence the stability of austenite: its chem. compn. and its grain size. The effect of a third parameter on its stability: prior deformation and high temp. work-hardening of austenite - are studied. Specimens were strained into the intercrit. temp. range and then submitted to a bainitic transformation dwell. Typical multiphase microstructure were obtained. The work-hardening behavior of the steel grades strained at room temp. was enhanced by the prior deformation of austenite.

Effect of the work hardening of retained austenite on the martensitic transformation in a TRIP-aided steel

This study deals with the hot deformation of TRIP-assisted multiphase steels. These steels consist typically of bainite and retained austenite grains dispersed in a ferritic matrix. The austenite, which is metastable at room temp., transforms to martensite during straining, bringing about the TRIP effect, which improves the strength-ductility balance of these steels. This work studies the effect of a prior deformation in the intercrit. temp. range on the mech. properties and work hardening behavior of these TRIP-assisted multiphase steels. Several deformation levels in the ferrite-austenite domain were applied during the thermomech. scheme. Work-hardening is strongly enhanced when the specimens were sufficiently deformed in the intercrit. temp. range. This study also shows that the beneficial influence of the prior deformation is more related to the TRIP phenomenon than to the strain-hardening of the ferritic matrix. These expts. prove that a displacive transformation such as the martensitic transformation is highly influenced by the dislocation d. within the parent phase.

Enhancement of the mechanical properties of TRIP-assisted multiphase steels by hot rolling in the intercritical range

Experimental assessment of the effect of work-hardening on the stability of austenite in Si-Mn TRIP-assisted multiphase steels