Ir. Odeline Dumas

Articles dans des revues avec comité de lecture

2025

Exploring the strength-ductility space of Ti-6Al-4V through a quenching and partitioning approach

The microstructure-mechanical property relationships of α+α′ dual phase microstructures are studied in Ti-6Al-4V. Various annealing temperatures in the α+β field are analyzed and the decomposition of the metastable martensite phase is also investigated. Low annealing temperatures lead to large work-hardening that is attributed to the occurrence of the concurrent deformation of a composite structure where the martensite is the softer phase and the occurrence of Reorientation Induced Plasticity in the martensite (RIP effect). This is studied by Electron Backscattered Diffraction (EBSD) and in-situ tensile testing combined with Digital Image Correlation (DIC). High annealing temperatures lead to a large decrease in work hardening due to the disappearance of RIP and of the mechanical contrast between α and α'. Further annealing of the α+α' microstructures goes along with the disappearance of RIP associated with the decomposition of the metastable martensite. The decomposition mechanism is shown to be a reversion phenomenon with the β phase precipitating within the martensite and at interfaces while retrieving its high temperature orientation. The precipitation of the β phase is shown to largely increase the strain to fracture of the samples annealed at high temperatures. The precipitates act as reinforcement of the larger laths that form within the large β grains associated with the high annealing temperatures.

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.

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.

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.

Communications publiées lors de congrès ou colloques nationaux et internationaux

2020

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