Enlarging the palette of mechanical properties of TI64 by a quenching and partitioning approach

The usual bimodal microstructure of wrought TI64 is known to provide an excellent yield strength to density ratio. However, its poor work-hardening capacities brings about limited ductility. Moreover, the post-uniform elongation is also very limited. In the present work, we report the beneficial influence of dual-phase microstructures obtained by quenching from the alpha+beta phase field. Under certain circumstances, such dual-phase microstructure exhibit martensite reorientation induced plasticity, that was never reported in this well-known industrial alloy. The dual-phase microstructures are further annealed allowing the partitioning of V from the supersaturated martensite. Depending on the initial dual-phase microstructure and the parameters of the annealing treatments, a very large palette of mechanical properties is obtained. The strength levels, work-hardening behaviors, ductilities and post-uniform elongations are discussed

Triggering new deformation mechanism in Ti alloys by heat treatments: a step forward into the improvement of the ductility and work-hardening of 3D printed parts

The plastic behavior of Ti alloys remains a major drawback: indeed “classical” Ti-based materials usually display a low work hardening rate, bringing rapid strain localization and low ductility level. Moreover, with the lightning growth of 3D printing, the damage tolerance criterion becomes critical since fabricated alloys contain defects inherent to the process, leading to early damage upon loading. In that context, a quenching strategy has been used to promote α + α’ dual-phase microstructures capable to induce martensite reorientation-induced plasticity, rather usually associated to the orthorhombic α’’ martensite. The occurrence of such non classical deformation mechanism was shown to be highly efficient to improve the work-hardening and the ductility of Ti-alloys while keeping a high mechanical resistance. The present study provides a fundamental understanding of the crystallography and the microscale behavior of such martensite. The critical influence of the chemical enrichment, the texture and the morphology of the martensite on reorientation is highlighted

Work-hardening by reorientation induced plasticity in titanium alloys: from a fundamental understanding in Ti-6Al-4V towards new alloy design rules

Les alliages de titane destinés aux applications aéronautiques souffrent de faibles capacités d'écrouissage. L’augmentation de l'écrouissage via le déclenchement de mécanismes de déformation alternatifs dans les alliages β-métastables, tels que la précipitation induite sous contrainte (TRIP) et/ou le maclage mécanique (TWIP), s'accompagnent souvent d'une diminution drastique de la limite d'élasticité et imposent des quantités élevées d'éléments d'alliage coûteux. Le présent travail a montré qu’il est également possible d’obtenir un écrouissage important dans les alliages alpha+beta peu alliés (typiquement plus de 500 MPa), ainsi que de l’obtenir simultanément à une limite élastique élevée (plus de 800 MPa). Cela, via le développement de microstructures dites « dual-phase » alpha+alpha’, lorsque la martensite se déforme par réorientation (RIP). Pour y parvenir, le concept de "dual-phase" a été étendu à des traitements de "quenching and partitioning" (Q&P). Ainsi, la fraction volumique respective de chaque phase, la taille, la chimie et la stabilité mécanique de la martensite ont été modulées de façon à décomposer leur contribution respective à l’écrouissage singulier de l’alliage. Des critères visant à l’obtention du meilleur compromis possible entre écrouissage, limite d'élasticité et ductilité ont pu en être déduit. En particulier, la réorientation de la martensite s’est avérée jouer un rôle majeur dans l’amélioration de l’écrouissage. Les conditions nécessaires au déclenchement de ce nouveau mécanisme de déformation ont été examinées de près. La martensite doit notamment présenter une quantité critique d'éléments bêtagènes de sorte qu'un seuil chimique déclenchant l'instabilité mécanique de la martensite soit atteint. L'augmentation de la proportion de martensite dans la microstructure dual-phase a permis d’augmenter la limite élastique tout en conservant un bon écrouissage. Cependant, une certaine proportion de phase α doit être conservée afin de limiter la croissance des plaques de martensite, sans laquelle des phénomènes de localisation des déformations y sont promus, conduisant fatalement à une rupture précoce. Les grandes plaques peuvent également être renforcées par le traitement de « partitioning » contrôlant la décomposition de la martensite. Initialement développé sur le Ti-6Al-4V puis transposé à d’autres alliages alpha+beta, ce travail a permis de définir un premier ensemble de critères afin de proposer et de tester avec succès une stratégie de design d'alliage pour développer des alliages de titane peu alliés présentant un fort écrouissage par réorientation.Doctorat en Sciences de l'ingénieur et technologieinfo:eu-repo/semantics/nonPublishe

Towards work-hardenability of Ti-6Al-4V using the α – α’ dual-phase microstructure

In this work, a quenching and partitioning strategy has been developed to provide insight into the possibilities to reach a work-hardening capacity very rarely reported in Ti-6Al-4V. Indeed, it was recently demonstrated that a sub-transus thermal treatment followed by water quenching could generate an + ’ microstructure displaying a high work-hardening and a promising increase in both strength and ductility. We performed a series of ‘quenching’ treatments using several solutionizing temperatures. In such way, the volume fraction of each phase, the size, the chemistry and the interfaces of the martensite are taken as microstructural variables to decompose the peculiar work-hardenability of dual-phase Ti-6Al-4V into respective contributions. Then, annealing of the dual-phase microstructure was performed for different times to bring about the martensite decomposition involving a ‘partitioning’ of the alloying elements. Martensite was shown to be a very heterogeneous and discontinuous medium in which interfaces play a crucial role in the capacity of the material to progressively transform during the deformation and harden at a macroscopic scale. The capacity of the martensite islands to progressively decompose into an equilibrium and very fine microstructure provides a broad range of surprisingly high mechanical properties. These results completed the dual-phase microstructure comprehension and highlighted the martensite yet unknown decomposition mechanisms.info:eu-repo/semantics/nonPublishe

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.info:eu-repo/semantics/publishe

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

info:eu-repo/semantics/publishe

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

In this work, the work hardenability of Ti-6Al-4V alloy has been investigated using a quenching and partitioning strategy on dual-phase Ti-6Al-4V samples. Indeed, it was recently demonstrated that a sub-transus thermal treatment followed by water quenching could generate a dual phase a + a’ microstructure displaying a high work-hardening capacity and leading to a promising increase in both strength and ductility.On as-quenched samples, work-hardening was mainly attributed to a mechanical contrast between the primary a phase and the transformed b phase (mainly a’ martensite). However, the actual micromechanical contribution of the complex martensitic medium, consisting of a discontinuous network of self-accommodated a’ needles (in the as-quenched state) is still not completely clarified, regarding the work-hardening behavior. Respectively, the role of both the interphases between the a’ needles and between the a and a’ media needs to be better understood to explain the deformation mechanisms and subsequent work-hardening capacities observed in those microstructures.In the present work, we performed a series of ‘quenching’ treatments using several sub- transus solutionizing temperatures and cooling rates. In such a way, the respective volume fraction of each phase and both the size and the chemistry of the quenched martensite are taken as microstructural variables to decompose the peculiar work hardenability of dual- phase Ti-6Al-4V alloys into respective contributions. Then, annealing of the metastable a + a’ phases was performed for different annealing temperatures and times to bring about the a’ martensite decomposition involving a ‘partitioning’ of the alloying elements.The quenching and partitioning parameters led to a very wide range of mechanical properties and associated work-hardening behaviour. The strain partitioning between the phases was characterized using digital image correlation during in-situ tensile testing in a SEM. The hardness of the microstructural constituents was probed using micro- and nano-indentation. In order to shed some light on the behaviour of the complex martensitic phase and associated interphases, in-situ tensile testing in a TEM was also performed. The macroscopic mechanical behaviour is finally discussed based on this multiscale characterization approach.info:eu-repo/semantics/nonPublishe

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

In this work, the work hardenability of Ti-6Al-4V alloy has been investigated using a quenching and partitioning strategy on dual-phase Ti-6Al-4V samples. Indeed, it was recently demonstrated that a sub-transus thermal treatment followed by water quenching could generate a dual phase a + a’ microstructure displaying a high work-hardening capacity and leading to a promising increase in both strength and ductility.In the present work, we performed a series of ‘quenching’ treatments using several sub-transus solutionizing temperatures and cooling rates. In such a way, the respective volume fraction of each phase and both the size and the chemistry of the quenched martensite are taken as microstructural variables to decompose the peculiar work hardenability of dual-phase Ti-6Al-4V alloys into respective contributions. Then, annealing of the metastable a + a’ phases was performed for different annealing temperatures and times to bring about the a’ martensite decomposition involving a ‘partitioning’ of the alloying elements.The quenching and partitioning parameters led to a very wide range of mechanical properties and associated work-hardening behaviour. In order to shed some light on the behaviour of the complex martensitic phase and associated interphases, TEM was performed. The macroscopic mechanical behaviour is also discussed based on this multiscale characterization approach.info:eu-repo/semantics/nonPublishe

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

info:eu-repo/semantics/publishe