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

par Dumas, Odeline ;Hary, Benjamin ;Martin, Guilhem;Sun, Fan;De Formanoir De La Caze, Charlotte ;Prima, Frédéric;Godet, Stéphane
Référence The 14th World Conference on Titanium (2019-06-(10-14): Nantes - France)
Publication Non publié, 2019-06-10

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Résumé :

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.