EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Titanium alloys and Ti-6Al-4V in particular, are widely used in the aerospace for their excellent mechanical strength to density ratio and high corrosion resistance. However, the relatively low work-hardening capabilities of these alloys compared to other metals still restricts their usage. It was recently shown that a subtransus annealing treatment followed by water quenching could generate an α-α’ dual-phase microstructure displaying a high work-hardening. This work-hardening improvement was attributed to the kinematic hardening that arises from the mechanical contrast between the hard α and the soft α’ phases. Nevertheless, there is a lack of fundamental understanding of this V-rich and Al-poor α’ martensite which is a particularly complex phase characterized by an extremely large number of interfaces between neighboring variants of martensitic plates. The present study provides further insight into the crystallography and the mechanical behavior of such α’. The Phenomenological Theory of the Martensite Crystallography (PTMC) coupled with EBSD and TEM analyses were used to rationalize the configuration adopted by the martensite during the dual-phase thermal treatment and to evidence the crucial role of the interfaces to explain its fine-scale mechanical behavior. It was shown that such martensite preferentially organized into 4 parallel groups of 3 variants that are self-accommodating. The variants of a same group were shown to be separated by a hitherto unobserved {13-41}α' type twin plane which is associated to the well-known misorientation of 63.26°/[-10 5 5 -3]α'. Such interface was shown to be mobile under deformation. This martensite is thus capable to reorientate under deformation, a property which is rather usually associated to the orthorhombic α’’ martensite. The characterization of this deformation mechanism in such martensite offered the opportunity to further investigate the links between the microstructure and the mechanical properties of the dual-phase microstructure.