A unique three-stage dependence of yielding behavior and strain-hardening ability in Ti-10V-2Fe-3Al alloy on phase fraction

Ti-10V-2Fe-3Al alloy represents one of the most widely adopted metastable beta titanium alloys, the mechanical properties of which were influenced by several factors, including the grain size, phase fraction, precipitates, etc. In this study, we systematically investigated the effect of phase fraction on the tensile properties of equiaxed alpha+beta dual-phase microstructures in Ti-10V-2Fe-3Al alloy. It is revealed that there existed a unique three-stage dependence of yielding behavior and strain-hardening ability on the phase fraction in this alloy. The change in beta phase fraction (f beta) and corresponding phase composition not only determined the mechanical stability of beta phase via equilibrium Mo content, but also altered the relative nano-hardness between the two phases by solid solution hardening. Specifically speaking, when f beta was larger than 80%, the beta phase generally exhibited stressinduced martensitic phase transformation (SIMT) during loading. However, the beta phase stability was continuously enhanced with decreasing f beta, leading to a progressive transition from double-yielding to single-yielding behavior. At 60% <= f beta < 80%, despite of the basically suppressed SIMT, there was a clear strain partitioning between the two phases due to a relatively large nano-hardness difference. This helped to maintain a reasonable strain-hardening ability by promoting geometrically necessary dislocations (GNDs) near the interphase boundaries. With further decrease of the beta phase fraction (f beta < 60%), the nano-hardness of the two phases became much closer and as a result, the strain-hardening abilities were substantially deteriorated due to a lack of strain partitioning. Also, both the yield and ultimate tensile strength became much less dependent on the beta phase fraction for microstructures in this region.