Microstructural evolution and strain-hardening in TWIP Ti alloys

A multiscale dislocation-based model was built to describe, for the first time, the microstructural evolution and strain-hardening of {332}⟨113⟩ TWIP (twinning-induced plasticity) Ti alloys. This model not only incorporates the reduced dislocation mean free path by emerging twin obstacles, but also quantifies the internal stress fields present at β-matrix/twin interfaces. The model was validated with the novel Ti-11Mo-5Sn-5Nb alloy (wt.%), as well as an extensive series of alloys undergoing {332}⟨113⟩ twinning at various deformation conditions. The quantitative model revealed that solid solution hardening is the main contributor to the yield stress, where multicomponent alloys or alloys containing eutectoid β-stabilisers exhibited higher yield strength. The evolution of twinning volume fraction, intertwin spacing, dislocation density and flow stress were successfully described. Particular attention was devoted to investigate the effect of strain rate on the twinning kinetics and dislocation annihilation. The modelling results clarified the role of each strengthening mechanism and established the influence of phase stability on twinning enhanced strain-hardening. Strain-hardening stems from the formation of twin obstacles in early stages, whereas the internal stress fields provide a long-lasting strengthening effect throughout the plastic deformation. A tool for alloy design by controlling TWIP is presented.