Deformation mechanisms in a metastable beta titanium twinning induced plasticity alloy with high yield strength and high strain hardening rate

Metastable β titanium alloys with both twinning (TWIP) and martensite transformation (TRIP) usually exhibit a low yield strength of between 200 and 500MPa, but high strain hardening rate and large uniform elongation. Alloys that exhibit twinning on a single system provide a higher yield strength, but a lower strain hardening rate. Here, for the first time, we report a new alloy (Ti-7Mo-3Cr wt%) with both high yield strength (695 MPa) and high work hardening rate (~1900 MPa) and a substantial 33.3% uniform elongation. The deformation mechanisms were systematically investigated using EBSD and TEM for samples strained to 1.3%, 5% and 16%. The high yield strength was achieved through initial deformation mechanisms of two twin systems, namely both {332}<113> and {112}<111> twinning. Importantly, the martensite transformation was suppressed at this stage of deformation. The combination of two twin systems, with approximately the same intensity, resulted in a high strain hardening rate (1600MPa to 1900MPa), much greater compared to alloys that exhibit a single twin system. Moreover, the TRIP effect was observed at strains greater than 5%, which also contributed to the high strain hardening rate large uniform elongation.