Work hardening and softening behavior driven by α′′-β interactions in a metastable β titanium alloy

In this study, a metastable β Ti–7Mo–3Al–3Cr–3Nb alloy with α s +β microstructure shows both TRIP and TWIP effects. Although the alloy has a low yield strength of 433 MPa, it shows a high work hardening rate of approximately 8.3 GPa. Besides, it has a total elongation of nearly 20%. The deformation mechanism was systematically investigated by combining EBSD and TEM analysis on specimens with tensile strains of 1.7%, 3.5%, 8.3%, until failure. The primary deformation mechanisms during tensile deformation are stress-induced α′′ martensitic transformation and dislocation slip. At strain initiation stage stress-induced lath martensite forms, accompanied by its growth, widening, and merging. At medium strain levels, martensitic domains form, accompanied by the appearance of a small number of martensitic twins. At high strain levels, a significant increase in martensitic twins is observed. The high work hardening rate is attributed to the continuous cutting of the β phase by martensitic variants that growing in different directions under low strain levels, which leads to a dynamic Hall-Petch effect. In the later stages of deformation, various types of martensitic twins form within the martensite domain until the matrix β phase is exhausted, playing a crucial role in achieving significant uniform elongation.