Dependency of mechanical properties in Ti-7Mo-3Al-3Cr-3Nb alloy on the β phase stability: Regulating primary α phase

In this study, the modification of the β-phase stability of Ti-7Mo-3Al-3Cr-3Nb alloy has been achieved by heat treatment at different temperatures. The synergy effects of primary α-phase and β-phase stability on the deformation behavior and mechanical properties were systematically studied based on these samples, combining the electron backscattering diffraction and transmission electron microscopy techniques. The content of the primary α-phase and stability of the β-phase increase when the heat treatment temperature is lowered from 860 °C to 770 °C, which increases the yield strength of the alloys. When the content of primary α-phase is less than 9 %, high strength and ultra-high work-hardening rates can be simultaneously achieved, with maximum work-hardening reaching 12.75 GPa and the ultimate tensile strengths reaching 846 MPa. Stress-induced α′′ martensitic (SIM α′′) transformation is observed in all experimental samples during tensile deformation, and the trigger stress of martensitic transformation decreases with the decrease of β-phase stability. When the stability of the β-phase decreases, the quantity of SIM α′′ increases, while martensitic variants and twinned martensite increase to coordinate local strains. Meanwhile, as the strain increases, the martensite laths grow into martensite domains, which trigger martensite twins during deformation, and the interaction of the primary α-phase and β-phase deformation products will achieve different work-hardening rates. This study provides a possible strategy for the design and modification of new high-strength and, work-hardening rate titanium alloys.