Tuning strength and plasticity combination on metastable β-Ti-7Mo-3Cr-3Al-3Nb alloy via coupling α_s precipitation strengthening and TRIP effect

This study systematically investigates the influence of secondary α (α_S) phase fraction on the mechanical properties and deformation behavior of a metastable dual-phase Ti-7Mo-3Cr-3Al-3Nb (wt%) alloy. The microstructural responses of samples with varying α_S phase fractions to uniaxial tensile loading were investigated using optical microscopy, electron backscatter diffraction, and transmission electron microscopy. The results indicate that stress-induced martensite and dislocation slip were observed in samples with an α_S phase content of 14.6 %-48.4 %. In this range, the yield strength doubles, while the elongation decreases slowly. When the α_S content exceeds 48.4 %, dislocation slip becomes the dominant deformation mode, resulting in a sharp decrease in elongation and an increase in strength. Additionally, as the α_S phase fraction increases, the work hardening rate curve transitions from a pronounced parabolic shape to a monotonically decreasing trend, indicating a gradual weakening of the work hardening effect. In samples with a low (14.6 %) α_S phase fraction, stress induced α′′ martensite coexists with {111}_α′′ Type I twins, < 211 >_α′′ Type Ⅱ twins, {011}_α′′ compound twins, {112}_α′′ Type I twins, and {130} < 31̅0 >_α′′ compound twins. In samples with moderate (25.8 %) α_S phase fraction, {111}_α′′ Type I twins induce secondary {111}_α′′ Type I twins and {112}_α′′ Type I twins. The type and number of martensitic twins decrease as the α_S phase fraction increases. Maintaining martensite activation in the residual β of metastable β-titanium alloys containing α_S and β mixed phases is considered an effective strategy for optimizing the balance between strength and ductility.