Oxygen interstitials mediated strength-ductility enhancement by triggering multiple stress-induced α" twins in metastable β-Ti alloys

The current investigation presents a novel approach for designing strong and ductile metastable β-Ti alloys by coupling oxygen interstitial strengthening with transformation-induced plasticity effect (TRIP). The influence of oxygen on mechanical behavior responses, stress-induced α" martensitic phase transformation (SIM α"), SIM α" twinning and facture mechanisms were systematically investigated using XRD, SEM, FIB-TEM and theoretical calculations. The results indicate that a complex hierarchical microstructure was obtained after deformation, due to the activation of SIM α" with multiple variants and twins. Notably, in addition to {130}<310>_α", {011}<0–11>_α", and {111}_α" type I twinning, a novel {021}_α" type I twinning is reported for the first time to operate during the plastic deformation of oxygen-doped β-Ti alloys. The formation mechanism of {021}_α" type I twinning was further rationalized by considering both shear and atomic shuffle. The results suggest that the most plausible mechanisms to form {021}_α" twins involve a shear of 0.3959 and atomic shuffle through mechanisms of Δ_Ia: 0.6044 Å and Δ_Ic: 1.9346 Å. Oxygen atoms not only promote SIM α" twinning but also inhibit extensive SIM α" growth. The synergistic improvement in strength, ductility and work hardening of the as-designed Ti–7Mo–3Cr–3Nb–3Al-0.23O (wt.%) alloy, was attributed to the coupling of oxygen interstitial strengthening, SIM α" transformation, SIM α" twinning-induced TRIP effects, and oxygen-induced strengthening of grain boundaries and twin boundaries.