Achieving an excellent combination of strength and ductility in metastable β titanium alloys via coupling isothermal ω phase and TRIP/TWIP effects

Abstract

TRIP/TWIP metastable β titanium alloys demonstrate high strain hardening rates and excellent tensile ductility. However, the precipitation of nanometer-sized ω phase through microstructural control significantly improves strength but often results in a significant decrease in ductility. This research proposes a novel strategy by precipitating isothermal ω phase (ω_iso) and integrating mechanical twinning/martensitic transformation to address these challenges. The single-phase β coarse-grained (CG) specimens of metastable Ti25Nb (at.%) alloy were subjected to solution treatment in the β phase region, followed by aging at 300 °C for 60 min to obtain CG60. The ω_iso-reinforced CG60 specimen exhibited a 12 % uniform elongation (1 % higher than CG specimen) and a yield strength of 857 MPa (approximately 67 % higher than CG specimens). In the CG60 specimen, deformation mechanisms were mainly attributed to the TRIP, TWIP and dislocation slip, with TWIP being predominant. As aging time increased, ω phase (localized barriers) and improved β matrix stability progressively suppressed TRIP and TWIP effects, with TWIP being completely inhibited first. Transmission electron microscopy and computational findings suggest that larger ω phase contributes more significantly to the precipitation strengthening.