Making high-strength bimodal Ti–5Al–5V–5Mo–3Cr alloy more ductile by grain-boundary shearing

The conflict between strength and ductility of bimodal Ti–5Al–5V–5Mo–3Cr (Ti-5553) becomes more evident as yield strength attains 1400 MPa, which limits the further application of the alloy in the ultra-high-strength condition. In this paper, we found that the conventional bimodal Ti-5553 alloy with 1400 MPa yield strength processes four typical kinds of grain boundary microstructures. However, two kinds of grain boundaries with the short α side-plate are almost un-deformable in tension, contributing to the strength-versus-ductility conflict. Fortunately, short α side-plate might transform to α laminate that is parallel to the grain boundary of prior β grain after the dual-ageing consisting of the high-temperature ageing and the low-temperature ageing. It is found that α laminate is capable of shearing, which is beneficial to the ductility of bimodal Ti-5553 alloy. Base on the dual-ageing strategy, a novel bimodal Ti-5553 alloy with excellent combination of yield strength (1391 MPa), ultimate tensile strength (1484 MPa) and tensile elongation (12%) has been fabricated. The strength versus ductility data of the novel alloy might extend beyond the benchmark range established by the strongest and most ductile Ti-alloys known. The result of the present work is instructive for designing the grain-boundary microstructure of high-strength bimodal near-β titanium alloys.