Novel approach to achieve gigapascal level yield strength and large uniform elongation in metastable β-Ti alloys

Metastable β titanium alloys exhibit an excellent strain hardening ability and a large uniform elongation, but their widespread use is challenged by a relatively low yield strength. The low yield strength of metastable β titanium alloys is partly due to an early initiation of strain-induced β-to-α″ phase transformation. Here, we propose a novel and cost-effective approach to solve this problem in a model Ti-10 wt.% Mo alloy by refining the grain size and adding oxygen solute. An unprecedented high yield strength of 1040 MPa and a large uniform elongation of 20 % are realized in a fine-grained (average grain size: 5 µm) Ti-10 wt.% Mo-0.5 wt.% O alloy. The superior strength-ductility balance is attributed to a ‘delayed and partially suppressed’ β-to-α″ phase transformation, due to the combined effects of fine grain size and oxygen solute. The delayed β-to-α″ phase transformation (until a plastic strain of 2.1 %) endows dislocation slips as the main factor determining the yield strength, thus enabling a full harness of grain boundary strengthening and oxygen solute hardening. Moreover, {332} twinning activated at the early stage and strain-induced α″ martensite initiated at the later stage of deformation provide continuous strain-hardening capabilities up to a relatively larger strain. Finally, the formation of relatively soft α″ martensite helps to relax the stress localization at slip bands, delaying the formation of microcracks, compared to the coarse-grained counterparts. The novel approach in the present study provides a general strategy to manage both high yield strength and large uniform elongation in metastable β titanium alloys, via tailoring both structural (grain size) and compositional (oxygen content) parameters of the material.