Abstract: Metastable β-Ti alloys exhibiting twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) generally have excellent ductility, but typically at the expense of relatively low yield strengths which has restricted their widespread use. Our work shows that interstitial oxygen can be employed to regulate β phase stability to significantly enhance both strength and ductility of TWIP/TRIP alloys. For a Ti-32Nb wt.% base alloy, inclusion of 0.3 wt.% O enhanced ductility by more than 140 %, reaching up to 54 % strain, and improved the tensile yield strength by over 95 % to 632 MPa. Compared to other common engineering alloys such as Ti-45Nb, elongation was increased by 29 %, and the yield strength increased by 182 MPa, respectively. Here, we elucidate on impacts of oxygen doping on TWIP/TRIP behaviors in the Ti-32Nb alloy. We reveal that oxygen regulates the critical stress for martensitic transformation, twinning, and dislocation slip. At lower oxygen doping concentrations (≤0.3 wt.% O), multi-stage martensitic transformation and martensitic twinning resulted in high ductility. In higher oxygen content alloys (≥0.5 wt.% O), deformation occurred initially via twinning, while strain induced martensite was subsequently induced in retained β phase regions. Oxygen concentrations control the deformation mechanisms, providing a flexible means to synergistically balance an alloy's strength and ductility. The use of oxygen to enhance stability of the β phase and regulate deformation behaviors is a promising new approach for creating high-performance TWIP/TRIP metastable β-Ti alloys with outstanding mechanical properties.

Key words: Titanium alloy, TWIP/TRIP, Oxygen-doping, Deformation behavior, Critical stress, High-energy X-ray diffraction