Mechanisms underlying enhanced strength-ductility combinations in TRIP/TWIP Ti-12Mo alloy engineered via isothermal omega precipitation

β Ti-alloys can achieve a high strain-hardening rate and tensile ductility by taking advantage of transformation induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects. While nano-precipitation of isothermal ω (ωiso) can have a substantial strengthening effect in these alloys, it usually has a detrimental effect on ductility leading to embrittlement. To overcome the above problem, this work proposes as a novel strategy based on coupling of ωiso formation and mechanical twinning/martensitic transformation to enhance the strength while preserving good ductility in case of the TRIP/TWIP Ti-12Mo alloy. An unprecedented combination of tensile properties, yield stress at 865MPa (80% higher than classic Ti-12Mo) with uniform elongation of 0.35, are recorded after 200°C aging for 60s. Higher yielding stress (990MPa) is achieved when increasing the aging duration to 150s where mechanical twinning is still active. In-situ investigations under traction/heating, and atom probe tomography are performed to clarify the ωiso formation process and the interactions between ω phase and the operating deformation mechanisms, i.e. mechanical {332}<113> twinning, β → α″ martensitic transformation and dislocation glide. The early stages of formation of ωiso precipitates, mediated via Mo partitioning at low aging temperature, and its consequent impact on the deformation mechanisms operative in the β matrix has been characterized. The transformation partition mapping method, based on statistical electron backscatter diffraction characterization developed in our previous work, is employed to individually assess the evolution of the critical resolved shear stresses of each operating deformation mechanism as a function of the ωiso nucleation.