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

beta 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 omega (omega(iso)) 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 omega(iso) 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 degrees 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 omega(iso) formation process and the interactions between omega phase and the operating deformation mechanisms, i.e. mechanical {332}< 113 > twinning, beta-* alpha '' martensitic trans-formation and dislocation glide. The early stages of formation of omega(iso) precipitates, mediated via Mo partitioning at low aging temperature, and its consequent impact on the deformation mechanisms operative in the beta 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 omega(iso) nucleation.

Automated summary

The work proposes a novel strategy to enhance the strength while preserving good ductility in the TRIP/TWIP Ti-12Mo alloy by coupling omega(iso) formation and mechanical twinning/martensitic transformation. After aging at 200 degrees C for 60s, an unprecedented combination of tensile properties is recorded, with a yield stress of 865MPa (80% higher than classic Ti-12Mo) and a uniform elongation of 0.35. Higher yielding stress (990MPa) is achieved with an increased aging duration of 150s, where mechanical twinning is still active.