Preparation of multiscale α phase by heat treatments and its effect on tensile properties in metastable β titanium alloy sheet

This study presents a strategy for the preparation of multiscale α phase by high/low-temperature two-step aging in metastable β titanium alloy, Ti-15Mo-3Al-2.7Nb-0.2Si. The purpose of high-temperature aging is to obtain a number of micro-scale secondary α phases (αS), besides precipitation-free zones (PFZs). After that, in the second stage of low-temperature aging, the nano-scale tertiary α phase (αT) precipitates in the PFZs. The second-step aging temperature and the time at low temperature will have an important effect on αT. On the one hand, with the extension of aging time, the morphology of αT changes from dot-like to needle-like. Secondly, the increase of the second-step aging temperature will promote precipitation of αT in PFZs. The strength increases gradually with the accelerated precipitation of αT, and good plasticity is still maintained. The effect of multiscale α on fracture behavior was investigated by in situ tensile testing. Results show that cracks form a transgranular fracture along the slip line after solution treatment. After first-step aging, the crack easily passes through αS and forms an intergranular fracture. After second-step aging, the formation of multiscale α phase has a stronger effect in hindering the crack growth, which leads to crack propagation mainly by transgranular fracture. Overall, this study demonstrates that the alloy can be heat-treated to achieve an excellent match between strength and ductility, which is expected to meet the application requirements of structural materials of the new generation of aircraft.