Dynamic β to α transformation and deformation accommodation in a metastable β-Ti alloy during tensile deformation at 520 °C

Balancing tensile strength and ductility at elevated temperature remains challenging for metastable β titanium alloys because precipitation, recovery, plastic localization and damage evolution may occur concurrently during deformation. In this work, the effect of aging time at 520 °C on the initial microstructure and subsequent tensile response of a Ti-3.5Al-5Mo-4V alloy was investigated at 520 °C. After solution treatment and short-time aging for 5 min, the alloy remains α-lean before deformation but undergoes pronounced thermomechanically assisted β to α transformation during tensile deformation at 520 °C. This transformable initial state enables a UTS of 1048.4 MPa and a total elongation of 34.6%. In contrast, prolonged aging produces α-rich microstructures before loading, which exhibit higher hardness and yield strength but reduced tensile ductility. Post-tensile SEM and TEM observations reveal deformation bands, band-confined nano-equiaxed (α+β) phases, stacking-fault-associated FCC lamellae and local nanoscale twinning in the 5 min-aged specimen. These features indicate that the tensile response is governed not only by the pre-aged microstructure, but also by thermomechanically assisted phase and defect evolution during high-temperature deformation. The results clarify the coupling between initial α precipitation state, dynamic β to α transformation and high-temperature tensile behavior in a metastable β titanium alloy.