Influence of multi-stage heat treatment on the microstructure and mechanical properties of TC21 titanium alloy

Duplex-structured TC21 alloy samples were first solution-treated at a higher temperature in the α + β region (940°C) with furnace cooling (FC), air cooling (AC), and water cooling (WC), followed by a second-stage solution treatment at a lower temperature in the α + β region (900°C), and then finally aged at 590°C. The effects of the morphology and quantity of α phases on the structure and properties of the TC21 alloy after the different heat treatments were analyzed. The in-situ tensile deformation process and crack propagation behavior were observed using scanning electron microscopy (SEM). The quantity of equiaxed α phases as well as the thickness of lamellar α phases reduced, the tensile strength increased firstly and then decreased, the elongation decreased with the increasing cooling rate after the first-stage solution treatment. The amount and size of lamellar α phases increased after the second-stage solution treatment because of sufficient diffusion of the alloying elements, thereby leading to increased tensile strength. The amount of dispersed α phases increased after the third-stage aging treatment owing to the increase in the nucleation rate, resulting in a noteworthy strengthening effect. After the third-stage aging treatment, the first-stage FC sample exhibited better mechanical properties because it contained more equiaxed α and β_trans phases than the first-stage AC and WC samples.