Effect of Hot Rolling and Annealing on Phase Component, Recrystallization, and Mechanical Properties of TC21 Titanium Alloy

The effects of hot rolling and subsequent annealing on microstructure and mechanical behavior of an equiaxed-structure TC21 titanium alloy were investigated. The microstructure observation showed that no phase transformation and dynamic recrystallization occurred during rolling due to the low rolling temperature (750 °C). During annealing, the annealing temperature influenced the microstructure and mechanical properties more remarkably than the annealing time. At the annealing temperature of 820 °C, compared with the rolled samples, the value of strength decreased and the value of plasticity increased, which was largely related to the reduction of strain hardening effect. At the annealing temperature of 880 °C, the increase of β phase content induced the simultaneous occurrence of stress-induced martensitic transformation (SIMT) and dislocation slip during tensile and then resulted in the double yield phenomenon. At the annealing temperature of 920 °C, the value of strength increased, and the value of plasticity decreased obviously, which was related to the precipitation of secondary α phase (αs) and martensitic (α″). However, prolonging annealing time from 1 h to 6 h had a little effect on mechanical properties, because the phase content and grain morphology had a little change with the variation of annealing time. In addition, the fracture morphologies of all annealed samples were composed of equiaxed dimple, indicating a typical ductile fracture.