Intermediate annealing of severely deformed pure titanium by multi-directional forging: Effect on mechanical properties and microstructure

Abstract

Multi-directional forging (MDF) is conducted on commercially pure titanium (CP-Ti) with intermediate annealing at 450 °C and 650 °C to investigate the effectiveness of intermediate annealing on mechanical properties improvement. The macrostructural and microstructural features, mechanical properties, fracture surface, and thermal stability of the samples are thoroughly investigated. The findings reveal a novel approach, employing intermediate annealing at 650 °C between MDF cycles, which is an effective method for significantly enhancing the mechanical properties of CP-Ti. This technique led to the highest strength of CP-Ti achieved by utilizing three MDF cycles with intermediate annealing at 650 °C, increasing the tensile strength and hardness by 70 % (up to 736 MPa) and 55 % (up to 268 HV), respectively, compared to the as-received material, and with acceptable elongation of 18.5 %. Maintenance of acceptable elongation is related to the second linear work hardening stage, delaying the necking occurrence. The improvement of tensile properties is ascribed to the increase in dislocation density and stored energy estimated by microhardness results. Calorimetry of MDFed samples showed a decrease in recrystallization temperature, indicating that annealing at 650 °C almost fully recrystallized the previously inhomogeneous deformed microstructure. The microstructure included equiaxed grains at the center and deformation twins at the sides. Annealing at 650 °C produced an equiaxed microstructure that allowed for the continuation of MDF cycles, while 450 °C did not. However, the absence of intermediate annealing during two consecutive MDF cycles resulted in undesirable microcracks development. The pile-up of dislocations at grain boundaries in adiabatic shear bands caused the microcrack formation characterized by void coalescence.