Dynamic recrystallization mechanism and twinning behavior in hot deformation of Ti-46.5Al-2Nb-2Cr with initial duplex microstructure

The dynamic recrystallization (DRX) mechanism and twinning behavior of Ti-46.5Al-2Nb-2Cr in the α + γ two-phase region were investigated in this work. Isothermal compression of Ti-46.5Al-2Nb-2Cr with initial duplex microstructure was performed at the deformation temperatures of 1180–1260 °C, the height reductions of 20–60% and strain rates of 0.001–1.0 s⁻¹. Quantitative characterization of grain boundary misorientation, local misorientation and geometrically necessary dislocation (GND) density in the γ phase was made to investigate the DRX behavior. The results showed that DRX occurred in the γ grains and the γ lamellae of the α₂/γ lamellar colonies as the height reduction increased, and the interaction between the deformation temperature and the strain rate influenced the volume fraction of the recrystallized γ phase. Discontinuous DRX (DDRX) via nucleation and grain growth dominated the DRX in the γ phase at a strain rate of 0.001 s⁻¹, in which the volume fraction of the recrystallized γ phase increased with the increasing of deformation temperature. Continuous DRX (CDRX) via transition from medium-angle grain boundaries (MABs) to high-angle grain boundaries (HABs) dominated the DRX in the γ phase at a strain rate of 1 s⁻¹, in which the volume fraction of the recrystallized γ phase decreased with the increasing of deformation temperature. γ/γ twin boundaries were quantitatively investigated. It was shown that γ true twin (TT) was the main twin component formed in the recrystallized γ grains. The fraction of the γ/γ twin boundaries increased with the increasing of deformation temperature at a strain rate of 0.001 s⁻¹ and height reduction of 60%, which decreased with the increasing of strain rate at a deformation temperature of 1260 °C and height reduction of 60%. Transmission electron microscopy and high-resolution TEM (HRTEM) characterizations of the dislocations and stacking fault (SSF) in the γ phase showed that the growth of the DDRX γ grains promoted the occurrence of γ twin.