Microstructural and textural evolution of high-purity Ti sheet with dense preexisting twins during isochronal and isothermal annealing

Abstract

A high-purity Ti sheet with dense preexisting twins (introduced by 10% cold rolling) was subjected to isochronal annealing at 500–800 °C for 1 h and isothermal annealing at 600 °C for 0.17–100 h, respectively. By mainly utilizing electron backscatter diffraction (EBSD) and electron channel contrast (ECC) imaging techniques, the microstructure and texture evolution during the isochronal and isothermal annealing were investigated systematically. Results show that recrystallization nuclei appear in the specimen annealed at 600 °C for 1 h. In contrast, recrystallization cannot be initiated for those annealed at lower temperatures or for a shorter time. With the increase in temperature or time, the fraction of the recrystallized structure increases with gradual grain coarsening. Nearly complete recrystallization is reached after 800 °C-1 h or 600 °C-100 h annealing. Due to the distribution heterogeneity of microstructure and stored energy induced by the dense preexisting twins, recrystallization nucleation preferentially occurs in some specific regions (twin-twin or twin-grain boundary junctions). Then, they selectively consume twin lamellar structures, leading to non-uniform grain growth. It is demonstrated that the recrystallization nucleation is dominated by the strain-induced boundary migration mechanism, allowing scattered texture components corresponding to the twin lamellar structures to be gradually encroached by those un-twinned structures with the initial bimodal basal texture (BBT). Eventually, a strong BBT is always obtained after sufficient recrystallization.