Microstructure, Texture, and Hardness Evolution of Cold-Rolled High-Purity Ti Sheet During Annealing at 350 °C to 550 °C

The microstructural, textural, and hardness evolution of a cold-rolled high-purity Ti sheet (50 pct reduction in thickness) during isochronal annealing at 350 °C to 550 °C was investigated by jointly using electron backscatter diffraction, electron channel contrast imaging, X-ray diffraction, and hardness test, with a dedicated correlation analysis between them performed. Results show that recrystallization nucleation starts at 400 °C with prior grain boundaries as preferential sites while fully recrystallized structures were obtained after annealing at 550 °C for 1 hours. During the primary recrystallization, contrary to earlier reports, considerable textural changes occur: the prior bimodal basal texture characteristic is essentially replaced by a ring-like basal texture. Such a textural change is attributed to the orientation-gradient-related subgrain coalescence mechanism that dominates the recrystallization nucleation of the high-purity Ti sheet. Specimen hardness always decreases with increasing the annealing temperature (from 234.3 ± 5.8 to 135.9 ± 2.9 HV), which is closely related to continuous variations of grain sizes and dislocation densities (mainly stored in low-angle boundaries). Quantitative analyses reveal that both grain sizes and dislocation densities have mild effects on the hardness change when annealed below 450 °C. However, the decrease in dislocation density leads to more significant softening than grain coarsening when the annealing temperature exceeds 450 °C.