Correlation of microstructural, textural characteristics and hardness of Ti–6Al–4V sheet β-cooled at different rates

In this work, a hot-rolled Ti–6Al–4V (Ti-64) sheet was annealed at 1050 °C (β field) and then subjected to different coolings (in water, air and furnace). Electron backscatter diffraction, electron channeling contrast imaging, energy-dispersive spectroscopy and X-ray diffraction techniques were jointly utilized to comprehensively reveal microstructural and textural characteristics of the specimens following various β-cooling rates. Hardness measurements were also performed to probe the hardness variation associated with the β-cooling rates and were further rationalized based on the revealed microstructures. Results show that the β-cooled specimens in water, air and furnace exhibited twinned martensite, basket-weave and parallel-plate Widmanstatten structures, respectively. The Burgers orientation relationship is always obeyed during the β → α transformation at all cooling rates. After phase transformation, major α texture components ((0°, 60°, 0°), (90°, 30°, 30°) and (90°, 90°, 30°)) are completely different from those of the as-received material ((0°, 90°, 0°) and (90°, 30°, 0°)). Slow cooling facilitates preferable nucleation of α phases at prior β boundaries and strengthens α-variant selection, leading to greatly intensified transformation texture (the maximum density from 9.5 to 31.1 times of random). Hardness values of the β-cooled specimens drop from 416.8 ± 4.3 HV for water cooling to 329.2 ± 23.6 HV for furnace cooling. Quantitative calculations suggest such hardness variation can be more related to differences in grain size between them, as compared to specific contributions from other microstructural and textural characteristics.