Revealing Microstructural, Textural, and Hardness Evolution of Ti–6Al–4V Sheet Cooled From Sub β-Transus Temperature at Different Rates

A hot-rolled Ti–6Al–4V sheet was annealed at 960 °C (sub β-transus temperature) and then subjected to different coolings [in water (WC), air (AC), and furnace (FC)]. Microstructural and textural characteristics were systematically characterized and quantitatively analyzed by combined use of multiple characterization techniques, with hardness also examined. Results indicate that both WC and AC specimens possess mixed microstructures of primary bulk α (α_p) grains and transformed secondary α (α_s) plates. The rapid WC allows the β → α transformation to be complete (martensite) while a small amount of residual thin β film (V-enriched) exists in the AC specimen. Plate structures are absent in the FC specimen with its microstructure consisting of coarse-equiaxed α_p grains and residual β phase. For all the cooled specimens, their major textural feature is quite similar to that of the as-received material, while the textural intensity increases at lower cooling rates due to specific α_p grain growth and α_s variant selection. For rapid cooling in water, however, a single β orientation is able to give birth to twelve different α_s variants, producing new textural components and a globally weakened texture. The hardness of the WC specimen (379.7 ± 5.5 HV) is higher than that of the as-received material (366.1 ± 4.3 HV), which can be ascribed to combined contributions from grain refinement and solid solution of alloying elements due to the martensitic transformation. With decreasing cooling rates, specimen hardness is reduced from 342.5 ± 4.9 HV (AC specimen) to 327.2 ± 4.6 HV (FC specimen) because of increased grain sizes.