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 degrees C (sub beta-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 alpha (alpha(p)) grains and transformed secondary alpha (alpha(s)) plates. The rapid WC allows the beta -> alpha transformation to be complete (martensite) while a small amount of residual thin beta film (V-enriched) exists in the AC specimen. Plate structures are absent in the FC specimen with its microstructure consisting of coarse-equiaxed alpha(p) grains and residual beta 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 alpha(p) grain growth and alpha(s) variant selection. For rapid cooling in water, however, a single beta orientation is able to give birth to twelve different alpha(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.