Effects of β-cooling rates on microstructural characteristics and hardness of Ti-5Al-5Mo-5V-3Cr-1Fe metastable β Ti alloy

A typical metastable beta Ti alloy (Ti-5Al-5Mo-5V-3Cr-1Fe alloy) was beta-solution treated (890 degrees C for 10 min) and then cooled at different rates (in water (WC), air (AC) and furnace (FC)). To clarify specific effects of beta-cooling rates, multiple characterization techniques were jointly utilized to reveal their microstructural and orientation characteristics in detail, with hardnesses also measured and correlated with specific microstructures. Results show that both the WC and the AC specimens are mainly comprised of recrystallized beta grains with no alpha phase, while the FC specimen consists of coarse equiaxed beta grains, intragranular acicular alpha phase (width similar to 48 nm) and grain boundary alpha phase. The Burgers orientation relationship is strictly obeyed during beta ->alpha transformation in the FC specimen. When adjacent beta grains share a common 110 axis, the boundary alpha phase maintains such orientation relationship with both of them. Compared to the as-received specimen (428 +/- 9 HV), the WC (309 +/- 5 HV) and the AC specimens (297 +/- 3HV) are softened, which can be attributed to the coarsened beta grains as well as the disappearance of alpha phase and deformed structures. For the FC specimen, dense fine alpha precipitates appear inside beta grains during slow cooling, leading to increased hardness (444 +/- 9 HV).