Improvement in fracture toughness of Ti-4.5Al-3V-2Mo-2Fe through microstructural optimization

In order to improve the observed low fracture toughness of beta-rich alpha + beta? type Ti-4.5Al-3V-2Mo- 2Fe annealed at the temperature of 1123 K, a two-step cooling (TSC) after solution treatment at that temperature was proposed instead of air cooling. Solution treatment plus aging (STA) and slow furnace-cooling (SFC) treatments were also carried out on the same alloy for comparison. It was found that the relatively higher fracture toughness (JIC) is obtained by the TSC treatment, that is, by slow cooling the alloy at a cooling rate of 0.075 Ks-1 from a temperature of 1123 to 723 K and, subsequently, water quenching to room temperature. The JIC of the two-step-cooled alloy has the same value as that of the alloy annealed at 993 K after having first been annealed at 1123 K (duplex annealing (DA)), which has been previously observed to have high fracture toughness. The JIC value can also be improved by STA, but it is still lower than that of two-step-cooled alloy. The relatively lower JIC is obtained in slow-furnace-cooled alloys. The JIC of slow-furnace-cooled alloys decreases monotonously with decreasing cooling rate for cooling rates less than 0.075 Ks-1. It is suggested that the factors responsible for increasing JIC in two-step-cooled and solution treated?and-aged alloys are the plate-like aand secondary a, respectively, which appears in the bphase during cooling and aging, respectively. The presence of such kinds of transformation products increases the crack deflection effect and, thus, increases fracture toughness.