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

In order to improve the observed low fracture toughness of β-rich α+β-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 (J _IC ) 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 J _IC 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 J _IC value can also be improved by STA, but it is still lower than that of two-step-cooled alloy. The relatively lower J _IC is obtained in slow-furnace-cooled alloys. The J _IC 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 J _IC> in two-step-cooled and solution treated-and-aged alloys are the plate-like α and secondary α, respectively, which appears in the β phase during cooling and aging, respectively. The presence of such kinds of transformation products increases the crackdeflection effect and, thus, increases fracture toughness.