Impact toughness, crack initiation and propagation mechanism of Ti6422 alloy with multi-level lamellar microstructure

The influence of different solution and aging conditions on the microstructure, impact toughness, and crack initiation and propagation mechanisms of the novel α+β titanium alloy Ti6422 was systematically investigated in this article. By adjusting the furnace cooling time after solution treatment and the aging temperature, Ti6422 alloy samples were developed with a multi-level lamellar microstructure, including microscale α colonies and αp lamellae, as well as nanoscale αs phases. Extending the furnace cooling time after solution treatment at 920 °C for 1 hour from 240 minutes to 540 minutes, followed by aging at 600 °C for 6 hours, increased the αp lamella content, reduced the αs phase content, expanded the α colonies and αp lamellae size, and improved the impact toughness from 22.7 J/cm² to 53.8 J/cm². Additionally, under the same solution treatment, raising the aging temperature from 500 °C to 700 °C resulted in a decrease in the αs phase content and a growth in the thickness of the αp lamella and αs phase. The impact toughness increased significantly with these changes. Samples with higher αp lamellae content or larger αs phase size exhibited higher crack initiation and propagation energies. Impact deformation caused severe kinking of the αp lamellae in crack initiation and propagation areas, leading to a uniform and high-density KAM distribution, enhancing plastic deformation coordination and uniformity. Moreover, the multidirectional arrangement of coarser α colonies and αp lamellae continuously deflect the crack propagation direction, inhibiting crack propagation.