Hierarchically ultrafine grains with the embedded nanotwins structure produced in ultra-high strain rate deformation of coarse-grained titanium alloy

Grain boundaries and coherent twin boundaries are two-dimensional defects which can effectively impede dislocation movement and thus contribute to high strength in metals and alloys. However, the simultaneous engineering of the coherent twin boundaries and ultrafine grains in an originally coarse-grained material is believed to be challenging by conventional processing strategies. In this research, we applied a high-strain-rate scratching to obtaining a hierarchically Ultrafine-Grain with the Embedded NanoTwins (UGENTs) structure in the coarse-grained commercial titanium alloy. Systematic experiments revealed an intriguing transition from dislocation-based deformation to twin-mediated plasticity with increasing strain rate. The enhanced twinning activities at high strain rates facilitated the generation of intensive high-angle grain boundaries, leading to the formation of a complex UGENTs structure. It is thus believed that such a UGENTs structure endows the structural component with a strong and tough skin to withstand harsh environmental attacks, such as wear during service.