Effects of Strain and Strain Rates on Microstructure Evolution Within ASB of TC4

Abstract

Adiabatic shear band (ASB), a typical failure mechanism in a metal at high strain rates, is hardly controllable or predictable to some extent. The development of the microstructure plays a crucial role in its formation. In this paper, the effect of strain rate on the development of microstructure in ASB of titanium alloy TC4 is investigated using hat-shaped specimens with the split-Hopkinson pressure bar device. The results show that the fracture strength of TC4 is significantly dependent on the shear strain rate. The increase in fracture strength from a strain rate of 11,300 s⁻¹ to 24,930 s⁻¹ is much higher than that from 24,930 s⁻¹ to 35,620 s⁻¹, which can be attributed to the effect of strain rate on dislocation evolution. Microstructures in both as-received and deformed states are investigated using various characterization techniques such as electron backscatter diffraction and X-ray diffraction. The region of ASB clearly shows three different microstructural features: random distribution of coarse grains (as received), distribution of elongated grains (transition zone), and distribution of equiaxed nanocrystals (shear-localized zone). The width of ASB increases with the strain rate. The possible reason for this is that the higher the strain rate, the larger the region where dynamic recrystallization (DRX) occurs due to the accumulation of a large number of dislocations. In the middle of ASB, a significant decrease in low-angle grain boundaries (LAGBs) and a large increase in high-angle grain boundaries are observed. The texture of specimens, especially the {11–20} and {10–10} planes, changes significantly during shear deformation at high strain rates. The mechanism of continuous dynamic recrystallization can well explain the formation and evolution of DRX within the ASB.