High-strain-rate deformation: Stress-induced phase transformation and nanostructures in a titanium alloy

Abstract

Microstructural evolution in adiabatic shear bands of a Ti-10V-2Fe-3Al (wt.%) alloy subjected to high strain-rate dynamic loading was investigated in detail via high-resolution transmission electron microscopy (HRTEM), focusing on the underlying plastic deformation mechanisms. The alloy after solution treatment consisted of a coarse body-centered cubic β single-phase microstructure, which exhibited double-yield and stress-collapse characteristics. The former was a result of stress-induced martensite transformation accompanied by the formation of nanotwins in the martensite, and the latter was caused by thermal softening stemming from dynamic recovery. Substantial grain refinement with three orders of magnitude grain size reduction occurred in the center of adiabatic shear bands due to severe plastic deformation along with dynamic recovery. Multiple deformation mechanisms in the adiabatic shear bands were identified, including the formation of face-centered orthorhombic α″ martensite, {111}α″ type I nanotwins, and hexagonal close-packed α phase, where distinctive nano-twinned structures and phase transformation from α″ to α with an orientation relationship of {1¯11}α″//{1¯101}α were observed. The transition area of the adiabatic shear bands contained mainly high-density dislocation regions and plate-like structures with relatively fewer nanoscale grains, where the transition from nanokinks to nanotwins along with phase transformation was also revealed, corroborating that kink bands served as nanotwin nucleation sites. The mechanisms of the microstructural evolution in relation to the mechanical response during the high strain-rate dynamic loading were discussed.