Adiabatic shear instability in a titanium alloy: Extreme deformation-induced phase transformation, nanotwinning, and grain refinement

Abstract

Increasingly harsh service conditions place higher requirements for the high strain-rate performance of titanium alloys. Adiabatic shear band (ASB), a phenomenon prone to dynamic loading, is often accompanied by catastrophic damage. Yet, it is unclear how the internal nanostructures are related to shear instability. Here we report detailed microstructural evolution in the ASB of a titanium alloy via in-depth focused ion beam (FIB), transmission Kikuchi diffraction (TKD), and high-resolution transmission electron microscope (HRTEM) analyses, with the deformation instability phenomenon discussed from the energy perspective. The ASB interior undergoes multifaceted changes, namely deformation-induced beta-to-alpha transformation and deformation-induced martensitic transformation to form substantially refined and heterogeneous structures. Meanwhile, two types of extremely fine twins are identified to occur within both nano-sized martensite and alpha phase. The critical plastic work representing the onset of adiabatic shear instability and dynamic equilibrium is observed to be constant for a specific structure in the same deformation mode. The energy analysis could be extended to other materials subjected to high strain-rate dynamic deformation.