Strain localization and evolution of shear band microstructures in AA5754 sheet material subjected to quasi-static versus dynamic strain rates

The formation of adiabatic shear bands (ASBs) is a phenomenon of highly localized shear deformation in metallic materials that occurs especially at high shear strain rates. From a conventional thermo-mechanical perspective, local rapid heating induced by deformation at high strain rates leads to local thermal softening. In this study, we investigate the thermo-mechanical behavior of, and ASB formation in, the non-age-hardenable aluminum alloy AA5754. Compression tests with cylindrical specimens are performed at different temperatures (from 293 K to 673 K) and in a wide range of nominal strain rates between 10−4 and 103 s−1. Moreover, we use S-shaped samples and digital image correlation for the in-situ characterization of shear banding under quasi-static and dynamic conditions, and we perform microhardness measurements, electron backscatter diffraction analysis and transmission electron microscopy (TEM) for a post-mortem analysis of microstructural changes associated with different stages of shear deformation. Our results show that the material behavior is dominated by dynamic strain aging effects and micro-shear band formation under quasi-static loading, whereas dynamic loading conditions are associated with a pronounced localization of deformation, leading to local shear strain rates that exceed 30,000 s−1, and to substantial grain refinement as evidenced by nanocrystalline grains observed by TEM and an increase of subgrain boundary density inside the ASB. Clearly, formation and growth of shear bands depends strongly on the applied strain rate. Distinctly different microstructural deformation mechanisms occur and determine microstructural evolution as well as the resulting local material properties.