Structural softening mediated shear bands in high entropy alloys

•Magnetically-driven ramp compression experiments are applied to probe shear band initiation of classical face-centered cubic Cantor high entropy alloy (HEA).•Both experimental and simulated evidences validate that the structural softening plays a dominant role in the emergence of shear band in this chemically disordered HEA.•Configurational variation in the form of the formation of nanotwins as well as the generation of low-angle dislocation boundaries acts as the root cause of shear banding emergence.•The spatial and temporal sequence of structural softening driving shear band is given via the interconnection between nanotwins and adjacent low-angle dislocation boundary. Plastic flow localization is a fundamental and ubiquitous non-equilibrium phenomenon in metallic materials. Despite decades of extensive study, what derives its emergence remains elusive. Here, we tackle this problem in face-centered cubic (fcc) Cantor alloy by the newly developed ramp wave compression technique, which provides a unique quasi-isentropic loading path. By detailed microstructure characterizations, analytical estimation of temperature increment and large-scale atomistic simulations, we conclude that thermal softening is not a dominant driving force for shear band nucleation. Instead, nanotwinning triggers the initial transformation softening which is then accompanied with severe chemical fluctuations and the creation of low-angle dislocation boundaries (LADBs) associated with enhanced local dislocation slips in the adjacent regions. Such LADBs in turn lead to directional softening, acting as the catalytic mediating distortion between neighboring nanotwins. The interconnection between nanotwins and LADBs is thus regarded as structural origin of shear bands, whereas dynamic recrystallization only occurs later during shear band evolution, accelerating strain localization and thickening shear band. These findings shed new lights into fundamental understanding of shear banding and dynamic failure mechanisms in metallic materials.