Strain delocalization in a gradient-structured high entropy alloy under uniaxial tensile loading

Suppressing strain localization in hard layers was crucial for making gradient alloys ductile. Here, a typical structure in CrFeCoNiMn_0.75Cu_0.25 high entropy alloy (HEA), which combines gradient distributions along the depth for grain width and twin density, was fabricated using the laser shock peening (LSP) treatment. In situ digital image correlation methods elucidated the mechanisms of the excellent mechanical properties in HEAs treated by LSP for 4 cycles from the perspective of local strain. Dense shear bands were nucleated at the low-strain stage, and remained stable evolution during the entire plastic deformation, thus, leading to an excellent tensile ductility (∼40 %). The plastic deformation incompatibility results in two-dimensional stress states and lateral strain gradient near the plastic-elastic interfaces, which stimulates the interaction and accumulation of micro-defects, thus, improving the strain hardening capacity of alloys. These observations will reveal the mechanistic origin of the gradient structured alloys with superior strength-ductility from a new perspective, which also provide a guidance for optimizing mechanical performances of HEAs.