The effect of Al content and loading speed on Al x CoCrFeNi high entropy alloy and molecular dynamics simulation

The effects of Al content and loading speed on the mechanical properties of Al x CoCrFeNi high entropy alloy (HEA) during nanoindentation were studied by molecular dynamics (MD) simulation from hardness, von Mises stress, shear strain, dislocation, and phase transformation. The results show that when the Al content is lower, more atomic shear bands are formed in HEA, and the total dislocation length and density are more significant. When the loading speed is high, many dislocation tangles produce high-density dislocation cells, which hinder dislocation slip and bring a considerable strengthening effect. Al content is inversely proportional to hardness while loading speed is directly proportional to the hardness of materials. In addition, at the initial stage of nanoindentation, all HEAs undergo recoverable elastic deformation. The material experiences plastic deformation, dislocation nucleation, and growth with increased indentation depth. Due to the indenter force, the atoms in the material are rearranged, and the FCC structure gradually transforms into BCC, HCP, and disordered structures. The atomic strain in the region in contact with the indenter is high and is the first to transform into a disordered structure. After many loading-unloading cycles, the elastic deformation gradually changes into plastic deformation, the residual stress in the alloy increases, and Al x CoCrFeNi HEA exhibits an evident cyclic degradation phenomenon.