Elastic and plastic deformations in a high entropy alloy investigated using a nanoindentation method

We employed instrumented indentation technique to study the elastic and plastic deformations in a face-centered cubic (fcc) high-entropy alloy NiFeCoCrMn. Single-crystal Ni was also examined for direct comparison. Tests were carried out using indenters with different tip radii to investigate the effect of indented volume on deformation processes. It was found that when the tip radius increased, the shear stress required for the occurrence of indentation pop-in decreased, which was attributable to a higher probability to find dislocations under a larger tip radius. We proposed a statistical model to describe the results quantitatively. In the plastic region, NiFeCoCrMn was much stronger than Ni, presumably resulted from a large lattice distortion in the multicomponent NiFeCoCrMn alloy. We found that the response of both materials at large indentation depth could be described by the classical Nix-Gao model, but when the indentation depth was shallow, the indenter tip must be treated as a sphere and Swadener's model offered a better description. In any event, it was necessary to introduce a scaling factor f to describe the effective stressed volume underneath the indenter tip to compensate for the overestimated hardness values. A map of indentation pressure against dislocation density was also summarized in this study.