Pile-up and sink-in nanoindentation behaviors in AlCoCrFeNi multi-phase high entropy alloy

Abstract

Microstructures and nanoindentation behaviors were studied on annealed AlCoCrFeNi high entropy alloy. Both pile-up and sink-in characteristics were found in the grain boundary and grain regions, respectively. The multiple phases present in the AlCoCrFeNi high entropy alloy are the reasons behind the different nanoindentation behaviors, which were identified using electron microscopy. The identified phases showed the grain boundary segregation to have A1 lattice, viz., FCC structure while the grain was distributed with A2 and B2 lattices, viz., BCC and ordered BCC structures, forming the matrix with nano-precipitates of the other. The reason for the pile-up and sink-in is attributed to the dislocation activity in the individual crystal structure: large dislocation activities were found under the pile-up and little dislocation activities under the sink-in, only limited to the indenter tip. Results from a finite element analysis under an isotropic elasto-plastic condition by varying the hardness-to-modulus ratio show that high hardness-to-modulus ratio results in pile-up and the lower ratio results in sink-in. This was associated with the susceptibility to plasticity and the elastic recovery for individual phases of the AlCoCrFeNi high entropy alloy.