Dynamic nanoindentation and short-range order in equiatomic NiCoCr medium entropy alloy lead to novel density wave ordering

Abstract: Chemical short-range order (CSRO) is believed to be a key contributor to the exceptional properties of multicomponent alloys. However, direct validation and confirmation of CSRO has been highly elusive in most compounds. Recent studies for equiatomic NiCoCr alloys have shown that thermal treatments (i.e., annealing/aging) may facilitate and manipulate CSRO. In this work, by using molecular simulations, we show that nanomechanical probes, such as nanoindentation, may be utilized towards further manipulation of CSRO, providing explicit validation pathways. By using well established interatomic potentials, we perform hybrid Molecular-Dynamics/Monte-Carlo (MD/MC) at room temperature to demonstrate that particular dwell nanoindentation protocols can lead, through thermal MC equilibration, to the reorganization of CSRO under the indenter tip, to a density-wave stripe pattern (DWO). We characterize the novel DWO structures, that are directly correlated to incipient SRO but are highly anisotropic and dependent on local, nanoindentation-induced stress concentrations, and we show how they deeply originate from the peculiarities of the interatomic potentials. Furthermore, we show that the DWO patterns consistently scale up with the incipient plastic zone under the indenter tip, justifying the observation of the DWO patterning at any experimentally feasible nanoindentation depth.