Phase transformation and microstructure evolution of nanoimprinted NiCoCr medium entropy alloys

Abstract In this study, the mechanical and processing properties of nanoimprinted nickel-cobalt-chromium medium-entropy alloy (NiCoCr MEA) are investigated using molecular dynamics (MD) simulation. The effects of microstructure, loading velocity, the taper angle of mould, and processing temperature are analyzed by loading force, deformation characteristic, crystal evolution, dislocation distribution, displacement vector, and radial distribution function. Based on the results, both stacking faults and twinning structures appear in NiCoCr MEA during the imprinting process. The formation of stacking fault is affected by grain size, and it always propagates in large grains (grain size = 8.5 nm). Besides, the maximum loading force and the residual stress increase with an increase in loading velocity (200 m/s), caused by the shorter relaxation time for the atoms in NiCoCr MEA specimen with higher loading velocity. Moreover, the temperature is found to affect the microstructure, phase transformation, and dislocation evolutions of NiCoCr MEA specimen. Most of the atoms transform into HCP or amorphous phase at a temperature of 900 K during the equilibrium process. The maximum loading force decreases with an increase in temperature, and atoms strongly adhesion on the bottom of the mould at the higher temperature (900 K). By contrast, the taper angle of mould gives little effect to the mechanical characteristics. There is no obvious trend in the deformation behaviors of the specimens with different taper angles.