Chemical short range order and deformation mechanism of a refractory high entropy alloy HfNbTaZr under nanoindentation: An atomistic study

Abstract

Refractory high entropy alloys (RHEA) are promising potential used as structural materials owing to the remarkable mechanical properties, such as excellent thermal stability and strength. However, the dynamic nanoindentation response of the nanocrystalline RHEA at atomic scale is not fully revealed. Here, the mechanical behaviour of the as-cast and annealed nanocrystalline HfNbTaZr RHEA under nanoindentation is investigated for first time using a large scale molecular dynamics simulation, in the terms of indentation force, microstructural evolution, stress distribution, shear strain distribution, and surface topography. The results show the annealed nanocrystalline HfNbTaZr RHEA has obvious chemical short-range ordered structure (CSRO), including Ta enrichment and Hf–Nb segregation. The elastic modulus of the annealed RHEA is larger than that of random RHEA, agreeing with the previous experiment. CSRO improves the indentation force and work hardening, due to the inhibition of dislocation nucleation and motion. Interestingly, the hardening behavior is found in the unloading stage, which is rarely reported in traditional alloys. The crystal-to-amorphous phase transition takes place under the loading, and the inverse phase transformation occurs under the unloading. The increase rate of high Von Mises stress and hydrostatic stress region is lower than the change rate of the high strain region. Certainly, almost all atoms move to consume stored energy after unloading, leading to the decrease of stress and strain. The current study not only gives an insight into probing the mechanical behavior of the nanocrystalline RHEA, but also provides an avenue to design high performance RHEAs based on the heat treatment process.