Hybrid Monte Carlo/Molecular Dynamics Simulation of a Refractory Metal High Entropy Alloy

The high entropy alloy containing refractory metals Mo-Nb-Ta-W has a body-centered cubic structure, which is not surprising given the complete mutual solubility in BCC solid solutions of all pairs of the constituent elements. However, first principles total energy calculations for the binaries reveal a set of distinct energy minimizing structures implying the likelihood of chemically ordered low-temperature phases. We apply a hybrid Monte Carlo and molecular dynamics method to evaluate the temperature-dependent chemical order. Monte Carlo species swaps allow for equilibration of the structure that cannot be achieved by conventional molecular dynamics. At 300 K (27 degrees C), a cesium-chloride ordering emerges between mixed (Nb,Ta) sites and mixed (Mo,W) sites. This order is lost at elevated temperatures.