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In this study Mo-Nb-Ta-W refractory high-entropy alloys (R-HEAs) have beenstudied for their phase stability for a wide temperature range (100 K to 2000K). The equilibrium thermodynamic phases are determined by the changes inenthalpy and entropy. The enthalpy changes at any temperature are simulated byembedded atom method (EAM) potential based hybrid Monte Carlo moleculardynamics (MC/MD) simulations. Configurational entropy was calculated byquasichemical method. The EAM potentials were all calculated based on thephysical input parameters of elements like atomic volume, cohesive energy,elastic constants etc. It was found that the MC/MD evolved equilibriumstructures and the degree of local chemical short-range order/clustering(SRO/SRC) largely depend on the ordering enthalpies for various temperatures.The ordering transition temperature is close to 700 K, which is alsosubstantiated by the experimental powder X-ray diffractions done in synchrotronbeam. Large increase of degree of next-neighbor B2-type ordering of Mo-Ta pairsand decrease of that Nb-W pairs were observed. This was also expected from thetrends of density functional theory (DFT) based ab-initio simulations done inthe literature and discussed in this work. The simulated diffraction patternand changes in scattering intensity due to the chemical SRO and SRC wasinvestigated. The diffraction trends were explained with the help of thefeatures of the evolved nanostructure morphology. The developed methodologiesmay be helpful in prior prediction of long-term phase stability inmulti-element alloys by reducing the number of costly experiments.

Temperature dependent phase stability of Mo-Nb-Ta-W refractory high-entropy alloys