Origin of gamma surface asymmetries in body-centered cubic refractory high entropy alloys

In this study, we develop an understanding of the effect of high stacking fault elements in the γ-surfaces and twinnability of NbMoTaW refractory High Entropy Alloy (HEA) using first-principles density functional theory calculations. γ-surfaces developed low energy pathways leading to asymmetries by controlling the concentration of high stacking fault energy elements in the HEA. This effect was correlated to the strength of specific solid-state pair interactions using the projected crystal orbital Hamilton population analysis, and they were found to be weak except for W-Mo and W-Ta. Since the interactions involving Tungsten are stronger than the other interactions, stacking fault and twin formation are less likely to happen near tungsten-rich portions of the alloy. This suggests the formation of tungsten-deficient twin boundaries in NbMoTaW which will further improve the ductility of the system. Stable twin fault energy as low as 522 mJ/m 2 is reported for the equimolar random atomic configurations of the HEA system. The ratio between twin boundary migration energy and the difference in unstable stacking fault energies (γTBMΔus) was found to be as low as 0.13 suggesting deformation twinning can happen in the HEA system. Vibrational analysis of the alloy system has indicated that tungsten and tantalum contribute to the acoustic phonon frequencies of the HEA. Fundamental insights from this study will help in engineering high entropy alloys with high strength and ductility.