Stacking fault energies of face-centered cubic concentrated solid solution alloys

We report the stacking fault energy (SFE) for a series of face-centered cubic (fcc) equiatomic concentrated solid solution alloys (CSAs) derived as subsystems from the NiCoFeCrMn and NiCoFeCrPd high entropy alloys based on ab initio calculations. At low temperatures, these CSAs display very low even negative SFEs, indicating that hexagonal close-pack (hcp) is more energy favorable than fcc structure. The temperature dependence of SFE for some CSAs is studied. With increasing temperature, a hcp-to-fcc transition is revealed for those CSAs with negative SFEs, which can be attributed to the role of intrinsic vibrational entropy. The analysis of the vibrational modes suggests that the vibrational entropy arises from the high frequency states in the hcp structure that originate from local vibrational mode. Our results underscore the importance of vibrational entropy in determining the temperature dependence of SFE for CSAs.