A Thermodynamic Modelling of the Stability of Sigma Phase in the Cr-Fe-Ni-V High-Entropy Alloy System

The addition of vanadium (V) to the representative Co-Cr-Fe-Mn-Ni high-entropy alloy (HEA) system is attracting attention expecting a large solid solution hardening effect. For the design of V-added HEAs, prediction of the sigma (sigma) phase formation has been mainly issued because it affects a significant influence on the mechanical properties. Although the CALculation of PHAse Diagram (CALPHAD) approach can be a good tool for prediction of phase structures, robust thermodynamic database is still required for an accurate prediction of V-added HEA systems. The present work aims at providing a thermodynamic description for the Cr-Fe-Ni-V HEA system, focusing on the thermodynamic stability of the sigma phase. A parameterization technique which minimizes the number of fitting parameter and simplifies the extension into higher-order systems is proposed and applied to the sigma phase with multiple sublattice during modelling the Cr-Ni-V and Fe-Ni-V systems. The reliability of the developed thermodynamic description for the Cr-Fe-Ni-V quaternary system is experimentally confirmed by designing, fabricating and analysing the phase structures of a series of Cr-Fe-Ni-V HEAs.