A Regular Solution Model for a Single-Phase High Entropy and Enthalpy Alloy

A regular solution, 3-component model suggested by J.L. Meijering in which binary interaction parameters were equal and positive has been extended to 5 and 6-component high entropy alloys (HEAs). On cooling, Meijering’s model develops miscibility gaps containing a low temperature eutectoid at the equiatomic composition. Similar behavior is found in this work on HEAs with the eutectoid temperature decreasing, while both the entropy and enthalpy are increasing, as additional components are added to the system. An equation for the chemical spinodal at the equiatomic composition is derived from the same thermodynamic model that was used to predict miscibility gaps. The spinodal temperature is at a cone point where multiple spinodal surfaces meet and is dominated by entropy. A proposal is made to categorize HEAs as having low, medium or high enthalpy. Low enthalpy HEAs are defined as having mixing enthalpies less than 1.25 kJ/mol, high enthalpy HEAs having mixing enthalpies greater than 2.9 kJ/mol, and medium HEA as between the extremes. A possible approach for designing high enthalpy HEAs is suggested to incorporate Meijering’s method of analyzing potential HEAs according to their individual binary interaction parameters instead of their total mixing enthalpy.