Effect of Thermodynamic Stability Parameters on Tracer Diffusion Kinetics in High Entropy Alloys

The high entropy effect and sluggish diffusion effect postulate that the alloys with higher thermodynamic stability exhibit anomalously slow diffusion kinetics. This postulation assumed that the enthalpy of mixing and excess entropy in random single-phase solid solution based high entropy alloys (HEAs) are negligible and configurational entropy is the dominant contributor towards the overall thermodynamic stability. The highly stable HEAs were assumed to exhibit very rugged potential energy landscape characterized by larger potential energy fluctuations, which slows down the diffusion kinetics. These assumptions, however, were challenged by several investigations. Therefore, this work investigates the dependence of tracer diffusion kinetics on thermodynamic stability parameters by systematically investigating the underlying assumptions of high entropy and sluggish diffusion effects. The effect of enthalpy of mixing and excess entropy on the overall thermodynamic stability of several FCC HEAs was also examined. Findings suggest that: (1) FCC HEAs exhibit non-ideal solid solution behavior because enthalpy of mixing and excess entropy contributes significantly towards the overall thermodynamic stability; (2) contrary to sluggish diffusion effect, higher entropy do not always result in higher potential energy fluctuations; and (3) tracer diffusion in FCC alloys is composition dependent and exhibit no direct correlation with the thermodynamic stability parameters.