Thermodynamics of High Entropy Alloys

HEAs have brought a renewed interest to the science of thermodynamics. In HEAs, compared to conventional alloys, thermodynamic properties are a new and influential trend, although the science remains the same. Thermodynamics underpins conventional alloy design and paves the way for innovative HEA design strategies. Fundamental study of the thermodynamic trends in HEAs is vital for their effective exploitation and attractive for their academic exploration. This chapter first discusses such fundamental aspects of thermodynamic properties. The most striking thermodynamic trend is the HEA design idea, the high-entropy effect, which is configurational entropy augmenting the stability of a random solid solution (SS). It is also the most debated one. The complex interplay of various entropy contributions and enthalpy of competing phases in the form of Gibbs energy is found to be the ultimate cause of phase formation. These thermodynamic factors are discussed in this chapter. The difference in the term configurational entropy used in metallic glasses literature and in HEA literature is elucidated. New terms, structural configurational entropy and chemical configurational entropy respectively, are proposed. The recent calorimetric studies on thermal entropy and its excess mixing quantities in HEAs are discussed. This chapter also includes discussions on the application of thermodynamics to HEA compositional design. Simple parametric guidelines use thermodynamic parameters extensively. These guidelines, as well as phase diagram inspection and Gibbs energy-composition (G-x) plots methods are discussed. The best use of thermodynamics comes in the form of rigorous and robust phase diagram calculations using the Calphad (CALculation of PHAse Diagram) method. The extent of its success and possible causes for discrepancies are discussed.