High-Entropy Materials : Composition and Structural Aspects

High-entropy materials have a wide range of unique properties due to their vast composition space and versatile tunable microstructures, which may overcome the performance limitation of conventional materials and open a new paradigm for the development of novel materials for cutting-edge applications. These benefits do, however, present difficulties in the design of such materials. This chapter focuses on metallic alloy systems, i.e., high-entropy alloys (HEAs). We first provide an overview of various active approaches utilized for phase selection in these alloys, including the empirical models using physiochemical parameters, the CALculation of PHAse Diagrams (CALPHAD)-based calculation, as well as machine-learning modeling. Specifically, formation rules for solid solutions and intermetallic compounds with different crystal structures are described in detail. The reliability and accuracy of each method are evaluated. The results indicated that integrating machine learning with physical-based approaches such as DFT, thermodynamic calculations, and experiments presents the most satisfactory outcomes, which helps to speed up the discovery of new HEAs. Second, we concentrated on the microstructural heterogeneities in the HEAs, including short-range order (SRO), long-range order (LRO) superlattice structures, and heterogeneous grain structures. Their influence on the mechanical properties of the HEAs is discussed. In the end, some remaining issues and future prospects with regard to phase formation rules and microstructural modulations for HEAs with desirable properties are addressed. © 2024 Anuj Kumar and Ram K. Gupta.