High-Entropy Alloys and Related Compositionally Complex Alloys: Design Principles, Structures, Processing, and Applications

High-entropy alloys (HEAs), complex concentrated alloys (CCAs), medium-entropy alloys (MEAs), and multi-principal element alloys (MPEAs) are compositionally complex material classes in which design is based on multiple principal elements present in substantial, often comparable, fractions rather than on a single dominant constituent. In this Entry, the discussion focuses primarily on metallic alloy systems within this broader framework. HEAs are commonly described as alloys containing five or more principal elements and relatively high ideal configurational entropy; CCAs represent a broader, phase-agnostic category of chemically complex alloys in which multiple elements occupy significant fractions and can form solid solutions, ordered phases, intermetallic compounds, or multiphase microstructures; MEAs generally refer to systems containing fewer principal elements or lower configurational entropy than typical HEAs; and MPEAs describe alloys designed around multiple principal elements without requiring a specific entropy threshold. These terms overlap and are not universally hierarchical, and their use depends on composition, phase constitution, and research context. The large compositional space enabled by these materials provides opportunities for designing face-centered cubic, body-centered cubic, hexagonal close-packed, ordered, eutectic, refractory, interstitially alloyed, and precipitation-strengthened alloy systems. Their properties are governed by the complex interactions among composition, phase stability, chemical ordering, processing route, microstructure, and service environment. This Entry summarizes the fundamental concepts, design approaches, structural characteristics, processing methods, modeling strategies, properties, candidate engineering applications, limitations, and future perspectives of HEAs and related compositionally complex alloys.