Processing, Microstructural Evolution and Engineering Performance of High-Entropy Alloys: A Review

High-entropy alloys (HEAs) and multi-principal-element alloys (MPEAs) provide broad compositional flexibility for regulating phase stability, microstructure, and properties. However, nominal composition and average phase constitution alone are insufficient to describe the actual material state formed during processing and service. This review summarizes the thermodynamic and diffusion-kinetic basis of phase formation and compares five representative fabrication routes, including mechanical alloying, vacuum melting, severe plastic deformation, magnetron sputtering, and additive manufacturing. Particular attention is given to the effects of processing history on grain structure, texture, elemental segregation, defects, phase constitution, and local chemical order. Computational methods and multiscale characterization techniques are also discussed in relation to the identification and interpretation of processing-dependent material states. Current studies indicate that alloys with identical nominal compositions can exhibit different microstructures and properties because of differences in thermal history, strain path, elemental redistribution, defect populations, and post-processing conditions. The review further examines strength and ductility, corrosion resistance, oxidation resistance, irradiation tolerance, and catalytic performance, with emphasis on the evolution of microstructure and surface state under service conditions. These results indicate that reliable evaluation of HEAs and MPEAs requires consideration of processing reproducibility, structural heterogeneity, and long-term stability rather than isolated peak properties. This processing-structure-service perspective provides a basis for more reliable comparison, selection, and engineering assessment of HEAs and MPEAs under application-relevant conditions. Future research should focus on reproducible fabrication, integration of computational prediction with experimental validation, multiscale assessment of structural evolution, long-term service performance, scalable processing, and sustainable alloy design.