Recent progress in high-entropy alloys

The thirty or so presently used common alloy systems are typically based on one or, at most, two elements. It has been considered that alloys consisting of a greater number of principal elements will form complicated and brittle microstructures, and hence research regarding such multi-principal-element alloys has received very limited attention. It was suggested by Yeh in 1995, however, that alloy systems with five or more metallic elements will in fact possess higher mixing entropies, and therefore favour the formation of multielement solid-solution phases, as opposed to the inferred complex structures consisting of many intermetallic compounds. Indeed, work over the past decade into such multi-principal-element alloys, which have fittingly been coined as "high-entropy alloys" (HE alloys), has found them to form simple phases with nanocrystalline and even amorphous structures. In addition to the high-entropy effect, these structural characteristics are ascribed to the large lattice distortion and sluggish diffusion of such multielemental mixtures. Dependant upon the composition and/or processing route, HE alloys have been found to possess a wide spectrum of microstructures and properties. The superior properties exhibited by HE alloys over conventional alloys, along with the huge number of possible compositions that may give new phenomena and functional uses, makes these proposed HE alloys of great interest not only from a research perspective but also in numerous industrial applications. Therefore, this paper reviews the recent progress made on HE alloys, and possible future directions.