8 - Special subgroups of high-entropy alloys

Abstract

While a number of equiatomic and nonequiatomic high-entropy alloys (HEAs) have shown a single solid solution, in a number of them, various intermediate phases with the structure of intermetallic compounds such as B2, L12, sigma phase, and Laves phase are shown to occur. This has been attributed by many researchers to the competition among mixing entropy, mixing enthalpy, and atomic size difference between the constituent elements in these alloys. The HEAs have opened up the massive composition space for alloy design and development. It is therefore instructive to systematically categorize the HEAs based on the constituent elements. The major classes include transition metal HEAs, refractory HEAs, and HE-bulk metallic glasses. The present chapter gives an account of intermetallic phases, interstitial alloys, and amorphous phases observed in HEAs. The transitional metal HEAs are the largest group that show a wide range of chemical constituents, microstructures, and properties and can be further categorized into different subclasses. Refractory HEAs consist of high-melting-point refractory elements and are mainly targeted for the high-temperature applications. Emerging HEAs such as light HEAs present new challenges and opportunities for developing breakthrough materials for critical applications.