High Entropy Alloys

 “High Entropy Alloys” are characterized as alloys consisting of roughly equal concentrations of at least five metallic elements and are claimed to favor close-packed, disordered structures due to high configurational entropy. Such crystal structures, e.g. face-centered cubic (FCC), hexagonal close-packed (HCP) and body-centered cubic (BCC), are advantageous in that they should offer multiple active slip systems usually observed in ductile metals and alloys. This opens the door to a large number of rich chemistries which would otherwise contain unacceptable volume fractions of intermetallic compounds to be useful in structural applications.

Despite thermodynamic arguments for entropic stabilization of simple, disordered phases, the high entropy alloys studied to date are typically combinations of elements with extensive solid solubility. For example, many investigated alloys are based on a cast CoCrFeNiX type base chemistry, where X = Al, Cu, Mo or Ti. Isolated research in other high-solubility systems, such as TaNbHfZrTi, has also been conducted. In the majority of papers, FCC and/or BCC crystal structures have been observed to predominate. In some cases, one compound-forming element, such as Al, is selectively added and appears to destabilize the FCC phase. Hence, the question the degree of influence of configurational entropy on solid solubility persists, and systematic studies comparing enthalpy (and compound formation) versus entropy stabilization (and disordered, crystalline solid solutions) have not been reported. Another issue is the dearth of reported tensile property data, which suggests there may be limited true ductility in reported chemistries. This could be discouraging for the development of future structural alloys. However, whether or not entropy plays a significant role in phase selection, the richness of the alloy design space and the breadth of microstructures that have so far been produced appear to offer possibilities for new discoveries and much to consider.

This presentation will review high entropy research to date and offer commentary on potential directions and applications, and also dicsuss some standing questions on the topic.