Phase stability and mechanical properties of high- and medium-entropy alloys

There is considerable interest in a relatively new class of high- and medium-entropy alloys in which multiple elements are present in equal atomic concentrations. A few of them have the striking characteristic that they can crystallize as single-phase solid solutions with simple (FCC or BCC) crystal structures. These alloys are interesting from a fundamental scientific viewpoint since textbook theories of solute strengthening break down in equiatomic alloys where there are no “solute” or “solvent” atoms in the conventional sense. Some of the alloys also have unusual mechanical properties, which make them attractive for a wide range of applications. In this talk I will first examine the role of configurational entropy and show that it is not a useful predictor of which alloys are likely to be single-phase solid solutions, contrary to what is often asserted in the literature. Then I will present results of tensile and fracture toughness tests performed on a model FCC high-entropy alloy that exhibits a remarkable combination of high strength, ductility, and fracture toughness, all of which actually improve at cryogenic temperatures. To shed light on these phenomena, a family of lower-order equiatomic alloys (having medium to low entropies) comprising the same elements as those in the high-entropy alloy were synthesized and tested. From the behavior of these alloys, we have identified possible mechanisms for the observed temperature dependencies of mechanical properties in high-and medium-entropy alloys.