Strength, Ductility and Toughness in bcc vs. fcc High-Entropy Alloys

Face-centered cubic (<i>fcc</i>) medium- and high-entropy alloys (HEAs) can display exceptional combinations of strength, tensile ductility and fracture toughness, properties that can be further enhanced at cryogenic temperatures. Body-centered cubic (<i>bcc</i>) refractory RHEAs, conversely, can display exceptional strength and compressive ductility at elevated temperatures, but are often compromised by poor lower-temperature behavior. We examine here the strength, ductility and toughness of these two classes of multiple principal-element alloys, and show that whereas some of the <i>fcc</i> HEAs exhibit among highest toughness values on record, even under extreme conditions of ultrahigh strain rates and liquid-helium temperatures, the <i>bcc</i> HEAs can display extremely low toughness when subject to <i>tensile</i> loading. We explore the reasons for such distinct properties, and investigate why most <i>bcc</i> HEAs are markedly brittle, whereas a few, such as NbTaTiHf, can display remarkably high toughness over an extremely wide range of temperatures.