Face-centered-cubic (fcc) medium/high-entropy alloys (HEAs) can display exceptional strength, ductility and fracture toughness, properties that are further enhanced at cryogenic temperatures. Body-centered-cubic (bcc) refractory RHEAs, conversely, can display exceptional strength and compressive ductility at elevated temperatures, but are often compromised by poor lower-temperature behavior. We examine the damage-tolerance of these HEAs, and show that whereas some single-phase fcc HEAs exhibit the highest toughnesses on record, even at ultrahigh strain-rates and liquid-helium temperatures, bcc HEAs generally display extremely low ductility and toughness under tensile loads. However, the NbTaTiHf single-phase RHEA has remarkable toughness at cryogenic to elevated temperatures. With KIc values >250MPam at room temperature (with ~600MPa yield strengths, ~16% ductilities), this alloy exhibits ~90MPam toughnesses both at 77K and 1473K, which is highly unusual for bcc materials. The reasons for such properties are explored to investigate why most bcc RHEAs are brittle in tension, whereas some can display outstanding toughness.
*funded by the Department of Energy, Office of Science (Basic Energy Sciences), Materials Sciences and Engineering Division.