Gradient nanostructure induces exceptional cryogenic mechanical properties in an additively manufactured medium entropy alloy

Developing additively manufactured alloys with high cryogenic performance can facilitate the development of geometrically complex components for cryogenic applications. In this study, we have implemented ultrasonic surface rolling process (USRP) on an additively manufactured CoCrNi-based medium entropy alloy to introduce a gradient nanostructure therein. The resulting alloy exhibits a substantial strength-ductility balance at 88 K, a relatively high ductility (24.6 %), 2.07 times higher than that at 293 K, as well as a yield strength as high as 1274 MPa. The exceptional yield strength is attributed to the high-density dislocations induced by additive manufacturing and ultrasonic surface rolling process, while the origin of impressive ductility promotion lies in two aspects, i.e., enhanced hetero-deformation induced hardening effect and activated twinning/microband behaviors at matrix region at 88 K. The former mainly comes from suppressed dynamic recovery in gradient layer. The latter is resulted from the lowered stacking faulting energy and high flow stress.