Recent progress in the CoCrNi alloy system

Abstract

The exceptional mechanical properties, particularly at cryogenic temperatures, of the equiatomic CoCrNi alloy are documented in numerous published studies. Similar to the equiatomic CoCrFeMnNi (so called Cantor alloy), from which the ternary alloy was derived, the CoCrNi ternary possesses low stacking fault energy that promotes complex deformation modes, as well as the activation of deformation twinning at ambient temperatures and increased strain. In addition to outstanding deformation mechanisms, chemical short-range order and face-centered cubic (FCC)-hexagonal close packed (HCP) transitions have been verified in this alloy and prove to be key factors contributing to the alloy's notable properties. The relationship between stacking fault energy and FCC→HCP phase transitions has been developed over the years through other low stacking fault materials, but the question that arises is: do well established physical metallurgical mechanisms require modification when applied to systems such as CoCrNi given their compositional complexity? Local chemical order plays an important role in that it brings the deviation from the random solid solution behavior generally expected from complex concentrated alloys. In this review, the fundamental atomistic deformation mechanisms of the CoCrNi alloy will be reviewed with a focus on deformation substructures and chemical short-range ordering. Recent studies on microstructural engineering through thermo-mechanical processing and efforts to enhance the tensile properties of the CoCrNi derived systems with minor alloying additions are discussed. Finally, future directions of research, which involve applying current understanding of the underlying mechanisms towards alloy design strategies, are discussed.