  High-entropy alloys are an intriguing new class of metallic materials that
derive their properties from being multi-element systems that can crystallize
as a single phase, despite containing high concentrations of five or more
elements with different crystal structures. Here we examine an equiatomic
medium-entropy alloy containing only three elements, CrCoNi, as a single-phase
face-centered cubic (fcc) solid solution, which displays strength-toughness
properties that exceed those of all high-entropy alloys and most multi-phase
alloys. At room temperature the alloy shows tensile strengths of almost 1 GPa,
failure strains of ~70%, and KJIc fracture-toughness values above 200 MPa.m1/2;
at cryogenic temperatures strength, ductility and toughness of the CrCoNi alloy
improve to strength levels above 1.3 GPa, failure strains up to 90% and KJIc
values of 275 MPa.m1/2. Such properties appear to result from continuous steady
strain hardening, which acts to suppress plastic instability, resulting from
pronounced dislocation activity and deformation-induced nano-twinning.
