Overcoming the strength-ductility trade-off via the formation of nanoscale Cr-rich precipitates in an ultrafine-grained FCC CrFeNi medium entropy alloy matrix

FCC high- and medium-entropy alloys (HEAs and MEAs) have demonstrated high ductility and fracture toughness, but suffer from low strength. To overcome such strength-ductility trade-off, here, we present a strategy via the formation of a high density of nanoscale precipitates in an ultrafine-grained (UFG) FCC matrix. To realize this concept, we selected a cost-effective equiatomic CrFeNi MEA as our model system. The equimolar elemental powder mixture was first forced into the formation of a nanostructured supersaturated FCC solid solution, followed by densification via spark plasma sintering (SPS). During SPS, a high density of nanoscale Cr-rich precipitates were formed in the UFG FCC matrix (821 nm). Such a particular microstructure enabled the alloy to overcome the strength-ductility trade-off, with a high tensile strength of 826 MPa and elongation of 26%. Grain boundary strengthening and precipitation strengthening were found to be the main strengthening mechanisms. These results provide deep insight into the design of novel multi-principal element alloys with high strength and ductility for structural applications.