Transgranular highly localized deformation and fracture mechanism in ductile CrFeNi medium-entropy alloys

The fracture evolution and damage mechanism of an engineering candidate, CrFeNi medium-entropy alloy (MEA), which exhibits superior tensile properties with the ultimate tensile strength of -1.03 GPa and ductility of -55%, were studied. This work highlights the effect of defect evolution, including microcrack, geometric necessary dislocation (GND) density, lattice rotation and dislocation configuration, on fracture evolution. The result reveals that the fracture mechanism of the studied alloy is dominated by the coalescence of microcracks, which initiate at a certain distance from the main crack tip. It is distinct compared to common ductile metals and is attributed to a transgranular highly localized deformation behavior which provides potential sites for microcrack initiation. The microstructural characteristics of the microcrack embryo are evaluated and clarified to be the localized damage accumulation with a high density of dislocations at the crystal-defect scale. It can result in elevated internal stress locally, providing the driving force for microcrack initiation. In addition, the mechanical model based on distributed dislocation technique (DDT) demonstrates that the transgranular highly localized deformation leads to a crack propagation acceleration effect and the deterioration of crack propagation resistance. By relating the defect evolution and fracture performance of the CrFeNi MEA, it provides new insight on the toughening design of ductile metals.