Structural damage and phase stability of cobalt-free FeCrNi medium-entropy alloy under high-fluence ion irradiation

A cobalt-free FeCrNi MEA was successfully synthesized and irradiated with 7.5 MeV Au ions at room temperature over a wide fluence from 5 x 1015 to 5 x 1016 Au ions/cm2. Microstructural characterization shows that the FeCrNi MEA exhibits low structural damage and high phase stability under high-fluence ion irradiation, and diffuse dislocations and defect clusters, especially dislocation loops and stacking-faults (SFs), are the main microstructural feature after irradiation. Limited elemental segregation at grain-boundaries and nanoscale Au clusters can be observed only in the specimen irradiated at the highest fluence. Meanwhile, void formation and phase instability are absent in any irradiation condition. Cascade-collision simulation reveals that large-size vacancy cluster collapses into the stacking fault tetrahedrons (SFTs) and abundant dislocation structures, especially the high-fraction movable Shockley dislocations at the high-energy ion irradiation, contributing to the absence of voids and the easily activated dislocation networks. Owing to these microstructural features, the irradiated specimens only exhibit a slight hardness increase (26 % at 210 dpa), indicating a superior resistance to irradiation hardening. Overall, this work supports that the FeCrNi MEA possesses an outstanding irradiation tolerance especially under high-fluence ion irradiation, thereby having good application prospects in the field of advanced nuclear reactors.