Retarded relaxation of grain boundaries’ non-equilibrium state in medium-entropy CoFeNi alloy probed by radiotracer diffusion

Grain boundary diffusion of Co in a high-pressure torsion-processed equiatomic CoFeNi medium-entropy alloy was investigated using radiotracer technique. The results revealed a retarded relaxation of deformation-induced, non-equilibrium state of grain boundaries. With increasing temperature, the formation and subsequent dissolution of the BCC phase at grain boundaries were observed, while atom probe tomography revealed nanoscale phase separation into Ni-rich FCC and FeCo-rich BCC regions. The combined effects of segregation and chemical complexity are proposed to stabilize the deformation-induced non-equilibrium grain boundary state. High-energy/high-diffusivity grain boundaries are found to survive even after high-temperature annealing treatments due to the chemical complexity of the medium-entropy alloy.