Ni diffusion in the equiatomic CoFeMnNiV high-entropy alloy: Unexpected impact of second-phase precipitation on bulk diffusion rates

Ni diffusion in the equiatomic CoFeMnNiV high-entropy alloy is measured over an extended temperature range that covers both single-phase (face-centered cubic, FCC, solid solution) as well as two-phase (FCC solid solution + σ -phase) states. In addition, Ni diffusion experiments were performed on a single-phase σ -alloy. The σ -phase precipitation is found to affect significantly the self-diffusion kinetics in the FCC solid solution. The measured tracer diffusion coefficients follow an Arrhenius-type relationship when estimated in the single-phase solid solution, and a characteristic enhancement of the Ni diffusivity is observed at lower temperatures when the σ -phase precipitates during the diffusion annealing treatments. The enhancement is significantly suppressed when the alloy is pre-aged to approach a nearly equilibrium σ -phase volume fraction at a given temperature. Since Ni volume diffusion in the σ -phase was found to be similar to that in the FCC equiatomic counterpart, the observed diffusion enhancement is attributed to an excess vacancy concentration, generated during the growth of σ -precipitates. This excess gets eliminated when approaching thermodynamic equilibrium, i.e., when precipitation ends. A simple model of the diffusion enhancement during precipitation is proposed.