Disentangling diffusion heterogeneity in high-entropy alloys

Abstract Diffusion in the traditional single-crystalline solids is usually dynamically homogeneous characterized by a single-value or two characteristic activation energies. However, such a scenario breaks down at atomic-scale in the recently advanced high-entropy alloys, which are of unique structural features with multi-principal elements randomly occupying on lattice sites that induces strikingly local chemical heterogeneity. Here we uncover and decouple the possible dynamic heterogeneity accommodating the lattice diffusion in an archetypical high-entropy Cantor alloy CoCrFeMnNi via combined molecular statics, molecular dynamics, and a saddle-point sampling method. Wide distribution of vacancy formation energies and migration energies are revealed. We propose a single-vacancy and a vacancy-saturated model, respectively, to set up possible lower bound and upper bound of diffusivities. The models define a possible range of activation energies for the lattice diffusion in high-entropy alloys, which are comparable to experimental data. Finally, we argue that the conventional hypothesis of diffusion activation energy estimated from Arrhenius equation as the sum of the vacancy formation energy and migration energy becomes intractable in high-entropy alloys. These atomic-scale insights into diffusion heterogeneity, in contrast to the classical theory of homogeneous diffusion in conventional solid solutions, highlight the complexity of diffusion pathways and the intimate correlation between chemical, topological disorder and dynamic heterogeneity in the generic complex concentrated alloys.