Disentangling diffusion heterogeneity in high-entropy alloys

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 atomicscale in the recently advanced high-entropy alloys, which are of unique structural features with multiprincipal 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.(c) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

Diffusion in high-entropy alloys, which have unique structural features with randomly distributed multiprincipal elements, shows dynamic heterogeneity due to the local chemical heterogeneity induced by the random distribution of the elements. By combining molecular statics, molecular dynamics, and saddle-point sampling, we uncover and decouple the dynamic heterogeneity and reveal a wide distribution of vacancy formation energies and migration energies. We propose models that define a possible range of activation energies for lattice diffusion in high-entropy alloys, which are comparable to experimental data. We also argue that the conventional hypothesis of diffusion activation energy becomes intractable in high-entropy alloys. The atomic-scale insights into diffusion heterogeneity highlight the complexity of diffusion pathways in high-entropy alloys.