Systematic ab initio-based investigation of substitutional impurity diffusion and solute-defect flux coupling in metals

Diffusion of impurities in alloys plays an important role in driving the microscopic and macroscopic evolution during manufacturing, processing, and operation. Moreover, correlations between solute atoms and point defects (vacancies, interstitials) affect the diffusion of both species. A detailed knowledge of such properties is essential to predict the behavior of alloys in a wide range of applications. For instance, in irradiated materials the radiation-induced high concentration of defects can cause a chemical redistribution in the alloy, and endanger its mechanical properties and corrosion resistance. At low temperature, it is extremely challenging to investigate solute-defect kinetic coupling and diffusion experimentally, and the existing atomistic models are often unable to accurately describe the correct coupling tendencies.

We propose in this work an innovative method to investigate low-temperature tracer diffusion in dilute alloys, here applied to vacancy-assisted diffusion of substitutional impurities in bcc metals. Based on accurate and extensive ab initio calculations of defect jump frequencies, the full set of transport (Onsager) and diffusion coefficients can be obtained, thanks to a mean-field averaging of all possible diffusion paths. Then, we present the results of a broad diffusion study of 26 transition-metal impurities in bcc iron [1]. Vacancy drag of solute atoms is found to arise systematically below a solute-dependent temperature threshold. The obtained diffusion coefficients match well with the experimental measurements, and repeating trends found for several distinct quantities reveal the effect of the electronic structure on the diffusion and coupling properties. Finally, we compare the results with other bcc (W), fcc (Al, Cu, Ni), and hcp (Mg) metals, to investigate the influence of the matrix and the crystal structure on diffusion and reveal common trends.

[1] L. Messina et al., Phys. Rev. B 93 (2016).