Diffusion of multi-principal elements through stable Cr[formula omitted]O[formula omitted] and Al[formula omitted]O[formula omitted] scales

Abstract

Understanding the oxidation mechanisms in multi-principal elements and high-entropy alloys (HEAs) is critical for their potential applications in high-temperature oxidative environmnts. In addition to the compositional complexity, the counter-diffusion of cations and anions through the oxide contributes to the growth of the oxide scales in these materials. We examine the cationic and anionic diffusion through the stable chromium and aluminum oxides that form in a model HEA using atomistic simulations. In accord with experiments, we find that the tracer cations diffuse faster than the native cations through the oxide scales at high temperature (1000 K to 2000 K) and the dynamics are directly correlated to the respective migration energies of the diffusion pathways. The oxide scale growth is strongly influenced by the presence of tracer/impurity elements in alumina forming alloys relative to those that predominantly form chromia. A geometric analysis of the vacancy-induced diffusion paths for the cation migration relative to the location of the oxygen atoms reveals the influence of the latter on the preferential diffusion pathway, resulting in anisotropy. The predictions offer insights on the diffusion characteristics in the oxide scales formed in HEAs and aid in our understanding of the oxide growth kinetics.