Mechanistic Understanding of Speciated Oxide Growth in High Entropy Alloys

Abstract

Complex multi-element alloys are gaining prominence for structural applications, supplementing steels, and superalloys. Understanding the impact of each element on alloy surfaces due to oxidation is vital in maintaining material integrity. This study investigates oxidation mechanisms in these alloys using a model five-element equiatomic CoCrFeNiMn alloy, in a controlled oxygen environment. The oxidation-induced surface changes correlate with each element's interactive tendencies with the environment, guided by thermodynamics. Initial oxidation stages follow atomic size and redox potential, with the latter becoming dominant over time, causing composition inversion. The study employs in-situ atom probe tomography, transmission electron microscopy, and X-ray absorption near-edge structure techniques to elucidate the oxidation process and surface oxide structure evolution. Our findings deconvolute the mechanism for compositional and structural changes in the oxide film and will pave the way for a predictive design of complex alloys with improved resistance to oxidation under extreme conditions.

Predicting Emergence of Nanoscale Order in Surface Oxides through Preferential Interactivity Parameter

“…Domination by Mn follows our thermodynamic prediction from PIP as PIP Mn < PIP Cr . Similar systems albeit under different oxidizing condition have also revealed the presence of a small Fe oxide island on the surface layer, , indicating that within this solid system at an extended stage, slow oxidizing elements have the probability to diffuse through defects akin to TOCI (Figure c). It is therefore important that one does not apply PIP, a thermodynamic parameter, without consideration of kinetics, especially under the presence of extended reaction times …”