A systematic study of oxide passivation induced by chemical composition and microstructure regulation in high entropy alloys.

The oxidation behavior of high entropy alloys (HEAs) has garnered growing attention owing to their potential applications in extremely harsh environments. Although some efforts have been made in newly developed data science methods, bottlenecks have arisen regarding the complex effect of multi-component alloying on the oxidation behavior. To extend the prior knowledge of the physical process of oxidation, we systematically analyze the elemental composition, local chemical environment, microstructure, and thermal oxidation behavior from a theoretical perspective, along with their relationships. We reveal that the chemical short-range order (CSRO) of Cu-containing HEAs repels oxygen occupation regardless of Cr's advantageous oxygen affinity and the octahedral interstitial sites where oxygen is prone to reside, which is validly attributed to the impact of the d orbital of the principal elements. Microstructures, such as grain boundary (GB), serve as rapid pathways for oxygen ingress into the matrix and tune the chemical composition in a refined manner. We thus summarize the series of factors that play essential roles in passive quality, including microstructure design, the consequent elemental regulation, and the diversity of elements in chemical affinity and diffusion rate. These insights will provide a fundamental understanding of oxidation resistance optimization strategies in HEAs.