Atomistic Insights into Initial Oxidation and Mechanical Degradation of FeCoNiCrAl High-Entropy Alloy

In this study, a charge-variable potential capable of describing both metallic and ionic bonding was developed for the Fe/Co/Ni/Cr/Al/O system. On the basis of this potential, reactive molecular dynamics (MD) simulations were conducted to analyze the influences of lattice orientation and nanoscale chemical inhomogeneity (chemical short-range order (CSRO) and surface segregation). Oxidation is primarily driven by inward oxygen diffusion, exhibiting a transition from the growth of discrete islands to the thickening of a continuous film. This transition in oxide growth is suppressed by chemical inhomogeneity. Further analyses of oxidation pathways indicate preferential oxidation of Al, Cr, and Fe. The specific kinetic behavior and structural evolution can be attributed to the diffusion of metallic species, i.e., the outward transport of Al and the inward expulsion of Ni and Co. Moreover, ionic diffusion during oxidation causes not only internal stress at the GPa level but also point defect accumulation within subsurface regions, which are identified as potential failure locations. A theoretical model is established to describe the evolution of the growth stress. Finally, uniaxial tensile and compressive loading of HEA nanopillars further discloses the correlation between mechanical degradation and diffusion-induced subsurface evolution. These findings provide novel insights into the design and performance optimization of oxidation-resistant HEAs.