Compositional-structural inhomogeneity and multi-layered oxide film formation on high-entropy alloys

Abstract

The wide compositional range and diverse elemental combinations in high-entropy alloys give rise to complex oxide scales, posing significant challenges in understanding the mechanisms governing local compositional and structural evolution during oxidation. This work investigates the mechanisms underlying the formation of the multi-layered oxide scale on the AlCoFeNiTi high-entropy alloys. The results show that the thermodynamic predominance for the formation of protective Al₂O₃ and TiO₂, coupled with the insufficient conditions for the establishment of a continuous oxide film, results in the development of a mixed inner oxide layer. This mixed layer facilitates the outward diffusion of alloying elements. Incomplete filling of the cavities at the forefront of the inner oxide layer by oxide growth leads to partial backfilling of Ni, thereby creating a localized Ni-rich layer. The formation of multiple oxide layers is governed by the interplay of thermodynamic driving force, elemental diffusivities, and the homogeneity of oxygen distribution. Additionally, kinetically captured solute atoms can alter the formation energy of oxides, influencing the spatial arrangement of different phases within the scale. The enrichment of Ni and Fe—driven by extended structural defects in the oxide scale—further leads to the precipitation of NiO_x and FeO_x clusters within the TiO₂ lattice. These insights provide a broader understanding of multi-layer oxide formation in Ti-containing high-entropy alloys and other alloys incorporating 3d transition metals, contributing to the design of oxidation-resistant high-entropy materials.