The Role of Vacancies in Oxide Detachment and Mechanical Degradation of Subsurface for AlCoCrFeNi Alloy at High Temperature

The protective function of the oxide scale and the vulnerability of the subsurface region play key roles in determining the high-temperature durability of AlCoCrFeNi high-entropy alloys (HEAs), yet their degradation mechanisms remain insufficiently understood. This study investigates the mechanisms of oxide scale spallation and subsurface mechanical degradation in the AlCoCrFeNi HEA after high-temperature oxidation at 1000 °C. Severe oxide scale spallation is not solely caused by residual stress but is closely linked to vacancy coalescence at the oxide/alloy interface, primarily induced by outward Al diffusion and the Kirkendall effect. A bilayer Al₂O₃ scale (≈8.4 μm thick after 200 h oxidation) forms, consisting of equiaxed and columnar grains. Moreover, the subsurface region undergoes a body-centered cubic (BCC)-to- face-centered cubic (FCC) phase transformation, which reduces the diffusion coefficient of Al (from 4.17 × 10⁻¹¹ m² s⁻¹¹ in BCC to 1.8 × 10⁻¹¹ m² s⁻¹ in FCC), thereby enhancing oxidation resistance. Meanwhile, nanomechanical testing reveals an ≈30% reduction in yield strength in the subsurface layer, attributed to vacancy-induced reductions in stacking fault energy, which promote dislocation activity and plastic deformation. This study provides critical insights into the coupled effects of oxidation, vacancy dynamics, and phase transformation on the high-temperature performance of HEAs, offering valuable guidance for their application in extreme environments.