Sammanfattning: High-temperature coatings, such as metallic MCrAlY (M: Ni and/or Co) coatings, are widely employed to improve the oxidation resistance of superalloys. However, interdiffusion between the coatings and the superalloys under elevated service temperatures leads to microstructural degradation in both. Some of the underlying degradation mechanisms are still elusive, such as the γ' (Ni3Al) phase depletion in the superalloys, where a large amount of γ' precipitates dissolve in the γ matrix, even though the incoming Al from the coatings locally increases the Al content. In this research, we investigated the interdiffusion and oxidation behaviors in coating/superalloy systems of Amdry365 (MCrAlY)/IN792, Ta-containing (MCrAlYTa) coatings/IN792 and high-entropy-alloying (AlCrFeCoNi) coatings/IN792 at 1100 °C. Multiple microscopic techniques were employed to study microstructural evolution and chemical compositional changes. Thermodynamic calculations were used to reveal the underlying mechanisms of these changes.The experimental and simulation results reveal a dominant effect of fast-diffusion Al from the coating towards the superalloy in the early stage of phase transitions. This leads to β phase depletion on the coating side and the formation of β as well as and increment of γ' phase on the superalloy side. We propose an Al-Cr interference (ACI) effect to account for the pile-up behavior of Cr and the reduced Al content observed near the coating/superalloy interface. The incoming Al from the coating enhances the Cr potential in the superalloy, causing Cr diffusion from the interior of the superalloy to the coating/superalloy interface., Meanwhile, the pile-up of Cr increases the Al potential, which accelerates Al diffusion from this region into the interior of the superalloy. Furthermore, local-phase- equilibrium calculations reveal that γ' phase depletion in the substrate is ascribed to the loss of γ' forming elements, Ti and Ta. The incoming Al accelerates γ' phase depletion, since it enhances the potential of Ti and Ta promoting their diffusion from the superalloy side to the coating side. Adding Ta to coatings impedes γ' depletion in the substrate superalloys and also promotes the phase transition from β (NiAl) to γ' in the coatings. As for the oxidation behavior, a high Ta content in coatings, such as the coating with 7.2 wt.% Ta reduces the oxidation resistance due to the formation of excessive Ta-rich oxides. It is observed in the high entropy alloy coatings that a lower Al/Cr ratio benefits oxidation resistance as its protective scale consists of larger Al2O3 grains, reducing the number of grain boundaries that function as fast diffusion paths for oxygen.This research clarifies a confusing point: the reason for γ' phase depletion is the increase of Cr and Co related to interdiffusion. These results answer a long-standing but critical question in the coating/superalloy interdiffusion research, i.e., why the γ' precipitates dissolve in the γ matrix even though the incoming Al from coatings increases the Al content. The γ' phase depletion is ascribed to the loss of γ' forming elements, Ti and Ta, whereas the incoming Al cannot offset their effect. The diffusion simulation performed in this work still requires a long calculation time, as a large number of elements were considered in the diffusion model. According to our findings, γ' depletion is mainly dependent on the loss of Ta, Ti. Hence, a simplified prediction model for γ' depletion for fast computation can be established by considering only the diffusion of Ta, Ti and Al.