Temperature Range of Heating Rate Dependent Reactions Leading to Spinel Formation on a Ni-Based Superalloy

Abstract

Improved oxidation kinetics for a polycrystalline Ni-based superalloy used in turbine disc applications has been shown to be possible by controlling the heating rate of the first thermal exposure to 5 °C min⁻¹. The beneficial effect arises from the formation of a protective layer of NiCr₂O₄, instead of the more usually formed doped Cr₂O₃. This study shows that it was possible to form the NiCr₂O₄ at temperatures up to 725 °C, within the operational conditions for this alloy, and that at higher temperatures Cr₂O₃ formed. The improvements in alloy performance extended to the internal oxidation processes where reduced depths of degradation were observed. It is demonstrated here that Al₂O₃ formation is less thermodynamically stable when the highly protective NiCr₂O₄ oxide is present at the alloy surface compared to the doped Cr₂O₃. Synchrotron XRD was performed on samples removed during the heating stage and provided evidence of the oxidation sequence occurring, enabling refinement in the thermodynamic calculations and suggesting an additional route to the formation of the NiCr₂O₄.