Oxidation of nickel-based single-crystal superalloys for industrial gas turbine applications

The oxidation resistance of three prototype single-crystal nickel-based superalloys for industrial (electricity-generating) gas turbine applications is studied. All contain greater quantities of Cr than in most existing single-crystal superalloys; two are alloyed with Si. All alloys are found to be marginal Al2O3-formers, with the performance being better at 1000 °C rather than 900 °C, and when Si is added. Microstructural analysis indicates that the ability to form an Al2O3 layer is better in the interdendritic regions; the dendritic regions are prone to internal oxidation. In all cases, an outer scale of Cr2O3 is formed which is in contact with either Ta2O5 (at 1000 °C) or NiTa2O6 (at 900 °C). To explain the results, the factors known to influence the rate of Al2O3 scale formation are considered. A model is developed to predict whether any given alloy composition will form a continuous Al2O3 scale. This is used to rationalize the dependence of Al2O3 scale formation on alloy composition in these systems. It is useful for the purposes of alloy design.