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An assessment is made of the Wagner transition criteria for predicting the formation of a continuous Al2O3 scale in Ni-based superalloys. Predictions are compared with data from an experimental Ni-based superalloy as well as commercial superalloys for which published data are available. The methodology was generally successful in predicting the transition temperature of the commercial superalloys but underpredicted the transition temperature of the experimental superalloy by approximately 50–100 °C. The difference in the transition temperature of the experimental superalloy to form a continuous Al2O3 scale is primarily attributed to a complex oxide subscale that increased the internal volume fraction of oxide and led to reduced oxygen ingress. The sensitivity and limitations of the methodology are discussed, and recommendations are made to refine the methodology to facilitate the interpretation of oxidation behaviour in polycrystalline Ni-based superalloys.

The effect of Co:Ni ratio on the oxidation performance of γ-γ' alloys based on the (Ni,Co)75Al5Ti5Cr15 system (at.%) was investigated. Seven model superalloys of Co concentration 0, 9, 19, 28, 38, 47 and 56 at.% underwent isothermal oxidation in air at 800°C using both box-furnace exposure (1000 hours) and thermogravimetric analysis (100 hours). Following 1000 hours oxidation at 800°C, alloys containing 28, 38 and 47 at.% Co exhibited a flatter, more compact external scale and Cr2O3 layer as well as reduced oxygen ingress compared with alloys of lower Co content. The alloy containing 47 at.% Co exhibited two distinct morphologies.

Degradation of the mechanical integrity of cobalt-based superalloys can occur as the carbide network is progressively oxidised during high temperature service. In this study, a heat-treatment aimed at redistributing the carbides was tested on two similar commercial Co-based superalloys, one with high C content (Co-101) and one with low C content (Stellite-21), to determine its influence on oxidation resistance. It was found that the carbide phases in the lower C-containing alloy could be solutioned more readily than the higher C-containing alloy, enabling the continuity of the carbide network to be reduced. This resulted in a reduced attack of the carbides down the interdendritic channels during oxidation testing, but increased thickness of the oxide overscale.

Degradation of the mechanical integrity of cobalt-based superalloys can occur as the carbide network is progressively oxidised during high temperature service. In this study, a heat-treatment aimed at redistributing the carbides was tested on two similar commercial Co-based superalloys, one with high C content (Co-101) and one with low C content (Stellite-21), to determine its influence on oxidation resistance. It was found that the carbide phases in the lower C-containing alloy could be solutioned more readily than the higher C-containing alloy, enabling the continuity of the carbide network to be reduced. This resulted in a reduced attack of the carbides down the interdendritic channels during oxidation testing, but increased thickness of the oxide overscale.

The isothermal oxidation behaviour of a series of quinary Ni-Co-Al-Ti-Cr alloys were studied at 800˚C. Alloys with higher Cr concentrations exhibited lower mass gain after 100-hour exposure, as did the alloys richest in Ni & Al, for a given Cr concentration. Extensive internal oxidation and nitridation was also observed in all alloys, except those containing the highest concentrations of Ni & Al. All alloys studied generated continuous chromium oxide layers, beneath which alumina particles were observed. Compositional analysis of the subscales identified shallower Cr concentration gradients in alloys containing equiatomic levels of Ni & Co, suggesting increased availability of Cr in the alloy. Thermodynamic calculations confirmed that these alloys contained higher concentrations of Cr in their γ matrices as a result of a combination of both the partitioning behaviour of the elements between phases in the alloys studied and the increased mole fraction of γ′ precipitates forming in the alloy.