The effect of carbon and silicon variation on the oxidation resistance of a cobalt-based superalloy

Understanding the oxidation behaviour of Co-based superalloys is crucial for the deployment of such alloys in high-temperature structural applications. This study explores the oxidation behaviour of four cast Co-based superalloys with varying C (0.25 or 0.5 wt%) and Si (1 or 4 wt%) contents at 800°C, 1000°C, and 1200°C for up to 100 h. Alloys with higher Si content generally exhibited lower isothermal mass gains than the low-Si variants, although the benefit depended on temperature and was accompanied by differences in the scale loss during cooling. At 800°C, an oxidation-associated Laves phase formed within surface-breaking interdendritic oxidation channels in the high-Si alloys (associated with M 12 C), consistent with reduced short-circuit transport along these pathways and the lower measured rates of isothermal mass gain. At 1000°C, Laves formation persisted but occurred as coarser particles and did not produce a measurable separation in mass gain. Notably, the high-Si alloys exhibited increased oxide spallation during cooling from this temperature. At 1200°C, the high-Si alloys developed a more continuous silica subscale at the alloy-oxide interface, consistent with a modest reduction in isothermal mass gain relative to the low-Si alloys. For alloys with equivalent Si content, reduced C improved mass-gain behaviour at 800°C and 1200°C, consistent with a reduced extent of interdendritic network (greater interdendritic spacing), whereas at 1000°C all alloys exhibited broadly similar mass gains. These findings demonstrate that Si and C influence oxidation through coupled effects on scale constitution, microstructurally controlled transport and oxide-scale integrity, providing guidance for the design of next-generation high-temperature alloys. • 4 wt% Si alloys showed superior oxidation resistance to 1 wt% Si counterparts • Oxidation at 800°C formed Laves phase; oxidation at 1200°C gave a silica sub-scale • Quantified oxide scale thickness at 800°C supports the proposed mechanism • 4 wt% Si alloys showed shallower oxidised channel depths at all test temperatures