Carbon-containing single-crystal nickel-based superalloys: Segregation behavior and carbide formation

Carbon is added to large single-crystal nickel-based superalloy components to reduce grain defects. In this study, the effect of carbon additions on the microstructure of a model nickel-based superalloy, LMSX-1, was examined. Previous results have shown that as carbon content increased, the tendency of the alloy to form all types of solidification defects decreased. In addition, as the carbon content increased, the as-cast microstructures exhibited a decrease in the amount of γ-γ′ eutectic structure and an increase in the volume fraction of carbides and porosity. The carbides formed in these alloys were mostly script-type MC carbides which formed continuous, dendritic networks in the interdendritic region. Microprobe analysis of the as-cast structures showed that the partitioning coefficients did not change with carbon additions. Therefore, the reduction in defect formation with increasing carbon content could not be attributed to changes in segregation behavior of alloying elements. Instead, the presence of these carbides in the interdendritic regions of the alloy appeared to have prevented the thermosolutal fluid flow.