In an effort to reduce grain defects in large single crystal Ni-based superalloy components, carbon is intentionally added. In this study the effect of carbon additions on the microstructure of a model Ni-based superalloy, LMSX-1, was examined. Previous results have shown that the tendency of the alloy to form all types of solidification defects decreased as the carbon content increased. The as-cast microstructures also exhibited a decrease in the amount of γ-γ' eutectic structure and an increase in the volume fraction of carbides and porosity, as the carbon content was increased. 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 appears to have prevented the thermosolutal fluid flow.
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