Effect of minor alloying additions on the solidification of single-crystal Ni-base superalloys

Grain boundary elements, such as carbon and boron, were initially removed from single-crystal alloys, but more recently they have been added back into single-crystal alloys for increased castability and defect tolerance. The mechanism(s) for the increased castability is not completely understood. In this study, carbon was added to the second generation, single crystal, Ni-base superalloy and CMSX-4, to form MC-type carbides. Then either nitrogen or boron was added to the carbon-containing alloy to alter the carbide morphology and the castability of the alloy. The segregation coefficients of the alloying elements in CMSX-4 were measured with varying techniques, but no changes in partitioning were observed. The addition of carbon to CMSX-4 did reduce the number of solidification defects observed in the samples. The addition of carbon and boron to CMSX-4 did not significantly change the number of solidification defects compared to the carbon-only alloy. However, the addition of nitrogen and carbon resulted in an increase in the number of solidification defects in the CMSX-4 samples compared to the baseline CMSX-4 and carbon-containing CMSX-4 samples. The effects of these alloy additions on the carbide morphology and solidification defects are discussed.