Effect of carbon content on solidification behavior of a Mo- and Al-rich nickel-based single-crystal superalloy

• Carbon additions (0–270 ppm) decrease the solidus temperature while keeping the liquidus nearly unchanged, widening the mushy-zone range. • Carbon systematically shifts interdendritic constituents from σ-NiMoRe dominance to Mo-rich M 6 C predominance in a Mo–Al-rich single-crystal superalloy. • Scheil calculations link terminal precipitation of M 6 C-type carbide and the evolution of tantalum and molybdenum in the residual liquid to the tantalum incorporation observed in carbides of the highest-carbon alloy. • Increased secondary dendrite arm spacing and reduced solidification shrinkage coefficient jointly lower shrinkage-driven pressure drop, suppressing microporosity with higher carbon. Optimizing carbon content is critical for the design of Ni-based single-crystal superalloys, particularly for high-temperature applications. This study systematically investigated the influence of carbon on the solidification behavior and phase evolution of a Mo- and Al-rich Ni-based single-crystal superalloy. Increasing carbon content lowered the solidus temperature and widened the mushy zone, which is associated with the precipitation of low-melting-point carbides during the terminal stage of solidification. At 56 ppm carbon, no carbides were observed and the interdendritic regions contained the σ-NiMoRe phase with a tetragonal structure. At 180 ppm carbon, σ-NiMoRe and blocky primary M 6 C carbides coexisted adjacent to the large primary blocky γ′ phase, separated by distinct boundaries. At 270 ppm carbon, script-like, Mo- and Ta-enriched M 6 C carbides with a cubic (fcc-type) structure became the dominant interdendritic constituent; concurrently, elemental segregation became more pronounced. Increasing carbon content also increased the secondary dendrite arm spacing (SDAS) and reduced both the liquid viscosity ( μ ) and solidification shrinkage coefficient ( β ). These effects contributed to significant porosity reduction by decreasing the shrinkage pressure drop. Overall, controlled carbon addition optimizes solidification behavior and phase stability, providing practical guidance for alloy design and solidification control in Mo- and Al-rich Ni-based single-crystal superalloys.