Effects of Co and C on the Microstructure and Aging Stability of a New Type of Hot-Corrosion-Resistant Nickel-Based Single-Crystal Superalloy

Abstract:

This study investigates the effects of Co and C on the microstructural characteristics and the stability during long-term aging at 1000°C in a novel hot-corrosion resistant Ni-based single-crystal superalloy. Four single-crystal alloys with varying Co and C contents were prepared via directional solidification and characterized by SEM, EDS, EPMA, and TEM. The results show that C suppresses solidification micropores and reduces γ/γ′ eutectic fraction by promoting the precipitation of MC-type carbides, such as TaC. Co enhances the solid solubility of the γ-matrix, effectively inhibiting the segregation of Re, W, Ta, and Al, although carbides reduce the homogenization efficiency. Co lowers the γ′ solvus temperature and refines the size of γ′ precipitates, while C increases the solvus temperature and promotes γ′ coarsening due to the release of γ′-forming elements during carbide dissolution. During long-term aging, γ′ coarsening follows Lifshitz-Slyozov-Wagner (LSW) kinetics. Both Co and C reduce the absolute value of the γ/γ′ lattice misfit (|δ|) and increase the effective diffusion coefficient (Deff). Co reduces the γ′ coarsening rates, while C accelerates it. The misfit (|δ|) dominates coarsening at fixed C content, whereas Deff governs it at fixed Co content. Predictions from three models for topologically close-packed (TCP) phase precipitation show discrepancies with experimental data: Co promotes TCP formation, while C inhibits it through sequestration of Mo and W in primary MC carbides.