Mechanism of primary MC carbide decomposition in Ni-base superalloys

Three industrial gas turbine blades made of conventionally cast (CC) IN-738 and GTD-111 and directionally solidified GTD-111 Ni-base superalloys were examined after long-term exposures in service environments. All three blades exhibit similar, service-induced microstructural changes (MCs) including γ' coarsening and coalescence, excessive secondary M₂₃C₆ precipitation, and primary MC degeneration, regardless of the chemical composition and the grain size. Special attention was paid to the primary MC decomposition. It is shown that the primary MC decomposition occurs by carbon diffusion out of the carbide into the γ + γ' matrix, resulting in the formation of Cr-rich M₂₃C₆ carbides near the initial carbide/matrix interface. A transition zone is shown to develop between the original MC core and its perimeter, demonstrating the gradual outward diffusion of carbon and a slight inward increase in nickel concentration. The hexagonal Ni₃(TiTa) η-phase was also found in the MC transition zone and on the MC-γ/γ' interface. The primary MC decomposition can be expressed by the reaction MC + γ/γ' → M₂₃C₆ + η. Finally, it is shown that the grain-boundary (GB) MC decomposes more rapidly than that in the grain interiors. This is consistent with the more rapid GB diffusion that leads to the acceleration of the MC diffusional decomposition processes.