Dissolution behavior of intragranular M23C6 carbide in 617B Ni-base superalloy during long-term aging

The microstructure evolution of 617B superalloy especially the dissolution behavior of intragranular M₂₃C₆ carbide has been investigated after aging at 700 °C and 750 °C up to 12000 h utilizing a combination of field emission scanning electron microscope (FESEM), electronic probe microanalysis (EPMA), transmission electron microscope (TEM), and precise chemical analysis. During long-term aging process, the γ′ phase precipitates and coarsens gradually. The M₂₃C₆ carbide distributes on the grain boundaries as well as within the grains, which performs different evolution behavior. The M₂₃C₆ carbide on the grain boundaries coarsens to films and a γ′ phase layer forms at the M₂₃C₆/γ interface. However, the M₂₃C₆ carbide within the grains dissolves partially into the matrix resulting in different morphologies of phase boundary. Furthermore, mutual diffusion relationship of alloying element among γ′ phase, M₂₃C₆ carbide (both on the grain boundaries and within the grains) and the γ matrix during long-term aging process has been established based on precise micro-chemical analysis.