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

Abstract

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.