Microstructural characterization of the M23C6 carbide in a long-term aged Ni-based superalloy

Abstract

The microstructural features of M23C6 carbide in a long-term aged heat- and corrosion-resistant Ni-based superalloy have been investigated in detail using various kinds of transmission electron microscope (TEM) techniques. It is found that TEM contrast, which is related to structural and chemical inhomogeneities inside the grains, always exists in the interior of grains in the alloy. The structure of these inhomogeneous regions has been determined to be the same as that of the γ′ and t-M23C6 phases, where t-M23C6 indicates a transitional and metastable phase. Although possessing the same structure as the M23C6 phase, the chemical composition of the t-M23C6 is different from that of the M23C6 phase. Compared with M23C6, t-M23C6 is richer in Ni, Co, Al and Ti but poorer in W, Mo and Cr. This phenomenon of structural and chemical inhomogeneity demonstrates that pristine M23C6 carbide (p-M23C6) precipitated in standard heat-treated samples is unstable. Therefore, upon long-term ageing treatment, Ni, Co, Al and Ti may locally enrich inside the p-M23C6 phase, finally forming the γ′ phase, which can be described by the decomposition reaction p-M23C6 → M23C6 + γ′.