Carbides in an aluminised single crystal superalloy: Tracing the source of carbon

Single crystal Ni-base superalloys contain marginal amounts of carbon and as a consequence, diffusion coatings on these superalloys are generally expected to result in carbide free interdiffusion zones (IDZ) containing intermetallic precipitates. The experimentally observed presence of carbides in the IDZ of the aluminised (CVD) single crystal SC2000 led to a thorough investigation of the source of carbon as well as the precipitate phases occurring in the coating and in the IDZ. Scanning electron microscopy (SEM) combined with energy- and wavelength-dispersive X-ray spectroscopy (EDX/WDX) was employed to characterise the aluminised superalloy specimens. Average concentration profiles were determined by glow discharge optical emission spectroscopy (GDOES). Phases were then identified by X-Ray diffraction (XRD) and electron backscatter diffraction (EBSD). The carbides in the interdiffusion zone were found to be of the type M6C/M12C. By aluminising specimens of SC2000 with different thicknesses, the available carbon reservoir for the precipitation of carbides was varied. The amount of carbides in the IDZ after coating decreased with decreasing thickness of the substrate specimen. Hence it could be confirmed that carbon indeed diffused from the substrate to the coating/substrate interface and did not originate from the service environment.