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

• Published in: Surface & coatings technology Vol. 288; pp. 15 - 24
• Main Authors: Pillai, R., Chyrkin, A., Grüner, D., Nowak, W., Zheng, N., Kliewe, A., Quadakkers, W.J.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 25.02.2016
• Subjects: Aluminide coatings; Carbides; Carbon; Coatings; GDOES; Interdiffusion; Interdiffusion zone; Precipitates; Precipitation; Single crystal superalloy; Single crystals; Superalloys; GDOES; Single crystal superalloy; Aluminide coatings; Interdiffusion zone; Carbides
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2015.12.066

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. •Phases observed in the aluminised single crystal Ni-base superalloy SC2000 were reported.•Carbides (M6C & M12C) and μ-phase were identified in the interdiffusion zone (IDZ).•Carbon diffusion from the substrate core to the surface during aluminising was experimentally verified.•Thermodynamic calculations are consistent with the experimental results.