Effect of alloying elements in Ni-base substrate material on interdiffusion processes in MCrAlY-coated systems

The effect of alloying elements in the substrate on the interdiffusion processes between a NiCoCrAlY-coating and model Ni-base superalloys was studied. Four single-crystal substrates Ni-10Cr-8Al-X (wt%, X = 10Co, 9Ta, 6 W) were coated with a conventional NiCoCrAlY-coating and exposed to air for up to 1000 h at 1050 °C. The microstructural analyses (LOM/SEM/EDX) revealed a strong dependence of the coating degradation rate on the substrate composition. The coating applied to the ternary base Ni-10Cr-8Al alloy and to the quaternary Co-containing substrate degraded most rapidly while the Ta addition to the substrate effectively suppressed interdiffusion. The degradation phenomena such as β-NiAl depletion and propagation of the secondary reaction zone (SRZ) were interpreted in terms of a thermodynamical analysis in Thermo-Calc and by using a CALPHAD-based thermodynamic-kinetic diffusion model.