The morphology evolution of vapour phase aluminide coating on aCMSX4 single crystal superalloy

The effects of diffusion temperature and time on the mechanism of the surface layers formation during low activity vapour phase aluminizing process on a Ni-base superalloy CMSX4 single crystal was experimentally investigated. The coating morphology of as aluminized samples grown along the substrate [100] and [110] directions of the substrate crystal was analysed by means of SEM, EDS and EBSD techniques. The presence of three layers, where β-NiAl differently combines to γ’, γ and TCP phases, was always observed. The β-NiAl phase in particular displayed two different morphologies and textures depending on the interface movement mechanism generating this phase at its external/internal interface. The presence of secondary precipitated phases within the coating layer, their composition and morphology, helped to understand the coating development process. The surface layer formation mechanism was found to be slightly temperature-dependent.