MICROSTRUCTURAL DEVELOPMENT IN ALUMINIDE DIFFUSION COATINGS ON NICKEL-BASE SUPERALLOY SINGLE-CRYSTALS

An investigation has been made of the microstructural stability of aluminide diffusion coatings during post-coating thermal exposure. This study has employed edge-on transmission electron microscopy to examine high-activity pack aluminised single crystals of a gamma prime strengthened nickel-base superalloy. The influence of exposure temperature, duration and atmosphere as well as the initial coating thickness has been assessed. Two major processes have been found to contribute to microstructural changes in the coating. These are, firstly, the transformation of the coating matrix (beta-phase, nominally NiAl) to other Ni-Al based phases, especially gamma'(nominally Ni3(Al, Ti)) and, secondly, the precipitation of chromium containing phases. The work has enabled the roles of three processes contributing to gamma' formation, namely: oxidation of the coating surface, interdiffusion with the substrate and ageing of the coating, to be understood. In addition, the factors leading to the formation of a sequence of chromium-containing phases have been identified.