Implications of Diffusion on the Composition and Microstructures of Platinum Modified Aluminide Coatings on CMSX-4 Single Crystal Superalloy
State-of-the-art aluminide coatings for high pressure aeroturbines rely on their chemical stability through the addition of platinum. In this work, the influence of diffusion on the composition and microstructure of Pt/Al coatings is investigated upon the different processing steps. Upon heat treatment of the electrodeposited Pt layers, the ready solubility of Pt in the ’-Ni3Al and its concentration gradient allow fast ingress of this element in the coating in spite of the higher diffusion coefficients of the substrate elements. The relatively high Pt contents at the surface thereafter promote Al ingress upon aluminisation by creating a greater concentration gradient between the coating atmosphere and the coating. This results in a -NiAl based coating grown by both Ni outward and Al inward diffusion. The diffusivity of Pt, Ni and Al being dependant on the stoichiometry of the -NiAl coating, different metal particles may precipitate mainly at the NiAl grain boundaries. In addition, the diffusion of the refractory elements towards the interdiffusion layer seems to be hurdled by increasing the Pt content. As a result of the countercurrent diffusion of species, different microstructures and compositions will appear across the coatings.
A. Agüero, J.M. Albella, M.P. Hierro, J. Phillibert and F.J. Pérez Trujillo
F. Pedraza "Implications of Diffusion on the Composition and Microstructures of Platinum Modified Aluminide Coatings on CMSX-4 Single Crystal Superalloy", Defect and Diffusion Forum, Vols. 289-292, pp. 277-284, 2009