Microstructural stability of a Ni-Pt-Al coating on CMSX-10 alloy at 950 and 1100°C

Specimens of CMSX-10 alloy have been platinum aluminised to give a two phase PtAl₂-(Ni, Pt)Al coating. Specimens of the coated alloy were exposed in an oxidising environment for 188, 350 and 750 hours at temperatures of 950 and 1100°C. At 950°C the two phase coating equilibrates with the substrate and this results in the development of a single phase (Ni, Pt)Al coating. After 750 hours, β-γ′ transformation effects are observed but there is significant Ta present in the γ′ grains. At 1100°C more rapid outward diffusion of nickel and countercurrent diffusion of aluminium facilitates more extensive β-γ′ transformation and Ta substituted γ′ grains are clearly visible after 188 hours. At both temperatures, refractory element rich phases form in a secondary equilibration zone adjacent to the substrate as a result of nickel depletion. The development of these phases at 1100°C is significant, such that they extend to a depth of 0.2mm beneath the diffusion layer.