A comparative study of two inward grown Pt modified Al diffusion coatings on a single crystal Ni base superalloy

Two inward grown Pt modified aluminide diffusion coatings (RT22 and SS82A) on the same Ni-based superalloy substrate (CMSX-4) have been studied. The specimens were oxidised in still laboratory air at 1050°C for times up to 2000 h and investigated by gravimetry. Specimens oxidised up to 500 h were studied by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectrometry (EDS). The gravimetric investigations revealed that the oxide on RT22 grew faster than the oxide on SS82A. Moreover, it was found that in the as-coated condition, both coatings contained precipitates of α-W, α-Cr, μ-phase and σ-phase. After short time heat treatment at 1050°C, only μ-phase precipitates were present. The volume fraction of the precipitates was measured and compared for different heat treatments. Compositional profiles across the coatings were acquired by TEM-EDS before and after heat treatment. They showed that considerable redistribution of elements takes place during oxidation and the initially steep Pt and Al profiles became smeared out. The RT22 coating formed γ′-Ni 3Al grains at the coating/oxide interface.