Microstructure of oxide scales on aluminide diffusion coatings after short time oxidation at 1050 °C

Three commercial Pt-modified coatings (RT22, SS82A and MDC150L) and one aluminide diffusion coating without Pt (PWA73) applied on a Ni-based single crystal superalloy (CMSX4), were investigated after isothermal oxidation in laboratory air at 1050 °C for 1 h, using X-ray diffraction, scanning electron microscopy and transmission electron microscopy (TEM). TEM investigations reveal the presence of the metastable γ-Al2O3 phase on all the investigated coatings. The amount of the observed α-Al2O3 phase formed seems to depend on the Al and Pt contents in the coating. A high Al content in the coatings promotes formation of α-Al2O3. A continuous layer of α-Al2O3 grains was found at the coating/oxide interface of the coating containing 55 at.% Al. On the coating with a low Al content (≈40 at.%) and with low amount of Pt (4 at.%), no α-Al2O3 phase was observed while on the coating with a similar Al content but with much higher Pt level (16 at.%), grains of α-Al2O3 phase were detected.