Abnormal oxidation behavior of β-NiAl base aluminide coating on a nickel-base superalloy

Isothermal oxidation of β-NiAl base aluminide coating has been studied by TGA, XRD, and SEM over the temperature range of 800-1050°C. During the initial period of oxidation, γ-Al2O3 and θ-Al2O3 were growing at a relatively high rate. Then these metastable oxides transformed to stable α-Al2O3 accompanied by a distinct decrease of the Kp values by one or two orders of magnitude at 800-1000°C. This transformation also resulted in a dramatic change in the surface morphology of the scales from blade-like oxides to equiaxed grain oxides. The higher the temperature, the faster the transformation.