Effect of the θ–α-Al2O3 Transformation in Scales on the Oxidation Behavior of a Nickel-Base Superalloy with an Aluminide Diffusion Coating

The high-temperature oxidation behavior of a nickel-base superalloy andaluminide-diffusion coating has been investigated over the temperaturerange from 800–1100°C and analyzed by TGA, XRD, EDAX, andSEM. The β-NiAl coating was formed by low-pressure gas-phasecementation at 950°C for 3 hr. It was found that the formation ofα-Al2O3 from θ-Al2O3 on the β-NiAl coating resulted in asharp decrease in the parabolic rate constant kp by one order ofmagnitude at 1050°C during transient oxidation. The transformationcaused the anomalous behavior of the oxidation kinetics curves of thisdiffusion coating in the temperature range 800–1100°C withinthe first 100 hr. A change in the morphology of scales also occurredwith the transformation. A growth stress was characterized by theformation of convoluted scales, which were observed on the surfaceafter oxidation. The oxidation mechanism of this β-NiAl diffusioncoating is described.