Long-term oxidation and phase transformations in aluminized CMSX-4 superalloys

Twenty disk-shaped aluminized (β-NiAl) CMSX-4 single crystal superalloy specimens were oxidized in air at 788, 871, 954, and 1010 °C for 1000, 2500, 5000, 7500, and 10,000 h. Phase constituents and the residual stress of thermally grown oxide (TGO), as well as microstructural degradation in the aluminized layer of CMSX-4, were examined by using photostimulated luminescence spectroscopy (PSLS), optical microscopy, scanning and transmission electron microscopy (SEM and TEM). The compressive residual stress in the TGO scale increased with increasing oxidation temperature and time until local spallation, after which a large standard deviation in the magnitude of residual stress was observed due to stress relief associated with TGO scale spallation. Whisker-shaped metastable Al₂O₃ phases were detected by PSLS and were observed by SEM and TEM. The presence of metastable Al₂O₃ phases on scale developed at 788 °C caused localized spallation of TGO within the grains of β-NiAl. At higher temperatures, the TGO scale primarily consisted of equilibrium α-Al₂O₃, and spallation occurred along the vaulted asperities associated with grain boundary ridges. The Al-rich β-NiAl phase was dissolved and transformed into Al-depleted phases with high-temperature oxidation. A finite difference model was developed to simulate the change in Al concentration, the phase constituents in the diffusion aluminide coatings, and the thickness of the TGO scale during oxidation. This model can serve as a method to predict the lifetime of these coatings.