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

Twenty disk-shaped aluminized ( beta-NiAl) CMSX-4 single crystal superalloy specimens were oxidized in air at 788, 871, 954, and 1010 degreesC 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 degreesC caused localized spallation of TGO within the grains of beta-NiAl. At higher temperatures, the TGO scale primarily consisted of equilibrium alpha-Al2O3, and spallation occurred along the vaulted asperities associated with grain boundary ridges. The Al-rich beta-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. (C) 2004 Elsevier B.V. All rights reserved