Characterization of Long-Term Oxidized Nickel Aluminide Coating by Photoluminescence Spectroscopy

Abstract

Microstructures and residual stress of long-term oxidized nickel aluminide coating on superalloy were evaluated by photostimulated luminescence spectroscopy (PSLS). CMSX-4 superalloy coupons coated with single phase b-NiAl were oxidized in air at 788, 871, 954 and 1010°C for up to 10000 hours. The photoluminescence technique was applied for the analysis of the phase constituents and the residual stress of the thermally grown oxide (TGO) scale formed on the^coating, together with transmission electron microscopy. According to the PSLS analysis, TGO consisted of both stable a-Al2O3 and metastable Al2O3 phases in specimens oxidized at 788°C and 871°C. At higher temperatures, the TGO scale primarily consisted of equilibrium a-Al2O3. The compressive residual stress within TGO increased up to about 3GPa with increasing oxidation temperature and time until local spallation, after which a decrease as well as large standard deviation of residual stress was observed owing to the stress-relief associated with TGO scale spallation.