Characterization of thermally cycled alumina scales

Cross-sectional transmission electron microscopy was used to characterize t he alumina scales formed on several Ni-base alumina-formers. The alumina sc ale microstructure of Ni-20at%Cr-19Al-0.05Y after 100, 1 h cycles at 1,100 degrees C was compared to an isothermally-grown scale. Despite being near t he onset of mass loss in cyclic testing, very few defects were noted in eit her scale microstructure. The more adherent scales that form on Hf-doped Ni Al and Ni-49at%Al-2Cr were also characterized. With the addition of Cr, the formation of alpha-Cr precipitates at the metal-oxide interface coincided with increased long-term scale spallation. No similar precipitation mechani sm was observed to be associated with scale spallation on NiCrAlY.