Factors Affecting Cathodoluminescence Intensity of Internal <i>α</i>-alumina Scales on Heat-resistant Alloys

The identification of internal α-alumina (α-Al2O3) scales is crucial because α-Al2O3 determines the performance of heat-resistant alloys. A non-destructive method was recently developed to rapidly identify internal α-Al2O3 scales on the Ni–Cr–Al and Ni–Al alloys by obtaining their surface cathodoluminescence (CL) spectra. However, the peak intensities at 695 nm originating from α-Al2O3 considerably deviated from those estimated from the attenuation rates of overlaid scales (Cr2O3, NiO, and NiAl2O4) and the CL peak intensity at 695 nm for the α-Al2O3 scale on the Fe–Cr–Al alloy surface. In this study, factors causing the deviations in the CL peak intensity at 695 nm for the internal α-Al2O3 scales were investigated by obtaining images of the scales and their CL spectra. The surface Cr2O3 scale on the Ni–25Cr–5Al alloy exhibited a surface with the roughness of ~1 µm, causing the CL peak intensity at 695 nm to be one order of magnitude higher than the estimated value. The CL spectrum and scanning transmission electron microscopy observation revealed that θ-Al2O3 overlaid on α-Al2O3 in the internal Al2O3 scale on the Ni–14Al alloy reduced the CL peak intensity at 695 nm by two orders of magnitude.