Determination of the Maximum Surface Chromia Thickness for the Nondestructive Identification of Internal Alumina Scales on a Heat-resistant Alloy Using Cathodoluminescence

To develop a nondestructive analytical method for identifying internal α-Al2O3 scales formed in heat-resistant alloys, the maximum thickness of surface Cr2O3 scales for identifying internal α-Al2O3 scales via cathodoluminescence (CL) spectroscopy was estimated using integral intensity of CL peak at 695 nm (R1-R2 doublet) for Cr2O3 film-deposited sapphires and α-Al2O3 scales formed on the surface of Fe-Cr-Al alloys. The estimated maximum Cr2O3 thickness was validated by measuring the intensities of the R1-R2 doublet for Ni-25Cr-5Al alloys heated at 1000 °C under oxygen. Therefore, internal α-Al2O3 scales can be identified nondestructively by acquiring the surface CL spectra up to ~ 3-µm surface Cr2O3 thickness.