Characterization of α-Al2O3 in Structural Isomers of Alumina Formed by Oxidation of Fe–Cr–Al Alloys

To understand the formation of structural isomers of alumina (Al2O3) on the surfaces of Fe–Cr–Al alloys, cathodoluminescence (CL), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) were used. The alloys were annealed under oxygen at a low partial pressure to focus on the initial formation of Al2O3. XRD revealed that corundum (α-Al2O3) was predominantly formed on the alloy surfaces after annealing at approximately 1100°C, whereas γ-Al2O3 and θ-Al2O3 were formed at 1000°C. CL peaks around 700 nm, which originated from Cr3+ ions in Al2O3, was drastically increases by rising the annealing temperatures from 1000 to 1100°C because of the increase of the chromium content in the Al2O3 scales that was revealed by XPS. EPMA revealed that the surficial α-Al2O3 had striped patterns, which were similar to those observed using CL imaging, indicating that CL can be conveniently used for imaging α-Al2O3 and analyzing the inhomogeneous distribution of α-Al2O3 on the alloys.