Rapid Identification of Crystal Structure of Alumina Scale on Heat-resistant Alloy

Abstract

Identifying a crystal structure of alumina (Al₂O₃) scale is critical for evaluating the performance of heat-resistant alloys because α-Al₂O₃ is stable and protective against high-temperature oxidation and corrosion but θ- and γ-Al₂O₃ provide poor oxidation resistance. Conventional methods to identify crystal structures of Al₂O₃ scales are time-consuming. Herein, the author proposes a method to rapidly identify crystal structures of Al₂O₃ scales on β-NiAl by obtaining their cathodoluminescence (CL) spectra. α-, θ-, and γ-Al₂O₃ can be identified by detecting a sharp peak at 695.8 nm and 686.3 nm, and a broad peak at around 700 nm, respectively, in CL spectra. Concentrations of α-, θ-, and γ-Al₂O₃ scales can be determined roughly from intensities of these peaks. This method can be applied to areas ranging from the millimeter to micrometer scale, and the acquisition time for the CL spectra was less than 10 s. The results indicate that obtaining CL spectra contributes to the identification of crystal structures of Al₂O₃ scales on heat-resistant alloys and a reduction in time to evaluate the performance of heat-resistant alloys.