Al₂O₃scale development on iron aluminides

The structure and phase of the Al₂O₃scale that forms on an Fe₃Al-based alloy Fe-28Al-5Cr (at.%) was investigated by transmission electron microscopy and photoluminescence spectroscopy. Oxidation was performed at 900 °C and 1000 °C for up to 190 min. Transmission electron microscopy revealed that single-layer scales were formed after short oxidation times. Electron diffraction was used to show that the scales are composed of nanoscale crystallites of the θ, γ, and α phases of alumina. Band-like structure was observed extending along three 120°-separated directions within the surface plane. Textured θ and γ grains were the main components of the bands, whereas mixed α and transient phases were found between the bands. Extended oxidation produced a double-layered scale structure with a continuous α layer at the scale/alloy interface and a γ/θ layer at the gas surface. The mechanism for the formation of Al₂O₃scales on iron aluminide alloys is discussed and compared with that for nickel aluminide alloys.