Al2O3 scale development on iron aluminides

The structure and phase of the Al2O3 scale 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 degrees C and 1000 degrees 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 theta, gamma, and alpha phases of alumina. Band-like structure was observed extending along three 120 degrees-separated directions within the surface plane. Textured theta and gamma grains were the main components of the bands, whereas mixed a and transient phases were found between the bands. Extended oxidation produced a double-layered scale structure with a continuous alpha layer at the scale/alloy interface and a gamma/theta layer at the gas surface. The mechanism for the formation of Al2O3 scales on iron aluminide alloys is discussed and compared with that for nickel aluminide alloys.