1998 Volume 84 Issue 4 Pages 285-290
The development and phase transformations of Al2O3 scales formed on Al-deposited Fe-Cr-Al foil and Fe-20Cr-5Al foil have been studied during the oxidation at 1173K in air using TEM, SEM and XRD. The both kinds of foils had very similar chemical compositions, however the oxidation started on a surface of high Al content for the Al-deposited foil. The scale formed on it has a two-layer structure during the initial oxidation period : the outer layer consists of γ-Al2O3 and the inner layer of mainly fine-grain θ-Al2O3. The phase transformation from γ to θ takes place at the scale / substrate interface. As oxidation proceeds needle-like crystals start to grow at the interface of the two layers, pass through the outer layer, and extended out from the scale surface. They consist of twin crystals of γ-Al2O3, some of which transform to θ-Al2O3, with twin boundaries parallel to their growth directions. It is proposed that the twin boundaries provide fast diffusion paths for Al cations. During the growth of needle-like crystals, the inner layer consists of fine-grain θ-Al2O3. After about 180ks oxidation α-Al2O3 grains nucleate at the scale / substrate interface and after about 360ks the major part of the scale transform to α-Al2O3. Its grain size is 1 to 2, μ m and thus one or two grains occupy the scale thickness. Small voids are formed within α-Al2O3 grains. This is attributable to volume decrease by about 12% accompanying the θ to α transformation. Contrary to the Al-deposited foil, scales formed on the Fe-20Cr-5Al foil transform to α-Al2O3 in short periods.
