In-situ Measurement of the Phase Transformation Behavior of Al2O3 Scale during High-Temperature Oxidation using Synchrotron Radiation

Very thin Fe-coatings, similar to 50nm, were found to suppress metastable Al2O3 formation on Fe-50Al and Ni-50Al alloys in our previous study. The authors proposed a mechanism whereby alpha-Al2O3 precipitates from the Al-saturated Fe2O3, which was formed during initial oxidation, since alpha-Al2O3 and alpha-Fe2O3 have isomorphous structures. In order to confirm the proposed mechanism, in-situ measurements were made of structural changes in the oxide scales formed on FeAl with and without Fe coating during heating and subsequent isothermal high temperature oxidation by synchrotron radiation with a two-dimensional X-ray detector. Diffraction peaks from Fe2O3 were initially observed at around 350 degrees C on Fe-coated samples. The lattice parameter of the Fe2O3 initially increased linearly due to thermal expansion, but then rapidly decreased due to the formation of a solid solution of Fe2O3-Al2O3. alpha-Al2O3 started to appear at around 800 degrees C, but no peaks from metastable Al2O3 were observed. The diffraction peaks from the alpha-Al2O3 on Fe-coated samples consisted of two distinct peaks, indicating that the alpha-Al2O3 had two different lattice parameters. These results suggest that the alpha-Al2O3 was formed not only by precipitation from the Al-saturated Fe2O3, but also by oxidation of Al in the substrate.