In-situ Measurement of the Phase Transformation Behavior of Al sub(2)O sub(3) Scale during High-Temperature Oxidation using Synchrotron Radiation

Very thin Fe-coatings, similar to 50nm, were found to suppress metastable Al sub(2)O sub(3) formation on Fe-5OAl and Ni-5OAl alloys in our previous study. The authors proposed a mechanism whereby alpha -Al sub(2)O sub(3) precipitates from the Al-saturated Fe sub(2)C sub(3), which was formed during initial oxidation, since alpha -Al sub(2)O sub(3) and alpha -Fe sub(2)O sub(3) 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 Fe sub(2)O sub(3) were initially observed at around 350 degree C on Fe-coated samples. The lattice parameter of the Fe sub(2)O sub(3) initially increased linearly due to thermal expansion, but then rapidly decreased due to the formation of a solid solution of Fe sub(2)O sub(3)-Al super(2)O super(3). alpha -Al sub(2)O super(3) started to appear at around 800° C, but no peaks from metastable Al sub(2)O sub(3) were observed. The diffraction peaks from the alpha -Al sub(2)O super(3) on Fe-coated samples consisted of two distinct peaks, indicating that the alpha -Al sub(2)O sub(3) had two different lattice parameters. These results suggest that the alpha -Al sub(2)O sub(3) was formed not only by precipitation from the Al-saturated Fe sub(2)O sub(3), but also by oxidation of Al in the substrate.