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

Abstract:

Very thin Fe-coatings, ~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 α-Al₂O₃ precipitates from the Al-saturated Fe₂O₃, which was formed during initial oxidation, since α-Al₂O₃ and α-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°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. α-Al₂O₃ started to appear at around 800°C, but no peaks from metastable Al₂O₃ were observed. The diffraction peaks from the α-Al₂O₃ on Fe-coated samples consisted of two distinct peaks, indicating that the α-Al₂O₃ had two different lattice parameters. These results suggest that the α-Al₂O₃ was formed not only by precipitation from the Al-saturated Fe₂O₃, but also by oxidation of Al in the substrate.

• Periodical: Materials Science Forum (Volume 696)
• Pages: 63-69
• DOI: https://doi.org/10.4028/www.scientific.net/MSF.696.63
• Online since: September 2011
• Authors: Shigenari Hayashi, Isao Saeki, Yoshitaka Nishiyama, Takashi Doi, Shoji Kyo, Masayuki Segawa
• Keywords: Fe Nano-Coating, Metastable to Alpha Al₂O₃ Phase Transformation

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