Anfangsstadien und Kinetik der Oxidation von binären und ternären Eisenaluminiden
Final Report Abstract
Oxidation experiments were performed on binary (Fe-15at.%AI, Fe-26at.%AI, Fe-40at.%Al) and ternary (Fe-26at.%Al-4at.%X and Fe-40at.%Al-4at.%X with X = Ti, Nb and Ta) iron aluminides in the temperature range 700 to 1000 °C for various times between 5 min and 1000 h. In case of binary iron aluminides, oxidation in dry synthetic air leads to a double-layered oxide scale on the surface which consists of an outer, approximately 40 nm thin, Fe2O3-rich layer followed by an inner, nearly pure AI2O3 layer which grows continuously with increasing temperature and time. The content of Fe203 in the outer oxide layer decreases with increasing time, temperature, and Al content. The double oxide layer is already present after 5 min exposure to synthetic air at 700 °C. The high iron activity of iron aluminides leads to initial simultaneous oxidation of iron and aluminium. Due to the higher growth rate of Fe2O3 in comparison to Al2O3, Fe2O3 outgrows Al2O3 during the early stages of oxidation which results in the double layered oxide scale. The variation of the Fe2O3 and Al2O3 content of the outer oxide layer found in this study with increasing oxidation temperature after 5 min indicates that the growth rate of FezOs decreases with increasing temperature in relation to the Al2O3 growth rate. This is most probably due to the separation of Fe2O3 from the substrate by the inner Al203 layer, stopping further iron supply for oxidation. Additionally, increasing oxidation time leads to a decreasing amount of Fe203 in the outer oxide layer. The reason for that could be the reduction of Fe2O3 to metallic iron at the internal oxide interface. Indeed small amounts of metallic iron were found at the internal oxide interface for all oxidizing conditions. It seems reasonable that the metallic iron was formed during first exposure to oxygen, when the alumina scale was very thin and metallic aluminium was in contact with the preformed Fe203 allowing a reaction Fe2O3 + Al -> Al2O3 + Fe. The oxide crystals formed after short oxidation times up to 5 h are less than approximately 100 nm in diameter at the lower temperatures of 700 and 800 °C. The crystallographic structures of these crystals grown in the thin oxide layers were analyzed by grazing incidence X-ray diffraction. In case of the alloys containing 15 and 26 at.% Al, no metastable polymorphs of Al2O3 were found at any oxidation temperature and time. Even after 5 min of oxidation at 700 and 800 °C, only α-Al2O3 and α-Fe203 (and the iron aluminide substrate) were observed. In case of Fe-40at.% Al, metastable Al2O3 polymorphs were found at temperatures below 900 °C which transform to cc-A^Os with increasing time and temperature. TEM investigations reveal that metastable AI2O3 has grown epitaxially on the metallic surface and is still present as a thin layer along the metal/oxide interface after longer times. Similar thin layers (some nm in thickness) of metastable AI2O3 were also found at the metal/oxide interface of the Fe-15at.%AI and Fe-26at.%AI alloys after oxidation at 700 °C. The measured lattice parameters of a-AI2O3 and a-Fe2O3 show that there is a very small but non negligible mutual solubility of the two oxides. Similar as on the binary alloys, double-layered oxide scales are also observed on the surface of the ternary alloys containing 26 at.%AI and 4 at.% Ti, Nb or Ta. Fe-26at.%AI-4at.%Ti forms additionally rarely distributed Ti oxides on the outer layer and a very thin Ti-rich layer at the α-Al2O3/metal interface. Fe-26at.%Al-4at.%Nb and Fe-26at.%Al-4at.%Ta form more complex substructures due to the formation of Laves phase precipitates along the grain boundaries of the iron aluminide matrix and the rare formation of Nb and Ta oxides. Despite these differences, the thickness of the overall oxide layers is about the same as for the binary alloys. In order to understand the role of the outer Fe2O3-rich layer for the formation of α-AIlO3, experiments were also performed in a N2-5vol.% H2 (dew point -40 °C) atmosphere. As a result, a single oxide layer consisting of Al2O3 forms with a large number of iron crystals growing on top of this layer. The size of the Fe crystals as well as the thickness of the oxide layer increase with increasing oxidation time and temperature. EDX and XRD show that the phases are pure oc-Fe and α-Al2O3. Investigations of the long-term oxidation behaviour by thermogravimetric experiments clearly demonstrate that the ternary additions have a much smaller effect on the growth kinetics than changes of the Al content. The present results show that the phase composition of the oxide scales on the binary and ternary alloys is basically corundum type Al2O3 and Fe2O3. Metastable phases were detected especially on Fe-40at.%AI, indicating that the surface aluminium or the surface oxygen concentration content may play a role in metastable alumina formation. An important goal of further research would be to clarify whether metastable alumina modifications only form if the oxide is formed by nucleation and growth rather than by using other oxides as templates.
Publications
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B, Pöter, F. Stein, and M. Spiegel: "Characterization of Thin Oxide Layers Grown on Fe-15at.%AI", EUROCORR 2005, Sept. 4-8, 2005, Lisboa, Portugal.
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B. Pöter and M. Spiegel: "Studies on the Nucieation and Growth of Oxide Films", Gordon Research Conference - High Temperature Corrosion, July 24-27, 2005, New London, NH, USA.
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B. Pöter, C. Sternemann, and M. Spiegel: "Investigation on Initial Stages of Iron Aluminide Oxidation Regarding the Crystallisation of AIZO3 potytypes", DELTA User Meeting, Nov. 30, 2005, Dortmund.
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B. Pöter, F. Stein, and M. Spiegel: "Initial Stages of AI2O3 Growth on Fe-AI Intermetallic Phases", EMPG X Conf., April 4-7, 2004, Frankfurt/Main, Germany.
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B. Pöter, F. Stein, and M. Spiegel: "Oxidation Behaviour of Fe-AI Alloys Analysed Using In- and Ex-situ Techniques", EUROCORR 2004, Sept. 12-16, 2004, Nice, France.
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B. Pöter, F. Stein, D. Vogel, and M. Spiegel: "A/203 Growth on Fe-AI Intermetaliic Phases", ANKA User Meeting, Sept. 23-24, 2004, Karlsruhe, Germany.
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B. Pöter, F. Stein, M. Spiegel, and M. Stratmann: "Anfangsstadien der Oxidation von intermetallischen Fe-AI Phasen", 38. Metallographietagung der DGM, Sept. 29-Oct. 1, 2004, Bochum, Germany,
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B. Pöter, F. Stein, R. Wirth, and M. Spiegel: Early Sfages of Protective Oxide Layer Growth on Binary Iron Aluminides, Z. Phys. Chem. 219 (2005) 1489-1503.
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B. Pöter, I. Parezanovic, and M. Spiegel: fn-situ FE-SEM and EBSD Investigation on the Oxidation of Pure Iron, 6th International Conference on the Microscopy of Oxidation, April 4-6, 2005, Birmingham, UK.
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B. Pöter, I. Parezanovic, and M. Spiegel:In-situ FE-SEM and EBSD Investigation on the Oxidation of Pure Iron, Microsc. Oxid. 6 (2005) 7-15.
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C. Krywka, M. Paulus, C. Sternemann, M. Volmer, A. Remhof, G. Nowak, A. Nefedov, B. Pöter, M. Spiegel, M. Tolan: The New Diffractometer for Surface X-ray Diffraction at Beamline BL 9 of Delta, J. Synchrotron Rad. 13(2006)8-13.
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M. Spiegel, F. Stein, and B. Pöter: "Initial Stages of Oxkte Growth on Fe-AI Alloys", 3rd Disc.Meeting on the Development of Innovative Iron Aluminium Alloys, Jan. 22-24, 2006, Mettmann-Düsseldorf, Germany.
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T. Liapina, M. Spiegel, and F. Stein: "Short-Term Oxidation of Fe-AI: Effect of Ternary Elements and AI Content", 4th Discussion Meeting on the Development of Innovative Iron Aluminium Alloys", Oct. 24, 2007, Interlaken, Switzerland
