Fatigue resistance of Fe 3 Al-based alloys
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Fatigue resistance of Fe3Al-based alloys Florian Gang1, Manja Krüger1, Alexandra Laskowsky1, Heike Rühe1, Joachim H. Schneibel1, Martin Heilmaier2 1
Otto von Guericke University Magdeburg, Institute for Materials and Joining Technology, P.O. Box 4120, D-39016 Magdeburg, Germany 2 Technical University Darmstadt, Dept. Materials Science, D-64287 Darmstadt, Germany ABSTRACT The low cycle fatigue (LCF) behaviour of two cast as well as two hot extruded Fe3Al-based iron aluminide alloys, either with or without Cr, is investigated. All four alloys contain microalloying additions of Zr, Nb, C and B. Fatigue tests were carried out under strain control for strain amplitudes in the range of a = 0.1 – 0.4 % for the cast alloys and a = 0.1 – 0.7 % for the extruded materials, at frequencies of 1 Hz (extruded Fe3Al) and 3 Hz (all other alloys) and at room temperature and 300 °C. Within the first cycles all alloys show strong cyclic hardening. Furthermore the fatigue strain – fatigue life curves are steeper at 300 °C than at room temperature, showing increased fatigue strength at low cycle numbers due to increasing ductility and decreased fatigue strength at increasing cycle numbers because of reduced yield strength. Cr is found to have only a negligible influence on the fatigue behaviour of Fe3Al-based alloys. Comparison between the differently processed materials shows superior LCF properties of the hot extruded iron aluminides due to significantly smaller grain sizes. INTRODUCTION Iron aluminide alloys based on Fe3Al have been of interest for many years since they combine excellent corrosion and oxidation resistance with adequate strength at elevated temperatures and low density. Therefore they have been considered for a wide range of applications, e.g. in the aviation and automotive industry [1]. While fatigue can become an important issue in these structural applications of iron aluminides, it has not received much attention to date [2-6]. This work deals with the low cycle fatigue (LCF) behaviour, as well as the transition to high cycle fatigue (HCF), of two cast and two hot extruded Fe3Al alloys with and without Cr. Testing was performed at room temperature and 300 °C. As stated in [7], alloying with Cr tends to improve the room temperature ductility of iron aluminides. Hence the effect of Cr addition on the LCF behaviour seems to be worth determining. Finally, the study was completed by microstructural examinations using optical microscopy, scanning electron microscopy and X-ray diffraction. EXPERIMENT Processing and microstructure Two different processing routes were chosen to fabricate the samples. The production of the as-cast samples started with preparing two cylindrical ingots by induction melting under partial argon atmosphere (100 mbar).
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This was followed by a heat treatment for 5 hours at 700 °C and furnace cooling to keep internal stresses at a low level. Chemical analysis, using optical emission spectrometry (OES), showed their composition to be Fe-27.1Al and Fe-27.3Al-4.1Cr (at. %). The ingots also contai
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