Microstructure and Phase Transformation Temperatures of Two-Phase FeAl (B2) + FeAl 2 Alloys
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Microstructure and Phase Transformation Temperatures of Two-Phase FeAl (B2) + FeAl2 Alloys Xiaolin Li1, Martin Palm1, Anke Scherf2, Daniel Janda2, Martin Heilmaier2 and Frank Stein1 1 Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, Germany 2 Karlsruhe Institute of Technology, Karlsruhe, Germany ABSTRACT Fe-Al alloys with about 55 to 65 at.% Al undergo a eutectoid transformation at 1095 °C: Fe5Al8 (ε) ↔ FeAl + FeAl2. Hence, as-cast Fe-Al alloys in this composition range show a very fine-scaled lamellar microstructure (average lamellar spacing below 500 nm) consisting of the two phases FeAl and FeAl2. The microstructure looks similar to the α2 + γ lamellar microstructure of Ti-Al-based alloys, which is known for having well-balanced properties in terms of creep, ductility and strength. However, there is limited knowledge about the properties of Fe-Al-based alloys in this composition range. In this study, a series of as-cast as well as heattreated Fe-Al alloys with compositions between 57 and 63 at.% Al were investigated. The microstructures and crystal structures were analysed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The composition dependence of all transition temperatures was obtained by differential thermal analysis (DTA). INTRODUCTION Fe-Al based intermetallic alloys with up to 50 at.% Al have been studied as promising materials for structural applications due to their excellent corrosion resistance and low density compared to steels because of the high Al content [1-4]. At Al contents above 50 at.%, the density will further decrease and it can be expected that also the corrosion resistance is very good. The phase diagram in Fig. 1 [5] shows that in the composition range near 60 at.% Al, binary Fe-Al alloys contain the phase Fe5Al8 (ε) at high temperatures, which transforms to the two phases B2 FeAl and triclinic FeAl2 on cooling below 1095 °C due to the eutectoid reaction Fe5Al8 (ε) ↔ FeAl + FeAl2 [5,6]. Microstructures and properties of such alloys are not yet well studied and the homogeneity range of the Fe5Al8 phase is still unclear. Therefore, several alloys in this composition range were produced by vacuum induction melting for investigation of microstructures, crystal structures and transition temperatures.
Figure 1. Phase diagram of the Fe-Al system in the composition range up to 75 at.% Al [5].
EXPERIMENT A series of Fe-Al binary alloys was prepared from high purity Fe (99.98 wt.%) and Al (99.99 wt.%) by vacuum induction melting under Ar atmosphere. During casting a cold copper mould with a diameter of 18 mm was used. The chemical compositions of the investigated alloys are listed in Table 1 showing that nominal and analysed compositions agree well. Slices of the alloys were cut by electrical discharge machining (EDM) for heat treatment and SEM. Heat treatments were performed at 1000 °C for different times up to 216 h under Ar atmosphere. Table 1. Compositions of the investigated Fe-Al alloys as obtained by wet chemical analysis. Nominal composition (at.
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