The Effect of Li on Intermetallic Fe-Al Alloys
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The Effect of Li on Intermetallic Fe-Al Alloys Xiaolin Li1,2, Frank Stein2 and Martin Palm2 1 2
IEHK, RWTH Aachen, Germany Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, Germany
ABSTRACT A couple of FeAl alloys containing up to 1.4 at.% Li have been produced by vacuum induction melting. Though previous reports indicated a significant effect of Li on the properties of FeAl, no marked changes with respect to binary FeAl are observed. Specifically, no decrease of the lattice constant and no significant increase in ductility are found by alloying with Li. If at all, there is a slight increase of the lattice constant. INTRODUCTION Fe-Al intermetallic alloys based on Fe3Al or FeAl show an excellent corrosion and wear resistance, have relatively low densities compared to other Fe based alloys, and are cost effective as alloys – if at all – do contain only little amounts of strategic elements and they can be produced and processed by standard techniques [1-3]. However, low ductility at room temperature has limited their use to niche applications so far [4]. Because brittleness is not an inherent property of Fe-Al alloys but is caused by environmental embrittlement [2,5], numerous approaches have been made to enhance ductility through alloying (see e.g. [6, 7]). According to a couple of studies, alloying with up to 5 at.% Li considerably improves the mechanical properties of FeAl [8-10] and Fe3Al [11, 12]. Specifically for FeAl alloyed with Li a decrease of the lattice constant and the hardness [8], and increases of the compressive ductility [10], elongation and ultimate tensile strength [9] have been reported. However, no evidence showing that Li is actually in solid solution in the Fe-Al intermetallic alloys has been presented, e.g. by wet chemical analysis. It is difficult to alloy Fe-Al intermetallic alloys with Li, because of the low melting point (180.5 °C) and boiling point (1342 °C) of Li. Hence in this study, Fe-Al-Li alloys have been prepared in two different ways, and their compositions, microstructures and basic mechanical properties have been studied. EXPERIMENT Three alloys were produced by melting in an induction furnace under argon. The intended composition of all three alloys was Fe-45Al-2Li (all compositions in at.% except noted). Alloys 1 and 3 were produced by melting an Al-Li pre-alloy (Al-9.4 Li) and then adding chips of Fe (99.9 wt.%). Alloy 2 was produced by melting a mixture of Fe and Al (99.99 wt.%) and then adding the Al-Li alloy. All alloys were cast into a cold copper mould with diameters of 29 mm for alloys 1 and 2 and of 17 mm for alloy 3. Alloys were cut by electrical discharge machining (EDM). Microstructures were characterised by light optical microscopy (LOM) and scanning electron microscopy (SEM;
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JEOL JSM-6490). To this end, specimens were mechanically polished to 1 μm grade diamond powder finish and etched by “Ti2 solution” (68 vol.% glycerin, 16 vol.% 70% HNO3, 16 vol. % 40% HF). Wet-chemical analysis was complemented by energy dispersive spectrometry (EDS; EDAX Genesi
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