Microstructure and mechanical properties of Fe-Al-Nb-B alloys
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Microstructure and mechanical properties of Fe–Al–Nb–B alloys Shahbaz Ahmed Azmi, Alena Michalcová, Lucia Senčekova, Martin Palm Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Str. 1, D-40237 Düsseldorf, Germany ABSTRACT Doping of Fe–Al–Nb alloys with boron results in precipitation of stable C14 Laves phase Nb(Fe,Al)2 instead of metastable Heusler phase Fe2AlNb as in case of the ternary system. The boron stimulated precipitation of the Laves phase leads to preferential precipitation of the Laves phase along grain boundaries and – with higher supersaturation of Nb in the Fe-Al matrix – to an even distribution of additional precipitates within the grains. Though these microstructures seem to be more favourable than in the boron-free alloys, which show an uneven distribution of rather large Laves phase precipitates, no marked strengthening effect by the Laves phase in the Fe–Al– Nb–B alloys is observed. INTRODUCTION For the broader application of iron aluminide alloys, strengthening at high temperatures is a crucial issue [1, 2]. Strengthening by precipitates is one possibility to reach this objective. In many Fe–Al–X systems, extended two-phase fields between Fe3Al or FeAl and a Laves phase exist [3]. As a high-melting and rather strong phase, Laves phases are well suited as strengthening second phases. However, in the Fe–Al–X systems the Laves phase forms by a eutectic reaction L ↔ (Fe,Al) + Laves phase, which results in rather coarse and unevenly distributed Laves phase precipitates [4, 5]. Recently it has been shown that in Fe–Al–Ta alloys, where metastable formation of the Heusler phase Fe2TaAl precedes the formation of the stable Laves phase (Fe,Al)2Ta, distribution of the Laves phase within the microstructure can be widely varied through a heat treatment, previous thermomechanical treatments or doping by boron [6]. The latter method results in an even distribution of fine Laves phase precipitates within the matrix. It is known that also in the Fe–Al–Nb system precipitation of the Laves phase from the supersaturated (Fe,Al) matrix is kinetically retarded and metastable Heusler phase forms instead [7]. The current investigation was performed to see whether the precipitation of the Laves phase can also be influenced by B doping in the Fe–Al–Nb system and to evaluate the mechanical properties of these alloys. EXPERIMENT Four different alloys of about 800 g were produced from Fe (99.95 wt.% purity), Al (99.7 wt.%), Nb (99.9 wt.%), and ferro boron (Fe–18.2 wt.% B) by induction melting under argon. Crucibles of pure alumina were employed and the alloys were cast into a cylindrical cold copper mould with 25 mm diameter. Samples were sectioned by electrical-discharge machining (EDM) and microstructures were characterised by light optical and scanning electron microscopy (LOM, SEM Jeol JSM–6490). Electron probe microanalysis (EPMA) was carried out on a JXA 8100
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