Compositional Dependence of the Compressive Yield Strength of Fe-Nb(-Al) and Co-Nb Laves Phases
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Compositional Dependence of the Compressive Yield Strength of Fe-Nb(-Al) and Co-Nb Laves Phases Simon Voß, Martin Palm, Frank Stein and Dierk Raabe Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Str. 1, D-40237 Düsseldorf, Germany ABSTRACT Large, defect-free single-phase samples of the hexagonal C14 NbFe2 and Nb(Fe,Al)2, and the cubic C15 NbCo2 Laves phases have been produced by a modified levitation melting technique. The compressive strength of NbFe2 and NbCo2 has been determined in dependence on the Nb content, that of Nb(Fe,Al)2 in dependence on the Al content. The binary phases did not show either a maximum (defect softening) or minimum (defect hardening) in strength when the Nb content was varied. Instead, for both phases an increase of the compressive strength with increasing Nb content is observed. INTRODUCTION Though Laves phases are the most abundant type of intermetallic phases, only few data on mechanical properties of single-phase Laves phases have been reported. One reason is of course the difficulty to produce single-phase material of these brittle phases that is large enough to perform suitable tests. Accordingly, very limited information exists on the compositional dependence of mechanical properties, i.e. the effect of off-stoichiometric compositions. Almost all available information stems from hardness measurements and these show quite contradictory results. E.g. for the hexagonal C14-type Laves phase NbFe2 a minimum (defect hardening) [1] and a maximum (defect softening) [2] of the hardness at the stoichiometric composition have been reported. Based on measurements on a number of other Laves phases it had been concluded that off-stoichiometry leads to defect hardening [1,3], defect softening [4-7], or has no influence at all [5]. From compression tests defect softening was concluded for C14 MgZn2 [8] while no effect on strength was observed for off-stoichiometric C15 NbCr2 [9]. Within the framework of the large-scale research initiative “The Nature of Laves Phases” (http://laves.mpie.de/) a modified levitation melting technique has been developed by which large bars of single-phase Laves phase can be produced [10]. From these large bars compression samples could be prepared. In contrast to earlier investigations, which were limited to hardness measurements because only chips of these brittle materials were available, the compositional dependence of the strength of hexagonal C14 NbFe2 and Nb(Fe,Al)2 and cubic C15 NbCo2 could now be more properly investigated. EXPERIMENT Alloys of up to 350 g were produced from Nb (99.9 wt.% purity), Fe (99.95 wt.%), Co (99.92 wt.%), and Al (99.7 wt.%) by a modified levitation melting technique (see [10] for details). Each alloy was cast into a preheated mould, held at 1200 °C for 45 min and cooled down to room temperature with a rate of 5 K/min. Certain alloys with Nb-lean and Nb-rich compositions were additionally heat-treated at 1350 °C/50 h (NbFe2) or 1180 °C/50 h (NbCo2) to dissolve metastable phases stemming from solidification. Samples were cut by e
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