Materials Science and Engineering A 497 (2008) 174–180 Contents lists available at ScienceDirect Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea Unusual mechanical behaviour of the intermetallic phase Nb 2 Co 7 L. Siggelkow a,b , U. Burkhardt a , G. Kreiner a , M. Palm b , F. Stein b,∗ a Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, D-01187 Dresden, Germany b Max-Planck-Institut für Eisenforschung GmbH, Postfach 140444, D-40074 Düsseldorf, Germany article info Article history: Received 5 November 2007 Received in revised form 25 June 2008 Accepted 27 June 2008 Keywords: Nb2Co7 Mechanical properties Yield stress Brittleness and ductility Microhardness Plastic deformation abstract Single-phase alloys of the monoclinic intermetallic phase Nb 2 Co 7 were investigated with respect to their mechanical behaviour. The material can be strongly deformed at room temperature by hammering with- out shattering or fracturing and it can be strained in compression to at least 5% without fracture. At the same time, Nb 2 Co 7 behaves like a brittle material when tested under bending or tensile load. The mechanical properties of this phase were systematically studied by microhardness, compression, tensile and bending tests and the microstructure was analysed by light-optical and scanning electron microscopy before and after testing. © 2008 Elsevier B.V. All rights reserved. 1. Introduction In the binary Co–Nb system, a number of different intermetallic phases exist which are, in the order of increasing Nb content, Nb 2 Co 7 , the Laves phases Nb 1-x Co 2+x (hexagonal C36-type), NbCo 2 (cubic C15-type) and Nb 1+x Co 2-x (hexagonal C14-type), and the  phase Nb 7 Co 6 . The Co–Nb Laves phases have been studied in detail by Grüner et al. [1,2]. The phase Nb 2 Co 7 was observed for the first time in 1960 by Saito and Beck [3] and denoted as “X-phase”. Bataleva et al. [4], Raman [5], Shen et al. [6], Stein et al. [7,8] and Grüner [1] confirmed the existence of this phase, while it was not found by Pargeter and Hume-Rothery [9]. Nb 2 Co 7 does not melt congruently but forms very slowly by the peritectoid reaction Co(Nb) solid solution + C36-type Laves phase ↔ Nb 2 Co 7 at 1086 ◦ C and is stable in a very small homogeneity range of about 0.2 at.% [8]. The crystal structure of Nb 2 Co 7 is monoclinic with lattice parameters a = 0.45874 nm, b = 0.81509 nm, c = 0.62223 nm, ˇ = 107.18 ◦ [8,10]. The structure is closely related to that of Zr 2 Ni 7 , which was first described by Eshelman and Smith [11] and later by Parthé and Lemaire [12]. Both mention that Zr 2 Ni 7 may be a ductile phase but without further investigation of this phenomenon. The present paper reports results on the preparation, microstructure and mechanical properties of single-phase, poly- crystalline Nb 2 Co 7 . The mechanical behaviour is studied by ∗ Corresponding author. Tel.: +49 211 6792 557; fax: +49 211 6792 537. E-mail address: stein@mpie.de (F. Stein). microhardness, compression, tensile and bending tests. Details on its crystallographic structure and defects will be described else- where [10]. 2. Experimental Binary Co–Nb alloys with the nominal composition Nb 2 Co 7 (see Table 1) were prepared from pure Nb (purity 99.9 wt.%; GfE Metalle und Materialien GmbH) and Co (99.9 wt.%; H.C. Starck and Haines & Maassen). In order to ensure the reproducibility of the results con- cerning the mechanical properties, several alloys were synthesised using different preparation methods. By levitation melting or vac- uum induction melting in an argon atmosphere and solidification in cold Cu and Al 2 O 3 moulds, rods of 20 mm to 45 mm in diameter and masses up to 900 g were obtained (alloys No. 1–4). Especially alloys No. 3 and 4 were produced as larger ingots in order to have enough material for compression, bending and tensile tests. Two small samples (alloys No. 5 and 6) with masses below 3 g were prepared by arc melting in an argon atmosphere in order to get high-purity material. The peritectoid formation of the phase Nb 2 Co 7 turned out to be a very slow process [8]. Therefore, the as-cast alloys obtained from the different melting techniques are in a non-equilibrium state and contain the phases Co(Nb), C36-type and C15-type Laves phase. In order to obtain single-phase alloys of Nb 2 Co 7 , the as-cast samples were enclosed in evacuated quartz capsules back-filled with argon and heat-treated at 1000 ◦ C for at least 200 h. Small samples prepared by arc melting were additionally encapsulated in Nb-ampoules under argon prior to enclosing in quartz capsules. 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.06.034