Microstructural Investigations of the Unusual Deformation Behavior of Nb 2 Co 7
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1128-U08-03
Microstructural Investigations of the Unusual Deformation Behavior of Nb2Co7 F. Stein1, M. Palm1, G. Frommeyer1, P. Jain2, K.S. Kumar2, L. Siggelkow1,3,4, D. Grüner3,5, G. Kreiner3, and A. Leineweber6 1
Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Str. 1, D-40237 Düsseldorf, Germany 2 Division of Engineering, Brown University, Providence, RI 02912, U.S.A. 3 Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, D-01187 Dresden, Germany 4 now at: Technische Universität München, Lichtenbergstr. 4, D-85747 Garching, Germany 5 now at: Division of Physical, Inorganic and Structural Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden 6 Max-Planck-Institut für Metallforschung, Heisenbergstr. 3, D-70569 Stuttgart, Germany ABSTRACT Usually, single-phase intermetallics in bulk form can easily be crushed into powder by hammering. It was therefore quite a surprise when we found that a bulk sample of the monoclinic intermetallic compound Nb2Co7 could be extensively deformed at room temperature without shattering or fracturing. In a previous paper, results of microhardness, compression, tensile and bending tests were provided and discussed [1]. In order to understand the observed unusual deformation behavior of this intermetallic phase, its hitherto unknown crystal structure has been studied and the microstructure of undeformed and deformed samples has been analyzed in the present investigation by light-optical, scanning electron and transmission electron microscopy. Single-phase specimens deformed at very different strain rates (hammering and conventional compression testing) both show the occurrence of microcracks along grain boundaries which, in compression-deformed specimens, are strongly localized in extended shear bands oriented approximately 45° to the compression axis. The grains adjacent to the microcracks are heavily deformed whereas, away from the sheared regions, the samples remain free of any indication of plastic deformation. INTRODUCTION In a recent re-assessment of the binary Co-Nb system [2], the formation, thermodynamic stability and homogeneity range of the intermetallic phase Nb2Co7 was investigated. Nb2Co7 does not melt congruently but forms as a peritectoid reaction product between the Co(Nb) solid solution and the hexagonal C36 Laves phase Nb1-xCo2+x at 1086°C. As the peritectoid formation and dissolution is a very slow solid state reaction (see e.g. [3]), the phase does not occur in ascast alloys and only forms after heat treatments for sufficiently long times. Single-phase Nb2Co7 specimens showed an unexpected ‘tough’ behavior during attempts to produce powder for XRD measurements by crushing in a mortar. To study this unusual behavior in more detail, a number of mechanical tests were performed with single-phase Nb2Co7 specimens, and the results were reported in a recent paper by Siggelkow et al [1]. Rather intriguing was the observation that in compression (strain rate: 10-4 s-1), this phase exhibited “plastic deformation” (non-l
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