Self-Diffusion of Both Components in the Intermetallic Laves Phase NbCo 2
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and for Nb 29Co 71 by 2 Dc0 = 3.32 .10-4 exp(-279.5 kJ mol-E/RT) m S-1,
The diffusivity of 95Nb in Nb 31Co 69 is considerably lower than that of 57Co. The latter increases significantly with increasing Co content. The results are discussed in terms of vacancy mediated diffusion in connection with structural aspects of the Laves phase together with the scarce information available for defects in NbCo2 . The results are compared with interdiffusion data from our laboratory and with self-diffusion in the also close-packed L1 2 intermetallics. INTRODUCTION Laves phases constitute the most numerous group of intermetallic compounds and have an approximate composition AB 2. They belong to the family of the Frank-Kasper phases showing topologically close-packed structures [1, 2]. Laves compounds can be formed if the effective atomic radii of the constituent elements exhibit a ratio of approximately 1.225 [3] and crystallize in the three intimately related structures C15 (MgCu 2-type), C14 (MgZn2-type) and C36 (MgNi2 -type). All these
Figure 1. Structure of NbCo 2.
ONb 0 Co KK7.6.1 Mat Res. Soc. Symp. Proc. Vol. 552 0 1999 Materials Research Society
structures have average coordination numbers larger than 12 [4] and are controlled by both the atomic size factor and the electonic factor [5]. Which particular modification will be stable is determined by electronic considerations [2]. Laves phases are strong and brittle with a brittle-to-ductile transition at about two thirds of the melting temperature. Various Laves phases have been regarded as promising for structural as well as for functional applications (e.g. as superconducting, magnetic or hydrogen storage materials) [2]. NbCo 2 crystallizes in the cubic MgCu 2-type structure which is illustrated in Fig. 1. In the completely ordered compound the larger atoms of the minority component Nb (open circles) from a diamond-type sublattice The sublattice of the smaller Co atoms (full circles) can be considered as a network of interconnected tetrahedrons. The NbCo 2 phase mainly exists on the Co-rich side of the stoichiometric composition [7-9]. For 1184 K a homogeneity range from 68.2 to 73.6 at.-% Co has been reported. For 1453 K this range extends from 66.4 to 76.2 at.-% Co [8]. Tracer self-diffusion of 57Co and 95Nb in the cubic Laves phase NbCo 2 has been studied. Information on diffusion in intermetallic Laves compounds is very scarce. To our knowledge this is the first time that self-diffusion of both constituents in a Laves phase is reported. A preliminary account of some of our results of Co diffusion has been published elsewhere [9]. The results are discussed in terms of vacancy mediated diffusion and compared with interdiffusion data which had been previously obtained in our laboratory from multiphase diffusion experiments and with self-diffusion in Li 2 intermetallics. EXPERIMENTAL Two alloys-Nb 31 Co 69 and Nb29Co 7 -inside the phase field of NbCo 2 were prepared by 1 repeated electron beam melting from the pure elements. The preparation of cylindrical diffusion
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