Diffusion of copper and silver in zirconium

Abstract

B.c.c. zirconium (/3-Zr) is one of the well-known anomalous b.c.c, metals, which show a significant upward curvature in the Arrhenius plot of the self-diffusion coefficients [1]. H.c.p. zirconium (cr-Zr) also shows peculiar self-diffusion behaviour, i.e. downward curvature in the Arrhenius plot [2]. Furthermore, it is well known that impurity atoms with small size such as Fe and Co are fast diffusers in both fl-Zr [3] and cr-Zr [3]. However, according to a recent compilation of a vast amount of experimental data on diffusion in solid metals and alloys [4], a large scatter in the diffusion parameters (the preexponential factor Do and the activation energy Q) for impurity diffusion of some elements in/3-Zr and ol-Zr is reported. For example, the values of the activation energy for the diffusion of Ag in o~-Zr by various researchers lie in a wide range between 174 and 245 kJ mol -I, all of which have been obtained by the radiotracer method. Thus, use of another experimental method may be helpful to elucidate the diffusion behaviour. On the other hand, no data on the diffusion of Cu in/3-Zr have been compiled [4]. The atomic radius of Ag (0.1406 nm) is larger than that of Cu (0.1244 nm) but smaller than that of Zr (0.1558 nm) in the b.c.c, structure [5]. Thus, diffusion experiments of Cu and Ag in Zr are effective for confirming the size effect for the impurity diffusion. In the work reported here, interdiffusion experiments with couples of pure Zr and dilute binary alloys containing Cu and Ag were carried out. The impurity diffusion coefficients of Cu and Ag in Zr were determined by applying Darken's relation [6], i.e. the extrapolated value of the interdiffusion coefficient to the infinite dilution of the solute can be regarded as the impurity diffusion coefficient of the solute in Zr. This method avoids problems such as chemical reaction on the surface of reactive Zr specimen that occur with the radiotracer diffusion experiments. A pure Zr rod 7 mm in diameter was made by Ar arc melting of Zr sponge of 99.6% purity supplied by Nippon Mining Co. Ltd. The main impurities in this material were (ppm): 852 O, 393 Fe, 38 Mg, 100 Cr, <60 Si and <50 Hr. The grain size in the rod was 1-2 mm. The Zr rod was cut to make disc specimens 5mm in thickness. Rods of Zr-1.57 at% Cu, Zr-0.34 at % Ag and Zr l .27 at % Ag alloys were made by Ar arc melting of the pure Zr sponge with Cu oxygen-free chips of 99.99% purity and Ag