B.c.c. zirconium (β-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 (α-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 β-Zr [3] and α-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 pre-exponential factor Do and the activation energy Q) for impurity diffusion of some elements in β-Zr and tf-Zr is reported. For example, the values of the activation energy for the diffusion of Ag in α-Zr by various researchers lie in a wide range between 174 and 245 kJ mol-1, all of which have been obtained by the radiotracer method. Thus, use of another experi-mental method may be helpful to elucidate the diffusion behaviour. On the other hand, no data on the diffusion of Cu in β-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, diffu-sion experiments of Cu and Ag in Zr are effective for confirming the size effect for the impurity diffusion.
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