Atomic size and fast diffusion of metallic impurities in zirconium

The phenomenon of fast diffusion of metallic impurities in hcp- and bcc-Zr has been analyzed in terms of the atomic sizes of the solutes. Two different size criteria have been used, namely, the partial molar volume of the impurity at infinite dilution and the metallic radius for 12-fold coordination in the pure metallic state. The results obtained indicate that these two size criteria are complementary of each other. Also, it has been concluded that metallic impurities in hcp-Zr can be sharply classified into substitutional and interstitial ones as a function of atomic size. In turn, in the case of bcc-Zr such a sharp classification was not obtained, a situation which possibly is related to the particular softness of the bcc lattice. The solvents bcc-Ti and Nb have also been included in the discussion and found to behave in a similar way as in the case of bcc-Zr.