Thermomigration and Electromigration in Zirconium

Experiments on directed atom motions in $\beta -\mathrm{Zr}$ have been carried out with a view to bringing additional experimental evidence to bear on the current conflicting theories of the anomalous diffusion behavior in this and other bcc refractory metals. Thermomigration and electromigration effects have been studied primarily by the surface-marker technique in zirconium wires purchased from the Materials Research Corp. (MRC) (purity 99.9%) and in rods supplied by the Oak Ridge National Laboratory (ORNL) (purity 99.98%). The thermomigration effects were large, with the mass flow to the hot (central) region of the specimens. The effective activation energy for this flow was 26±3 and 29±3 kcal/mole for MRC and ORNL Zr, respectively. These values are quite close to the activation energy for the so-called "low-temperature diffusion mechanism" ($Q=27.7\mathrm{}$ kcal/mole) obtained from tracer diffusion data. If one assumes that this is the operative mechanism, then $\frac{Q^{*}}{f}$ turns out to be - 34±11 kcal/mole. The fact that the forced motion shows no evidence of a high-temperature mechanism lends support to the hypothesis that this motion is primarily by dislocation, or short-circuit paths. The electromigration, on the other hand, is a very small effect for this material, with motion apparently toward the cathode. The activation energy is in the neighborhood of 30 kcal/mole, and the effective charge $\frac{Z^{*}}{f}$ is about +0.3.