Diffusion in Gamma Uranium

The diffusion of radioactive Cr, Mn, Fe, Co, Ni, Cu, and Nb tracers in bcc gamma uranium has been measured from 780 to 1080°C, using the thin-layer sectioning technique. The Arrhenius plots are curved at low temperatures; the curvature is not due to diffusion along grain boundaries or dislocations, or to defects introduced by the $\beta -\gamma$ phase transformation. The activation energies and frequency factors calculated from the linear portion of the Arrhenius plots above the bend are very low (12-39 kcal/mol, ${10}^{-4\mathrm{}}$-${10}^{-2\mathrm{}}$ ${\mathrm{cm}}^2$/sec) and do not agree quantitatively with any known theory of impurity diffusion. The motion of Kirkendall markers in a very small chemical concentration gradient indicates that diffusion is by a defect mechanism; considerations of atomic size rule out interstitials as the defects. Co and Fe diffuse very fast ($D\approx 2\times {10}^{-6\mathrm{}}$ ${\mathrm{cm}}^2$/sec at 950°C), indicating a strong Co-(or Fe-) vacancy binding energy, and a high degree of correlation between the directions of successive vacancy jumps. This is substantiated by the acceleration of self-diffusion in $\gamma$-U by additions of Co.