Diffusion in Gamma Uranium

Abstract

The diffusion of radioactive Cr, Mn, Fe, Co, Ni, Cu, and Nb tracers in bcc gamma uranium has been measured from 780 to 1080\ifmmode^\circ\else\textdegree\fi{}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 $\ensuremath{\beta}\ensuremath{-}\ensuremath{\gamma}$ 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, ${10}^{\ensuremath{-}4}$-${10}^{\ensuremath{-}2}$ ${\mathrm{cm}}^{2}$/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 ($D\ensuremath{\approx}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ ${\mathrm{cm}}^{2}$/sec at 950\ifmmode^\circ\else\textdegree\fi{}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 $\ensuremath{\gamma}$-U by additions of Co.