The Snoek-Koester relaxation in niobium and tantalum containing oxygen

The relaxation phenomena in oxygen-doped high-purity niobium and tantalum after plastic deformation are investigated by means of internal-friction and mechanical after-effect measurements. It is shown that there may be two distinct Snoek-Koester (or 'cold-work') relaxations associated with interstitially dissolved oxygen, namely 'peak (1)' occurring at lower temperatures but possessing the higher activation enthalpy, and 'peak (2)' at higher temperatures but with the lower activation enthalpy. Peak (1) is much less stable against annealing than peak (2), which in the present work is studied in considerable detail. Its excess width is found to be due to a distribution in the pre-exponential factors of the relaxation times and not in the activation enthalpies, and its relaxation strength to be independent of temperature. The observations are quantitatively accounted for by a theory attributing the Snoek-Koester relaxation to the thermally activated formation of kink pairs on screw dislocations in the presence of foreign interstitial atoms. The expression for the activation enthalpy of the Snoek-Koester relaxation involves, in combinations that depend on the experimental conditions, the migration enthalpy of the foreign interstitial atoms, the kink-formation enthalpy, and the enthalpy of binding of the foreign interstitial atoms to screw dislocations, Hsup(B)sub(d). Comparison between theory and experiment gives us 0.23 eV <= Hsup(B)sub(d) <= 0.31 eV for Nb and 0.33 eV approx. <= Hsup(B)sub(d) approx. <= 0.55 eV for Ta. (orig.)