H. Schultz

The dislocation-internal friction peak γ in tantalum

Torsion-pendulum measurements were carried out on high-purity single crystal specimens of tantalum, having extremely low oxygen contents ( < 1 at. ppm), and on similar specimens doped with small, controlled amounts of oxygen (7 at. ppm). Our investigations are mainly concerned with the γ peak, which is usually explained by kink-pair formation in screw dislocations, and with the γ2 peak, which appears close to γ is small traces of oxygen are present. The γ2 peak was formerly explained as a “dislocation-enhanced Snoek peak”. The γ peak recovers at the peak temperature, whereas the γ2 peak is more stable. On the basis of our results, and making use of earlier investigations of Rodrian and Schultz, we suggest that γ2 is modified γ relaxation, related to screw-dislocation segments, stabilized by oxygen-decorated kinks. The stability of the γ2 peak allows an accurate determination of the activation energy, found to be 1.00 ± 0.03 eV. This value is distinctly lower than the activation energy of the oxygen Snoek effect (1.10 eV) and is related here to the mechanism of “kink-pair formation” in screw dislocations, as the original γ peak. The numerical value is compatible with recent values derived from flow-stress measurements. The peak γ2 shows increasing stability with increasing oxygen content. This is explained by single- and multi-decorated kinks.

Long-range migration of self-interstitials and dislocation pinning in niobium

The authors present experimental evidence for pronounced dislocation pinning in niobium beginning at 6 +- 0.2 K due to defects introduced by 2.3 MeV electron irradiation. This indicates long-range diffusion of radiation-induced interstitial atoms in niobium at 6 K. On the other hand, resistivity results to be presented elsewhere.