The dislocation-internal friction peak γ in tantalum

Abstract

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 γ 2 peak, which appears close to γ is small traces of oxygen are present. The γ 2 peak was formerly explained as a “dislocation-enhanced Snoek peak”. The γ peak recovers at the peak temperature, whereas the γ 2 peak is more stable. On the basis of our results, and making use of earlier investigations of Rodrian and Schultz, we suggest that γ 2 is modified γ relaxation, related to screw-dislocation segments, stabilized by oxygen-decorated kinks. The stability of the γ 2 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 γ 2 shows increasing stability with increasing oxygen content. This is explained by single- and multi-decorated kinks.