Dislocation relaxation processes in bcc metals: β versus γ relaxation

The ‘reversible’ γ relaxation observed by D’Anna and Benoit in Nb and the β relaxation (Chambers–Schultz notation) of plastically deformed bcc metals are shown to be caused by the same mechanism, namely the stress-assisted thermally activated generation of kink pairs on {110} slip planes in screw dislocations with Burgers vectors a 0⟨111⟩/2. The classical (‘irreversible’) γ relaxation is due to the analogous process on {112} planes. The different slip planes result from different core configurations of the screw dislocations. Internal-friction experiments on O-doped Nb by Igata et al. and the comparison of the flow stress of Nb of modest purity with that of high-purity bcc metals demonstrate that the transition between the low-temperature {110} configuration and the high-temperature {112} configuration may be shifted to higher temperatures by the incorporation of foreign interstitial atoms into the cores of the screw dislocation. This process may also account for the suppression of anomalous slip by interstitial impurities.