High-purity single crystals of stoichiometric composition were prepared by crucible-free induction melting under an Ar atmosphere. The internal friction was investigated, at temperatures ranging from 300 to 1400K, by using a low-frequency sub-resonance torsion apparatus operating under forced vibration conditions. In the case of non-deformed single crystals, the internal friction above 1000K exhibited a strongly increasing and frequency-dependent damping of up to 0.3 at 1400K. After plastic deformation, a maximum at about 1250K (0.1Hz) was superposed on this high-temperature damping. The activation enthalpy for the high-temperature increase was deduced to be 2.65eV, while that for the superposed maximum was 2.9eV. The results were explained in terms of the diffusion-controlled climb of dislocations via vacancies.

M.Hirscher, E.Schweizer, M.Weller, H.Kronmüller: Philosophical Magazine Letters, 1996, 74[3], 189-94