Abstract
Internal friction in ultrafine-grained Mg was measured by forced vibration method over a wide temperature interval from room temperature up to 750 K and at several frequencies. The specimens were prepared by milling procedure in an inert atmosphere and subsequent compaction and hot extrusion. The linear grain size of specimens used was estimated by X-ray line profile analysis to be about 100-150 nm. Two peaks were obtained in the internal friction spectrum, one at a temperature of ≈ 375 K and one at ≈ 631 K (at the frequency of 1Hz). The influence of prestraining and subsequent annealing was observed on the low temperature peak. The activation energy of the two peaks was estimated to be 1.16 eV and 1.77 eV, respectively. The activation volume has been estimated for low temperature peak. The low temperature peak has dislocation origin, while the high temperature peak is conditioned by the grain boundary sliding.

Key words
ultrafine-grained material, internal friction, dislocation motion, grain boundary sliding

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