The change of the elastic moduli and the internal friction of copper single crystals of various orientations during deformation at 78 K was measured. First the elastic moduli change due to the fact that the crystal orientation changes upon plastic extension. An analogous effect is the change of the moduli due to the change of orientation of part of the crystal by mechanical twinning. Finally the dislocations produced by the deformation give rise to a decrease of the moduli and an increase of the internal friction. The contribution of these mechanisms to the observed effects is discussed. The dislocation contribution is analyzed in terms of the Koehler-Granato-Lücke model yielding the density and line lengths of the contributing dislocations. The result is compared with literature data from transmission electron microscopy, leading to the conclusion that the dislocation part of the modulus effects and internal friction is due to the small fraction of total dislocation density lying between the dense dislocation tangles and having line lengths of the order of 1 μm. © 1975.
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