On the Damping of Screw Dislocation Motion in FCC Crystals by Phonon Viscosity

The phonon viscosity mechanism, as it applies to screw dislocations moving on the {111} planes of face-centered-cubic crystals, is examined. Formulas are derived for changes in the elastic stiffnesses of cubic crystals with the impression of an arbitrary elastic strain. The result is specialized to the case of the strain field of a screw dislocation in a face-centered-cubic crystal. Lattice energy absorption from a moving screw dislocation is then considered through the relaxation of the elastic stiffnesses. Using the formula for energy absorption, a dislocation damping coefficient, B, is found. B is independent of temperature above the Debye temperature, and at lower temperatures has different forms for anisotropic and isotropic crystals. The damping coefficient for an edge dislocation is qualitatively similar to that of the screw dislocation. The results are consistent with recent measurements of the damping coefficient for dislocations in copper.