A model for the time evolution of the internal friction observed in some fcc metals under irradiation: The peaking effect

The time evolution of the internal friction that has been observed in certain fcc metals (Cu, Ag, Al) under irradiation, and known generally as the ’’peaking effect,’’ has been shown by a recent set of experiments1–2 to be inadequately explained by the existing models which describe the energy dissipation through dislocation motion by the well-known analogy to a vibrating string with viscous damping. A phenomenological model is thus proposed here that introduces a source of energy dissipation through dislocation motion which is independent of the dislocation velocity, viz., nonviscous damping. With dislocation pinning by thermally diffusing point defects and strain- dependent depinning also included in the model, the observed internal-friction dependence upon irradiation time as well as upon the applied strain amplitude and frequency are well described.