Cell-Wall Dislocation Damping in Single-Crystal Cu

The dislocation substructure of 〈111〉 Cu single crystals has been controlled by varying crystal growth rates from 1.57 to 109 cm/h. Etch-pitting and x-ray studies reveal that the substructure consists of a random dislocation network, with density 1×106/cm1 independent of growth rate, and dislocation cells whose size and intercell misorientation angle increase with decreasing growth rate. Longitudinal resonance bar and ultrasonic pulse-echo damping measurements at frequencies from 25 kHz to 100 MHz show that the larger the cell size, the higher the damping over the entire frequency range. There is strong evidence that damping from cell-wall dislocations overshadows that due to the random network. The results are interpreted in terms of a modification of the Granato-Lücke damping theory, which treats cell walls as pseudomembranes vibrating under the action of the applied stress.