Observation of acoustic harmonics generated by long-range motion of dislocations

We report, for the first time, observations of the acoustic harmonics generated by steady-state vibration at amplitudes so large that long-range dislocation motion occurs. Most of the measurements have been made on the second and third harmonics A2 and A3. At low amplitudes the experimental results of Hikata and Elbaum et al. are confirmed, but there is some discrepancy with theory for A3. At amplitudes such that dislocation sources operate, the harmonic spectrum departs strongly from the Hikata-Elbaum predictions. The harmonic amplitudes do not always increase monotonically with strain; A2 occasionally shows a maximum, A3 sometimes, and A5 even more frequently, in a pattern sensitive to mechanical history. The spectrum depends on grain size and prestress, varying jointly with the modulus defect and internal friction as these indicators of dislocation motion vary. The results on strain dependence are in accord with a simplified model of dislocation breakaway and Frank-Read source operation proposed previously to account for the internal friction and modulus defect.