Ultrasonic dislocation dynamics in Al (0.2 at.% Zn) after elastic loading

Ultrasonic resonance measurements at frequencies near 1 MHz were performed as a function of time and temperature on polycrystalline Al (0.2 at.% Zn) subjected to uniaxial loading below the plastic regime. The application of loads induced a decrease in the resonant frequency (f) and an increase in the damping (Q−1) that fully recovered nonexponentially with time when the load was released. The relative magnitude of the changes in f and Q−1 cannot be explained by a single anelastic relaxation process, considering that no significant frequency dependence was observed. The changes in f may be dominated by a purely elastic effect or a second anelastic effect that is relatively rapid. Analysis of the measurements of f after loading is performed under the assumption that the changes are elastic and associated with the diffusion of point defects to dislocations. Data at three temperatures are fit to a theoretical expression for the time-dependent number of point defects bound to a dislocation, yielding an activation energy for diffusion of 0.69 eV. This activation energy is in the range of values reported for vacancies in aluminum.