High-velocity dislocation damping in aluminum

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Abstract
High‐velocity compression tests were conducted on pure polycrystalline aluminum. Data obtained at compressive strain rates ranging from 0.4×104 to 12×104 sec−1 could be divided into two regions of interest. For strain rates from 0.7×104 to 1.4×104 sec−1 a linear relationship was found to exist between the applied stress and the strain rate, indicating that the moving dislocations are viscously damped in this region. The values of the damping constant B obtained in this region ranged from 1.9×10−4 to 3.0×104 dyn sec/cm2 and showed good agreement with B determined by other investigators. Beyond these strain rates, the damping reduced at first and then increased, with very high stresses being obtained at the highest strain rates reached. The drop in the damping has been attributed to the reduction in the volume around the cores of the moving dislocations that can exchange energy with the lattice phonons. The rise in the damping at the higher strain rates may be due to relativistic effects as the moving dislocations approach the velocity of sound. Using the expression for the strain field around a fast‐moving dislocation in the phonon viscosity theory and the general expression for dislocation damping due to phonon scattering, an expression for the damping constant was obtained embodying these effects. This was found to show good agreement with the data.

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