Dislocation Damping in Macrosonic Fields

Abstract

The damping of high-amplitude ultrasonic waves (macrosound) in the 40 kc/sec range is observed in high-purity aluminum single crystals at room temperature and at strains above ${10}^{\ensuremath{-}4}$. At strain amplitudes between ${10}^{\ensuremath{-}4}$ and ${10}^{\ensuremath{-}3}$, the decrement ranges from 1 to 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}2}$ and levels off at \ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}2}$ for strains above ${10}^{\ensuremath{-}3}$. The dependence of decrement on irradiation time shows a rapid increase at the beginning of irradiation ($\ensuremath{\alpha}$ region), a somewhat reduced damping over a longer period of time ($\ensuremath{\beta}$ region), and another increase in damping before termination of the experiment ($\ensuremath{\gamma}$ region). The data lie by about two orders of magnitude above those from previous observations on internal friction to which the Granato-L\"ucke theory and associated concepts apply. The present experimental conditions are discussed from the viewpoint of a refined model which accounts for the irreversible changes in dislocation structures caused by macrosonic irradiation and revealed by transmission electron microscopy. Indications are given of points requiring further study.