Kilocycle-Range Dislocation Damping in Magnesium Oxide

Measurements of temperature, frequency, and amplitude dependence of dislocation damping in plastically deformed MgO have been made in the kilocycle range. The low-amplitude damping shows a broad maximum around 150°C, with little variation below room temperature down to 78°K. The damping decreases slightly with increasing frequency and the maximum moves to a slightly higher temperature. At strain amplitudes of greater than about 2×10−6 the damping increases steadily. The excess damping does not give a straight Granato-Lücke plot; it usually gives a relatively straight line of slope 0.8 on a log δ vs log ε plot. The amplitude-dependent damping varies more strongly with temperature than the low-amplitude damping. Annealing at temperatures between 350°C and 650°C causes a steady decrease of damping which may be associated with a process having an activation energy of 2.0 to 2.6 eV. A composite annealing curve, obtained from curves taken at different temperatures, yields a damping which varies as (annealing time)−0.4 over many decades of time. Varying concentrations of many impurities, including iron, seem to have little effect on the behavior, but the introduction of 0.05 at.% chromium reduces temperature dependence and amplitude dependence and introduces a further process of lower activation energy in the early stages of annealing. The results reported here cannot readily be interpreted in terms of the usual theories of dislocation damping.