The recovery of internal friction in sodium chloride

The internal friction of plastically deformed sodium chloride is due predominantly to the presence of mobile dislocations and is sensitively affected by the interaction between these and other lattice defects. This paper describes how this phenomenon has been used to study the interaction between dislocations and point defects in single crystals of sodium chloride. Internal friction has been measured in pure and cadmium-doped crystals as a function of vibration amplitude, temperature, pre-strain and time, and the results have been interpreted in terms of existing theories. The state of dispersion of point defects is an important factor to consider when interpreting internal friction measurements. Dielectric loss, d.c. conductivity and polarographic analysis measurements have been carried out in conjunction with the internal friction measurements to provide information about the distribution of point defects. From these measurements it has been shown that the predominant, mobile, dislocation pinning defects are divalent cations or associated divalent cation-cation vacancy pairs, and information has been obtained about the kinetics of the pinning process in the temperature range 0°c to 53°c.