Dislocation motion and generation in LiF single crystals subjected to plate impact

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Abstract
A plate‐impact technique is developed for subjecting low‐dislocation‐density single crystals to a submicrosecond square pulse under conditions which allow recovery of the crystal for examination of changes in dislocation configurations. Subsequent reflected waves in the crystal are minimized by use of a longitudinal‐momentum trap and a specially designed flyer plate shape. Transmitted wave profiles are monitored with a laser displacement interferometer. Dislocations are observed by means of etch pits. Experiments carried out on both annealed and irradiated single crystals of high‐purity LiF show newly generated dislocations occurring primarily in the form of glide bands. Average dislocation velocities computed from the lengths of interior glide bands are, for the annealed crystals, proportional to the resolved shear stress. The proportionality constant corresponds to a drag coefficient B?3×10−4 dyn sec/cm2, which agrees reasonably well with values determined previously at much lower (one to four orders of magnitude) stress levels. Thus, it appears that, in annealed high‐purity LiF at room temperature, the intrinsic resistance of the lattice to the motion of dislocations is characterized adequately by a linear viscous drag for dislocation velocities in the range from near zero to at least one‐fourth the elastic shear‐wave speed.

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