Effects of Point Defects on Elastic Precursor Decay in LiF

Experimental data for shock propagation along a 〈100〉 direction in single-crystal LiF show that elastic precursor decay is critically dependent on the origin of the sample. The most obvious differences among samples used are in their concentrations of impurities. It is suggested that divalent cation impurities are responsible for variations in precursor decay, and this is supported by results from a set of samples irradiated with γ rays to produce F centers. For the observed range of defect concentrations, quasistatic yield stresses varied monotonically with concentration from 0.02 kbar for pure crystals to 1.0 kbar for the hardest material studied. In the shock loading experiments both hard and soft crystals showed an initial rapid decay of the precursor to near-equilibrium values of about 2 kbar for the softest crystals and about 6 kbar for the hardest. For crystals of intermediate hardness the decay was much slower. From observed effects of annealing before shocking it is inferred that dislocation mechanisms in shock differ from those believed to operate at low strain rates. Impact stress for all experiments was about 28.6 kbar and sample thicknesses ranged from 0.27 to 15.44 mm.