Effect of Impurity Clustering on Elastic Precursor Decay in LiF

Abstract

Experimental data are presented for shock propagation along a $\text{〈100〉}$ crystallographic direction in single‐crystal LiF. The initial impact stress for these measurements was 29 kbar. For crystals containing two different divalent impurity concentrations, elastic precursor amplitudes were measured as a function of propagation distance. In agreement with quasistatic yield stresses, it was found that the degree of impurity clustering strongly influences dynamic yielding. For crystals containing 130 ppm of Mg⁺⁺ ions, the precursor amplitude at a propagation distance of 2 mm decreased from 17.3 kbar for annealed specimens to 5.6 kbar for samples air quenched from a high temperature. For crystals containing 270 ppm of Mg⁺⁺, a smaller change in precursor amplitudes occurred upon quenching. The dislocation density remained unchanged for the heat treatments mentioned here. From these measurements, it is concluded that the rate of attenuation of high‐amplitude elastic waves is reduced for a given defect concentration if the defects are clustered.