Ultrasonic Study of Creep in Sodium Chloride

Abstract

The attenuation and velocity change of a 10-MHz sound wave and the shear strain ε have been measured simultaneously during transient creep in single crystals of sodium chloride. Quantitative determinations of the dislocation den ity A and average dislocation segment length L were obtained from the ultrasonic measurements. A was found to be approximately proportional to ε, and during a given run, L went through a maximum several minutes after a run was started. A model which meets all the limitations imposed by the simultaneous ultrasonic and strain measurements is proposed, according to which transient creep in sodium chloride proceeds by a two-step process. First, edge dislocations move quickly away from the dislocation source, giving an elongated loop composed primarily of two screw segments. The slower screw portions of the loop then move to give the observed strain. The velocity of the screw dislocation thus determines the strain rate ε̇. This model is supported by an interpretation of directly observed shapes of dislocation loops in alkali halide crystals. As the increase in the internal stress σG, determined from the increase in A during a run, was found insufficient to account for the decrease in ε̇, the mobile screw-segment length Ls must decrease. An atomistic theory of transient (logarithmic) creep was developed from this finding. From the theory, the screw-dislocation-segment length Ls was found to be the same order-of-magnitude as that found ultrasonically, and the derived screw-dislocation velocity was found to be in good agreement with that determined independently from an etch-pit study.