Dislocation Mobility and Density in Zinc Single Crystals

Abstract

Experimental measurements of dislocation mobility and density, and the strain-rate sensitivity of the flow stress have been made on 99.999 per cent pure zinc crystals. Dislocation density and the strain-rate sensitivity of the flow stress were also measured on zone refined crystals and crystals containing 0.0025 and 0.02 wt per cent aluminum. Dislocation mobilities in the 11 2 0 {0001} (basal), and 1 2 1 3 {1 2 12} (nonbasal) slip systems were measured by observing slip band growth produced by load pulses of controlled amplitude and duration. The results of the experimental measurements of dislocation mobility are discussed in relation to current theories. A comparison of the strain-rate sensitivity and the mobility measurements shows that a significant change in the density of roving dislocations is associated with a change in strain-rate. This change in density has generally been ignored by previous investigators. A dislocation model is proposed to explain the observed strain-rate sensitivity. Observations were also made of the change of substructure and in particular the change of nonbasal dislocation density accompanying impurity additions of aluminum to the zinc. The effect of the aluminum on the basal stress-strain behavior is explained in terms of changes in nonbasal dislocation density which determines the separation distance of attractive and repulsive junctions between basal and nonbasal dislocations. The onset of basal slip is associated with the breaking of attractive junctions. The change in basal dislocation density produced by plastic shear strain is shown to obey the relation Δρ = C୪p1/3, and is independent of purity. A markedly different relation is indicated for the nonbasal dislocation density vs. strain. These results are explained by a significant difference in the average glide distance of dislocations in the basal and nonbasal slip systems.