Stress State Required for Pyramidal Dislocation Movement in Zinc

A tension or compression stress in such a direction that basal slip is minimized can produce second-order pyramidal slip bands in zinc single crystals. The stress required to initiate pyramidal dislocation motion is not sensitive to temperature. However, dislocation velocity at a given stress is sensitive to temperature and the very small dislocation velocity at low temperatures has lead to an erroneous estimate of a ``starting stress'' for pyramidal dislocations. Dislocation velocity at a constant temperature was found to be a function of the magnitude, but not the sense of the resolved shear stress

Twinning and Slip in Zinc by Indentation

Observations of twinning and slip deformation caused by indentation of zinc reveal that extensive slip on the basal and second-order pyramidal systems takes place at loads up to 5 kg. Prismatic punching through 1-cm crystals is observed at indentation loads in excess of about 2.5 kg. It is concluded that the stress at the tip of the twins cannot be obtained by use of an elastic stress analysis

Orientation Dependence of a Dislocation Etch for Zinc

The dislocation etch for (101-[bar]0] surfaces of zinc reported by Brandt, Adams, and Vreeland have been further explored. Additional surface orientations have been found where dislocation etching takes place. These orientations cover an area located between 3 degrees and 12.2 degrees to the [0001], and the area is symmetric about that axis. Attempts to produce dislocation etching on within 2 degrees of (0001) were generally unsuccessful. This is in contrast to etching of many crystals which takes place only within a few degrees of a low index plane

Dislocation velocity on the {1212}<1213> slip systems of zinc

Dislocation velocity on the {1212} slip systems of zinc monocrystals was deduced from the rate of growth of slip bands. Near 77°K dislocation velocity is directly proportional to stress, and screw dislocations move more rapidly than edge dislocations. The difference between edge and screw dislocation velocity can be interpreted in two ways. The pre-exponential factors in a thermal activation model may differ by a factor 4 while the common activation energy is 0.21 eV, or the pre-exponential factors are the same, but the activation energy for edge dislocations (0.22 eV) exceeds that for screws by 5%. Other experiments will be required to establish the appropriate model. The authors favor the second alternative since extra activation energy might be needed to change the core structure of the edge dislocations (which lie on the basal planes) to make them glissile. Near room temperature, dislocation velocity decreases and cross-glide increases with increasing temperature. It is suggested that dragging dipoles and debris caused by their dissociation are responsible for the decrease in dislocation velocity. Finally, it is shown that the temperature dependence of both the yield strength and the plastic modulus is similar to the temperature dependence of the stress required to produce a constant dislocation velocity

Dislocation Velocity on the {1212}〈1213〉Slip System of Zinc

Dislocation velocity on the {1212}〈1213〉slip systems of zinc monocrystals was deduced from the rate of growth of slip bands. Near 77°K dislocation velocity is directly proportional to stress, and screw dislocations move more rapidly than edge dislocations. The pre-exponential factor in a thermal activation model is the same for edge and screw dislocations, but the activation energy for edge dislocations (0.22eV) exceeds that for screws by 5%. It is postulated that the larger activation energy for edge dislocations is due to their dissociation in the basal plane. Near room temperature dislocation velocity decreases and cross-glide increases with increasing temperature. It is suggested that dragging dipoles are responsible for the decrease in dislocation velocity. Finally, it is shown that the temperature dependence of both the yield strength and the plastic modulus is similar to the temperature dependence of the stress required to produce a constant dislocation velocity

Dislocation density and rate effects in the twinning of zinc

This note presents several observations which indicate that twinning in zinc single crystals is influenced by the pre-existing dislocation structure and by the multiplication of slip dislocations which can occur prior to twinning. These observations were made during the course of an investigation of second-order pyramidal slip, i.e. the {1212}〈1213〉slip system. An order of magnitude increase in the density of basal dislocations is found to increase the [0001] axis compressive stress for twinning by a factor of 3