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Dislocation Velocity Measurements in Copper and Zinc

Vreeland, T., Jr. (1967) Dislocation Velocity Measurements in Copper and Zinc. , Pasadena, CA. (Unpublished)

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Studies of dislocation dynamics in a number of different materials have been reported, but no direct measurements in FCC or HCP metals have appeared in the literature. Studies of slip band growth in the basal system of zinc at this laboratory have indicated that individual dislocations achieve relatively high velocities at the yield stress. This implies that only very short duration loading (µsec) of low dislocation density crystals (<1000 cm^(-2)) will permit direct measurement of the dynamics of motion of individual basal dislocations in zinc. Suitable single crystal specimens of 99.999+% copper and zinc were prepared for this study. A torsion loading system was developed to apply single, short duration stress pulses to the specimens. This system generates a zero mode torsional wave front with a rise time of 2 µsec in a 1.25 cm diameter elastic rod. The loading wave front, after passing through a specimen crystal, is reflected at a free surface normal to the cylindrical axis. The reflected wave unloads the specimen, and the duration of the stress pulse at any point is just the round trip travel time of the wave front from that point to the free surface. The torsion strain on the cylindrical surface of the elastic rod is monitored using semiconductor strain gages. The stress on a cross section of the specimen crystal varies linearly from zero on the cylindrical axis to a maximum at the surface. Dislocations were observed before and after stressing. Copper cylinders with [100] axes, and 4, 3mm wide flats on {100} surfaces were tested at room temperature. Dislocation displacements up to 100µ into regions initially free of dislocations were measured on the {100} flats using the double etch technique. The dislocations were of mixed edge screw orientation. The behavior of fresh dislocations produced by scratching and of isolated aged dislocations was not significantly different. The displacement was found to be a linear function of distance from the free end, therefore, the displacement was linearly proportional to the time of stress application. A nearly linear relationship between dislocation velocity and stress was found, stresses from 2.8 Mdyne/cm^2 to 23.1 Mdyne/cm^2 produced velocities from 160 cm/sec to 710 cm/sec. Zinc crystals with a [0001] cylindrical axis and (0001) end surfaces were used to study basal dislocation mobility at room temperature. One end surface was scratched prior to stressing to produce fresh edge dislocations. The end surfaces were then bonded to elastic rods and a torsion pulse was applied. The bond on the scratched end of the crystal was removed, and displacement of dislocations from the scratches was observed using the Berg-Barrett X- ray diffraction technique. Dislocation displacement was found to be directly proportional to radial position from the cylindrical axis, which implies that the basal dislocation velocity is directly proportional to stress. Dislocation displacements up to approximately 400µ were measured. Interaction between different basal dislocations in these tests was negligible. The maximum test stress of 17.2 Mdynes/cm^2 produced a dislocation velocity of approximately 600 cm/sec. The mobility of basal dislocations in zinc was considerably reduced by aging for eight hours after scratching. This is attributed to the accumulation of jogs along the dislocations which were within 5µ of the observation surface. These results indicate that the dislocation drag in copper and the basal system in zinc is relatively small compared to that in other materials in which direct observations have been made. The drag agrees well with that predicted for phonon interaction with the moving dislocations, and with the value deduced from internal friction measurements in copper. Tests are in progress to see if the dislocation velocity increases as the phonon interaction is reduced at lower temperatures. Slip on the {1212} 〈1213〉 system of zinc occurs by the formation and growth of slip bands. A dislocation etch and the Berg-Barrett technique were used to observe the growth of the slip bands produced by compression pulse loading of specimens in the [0001] and 〈1210〉 directions. The majority of slip bands are nucleated at substructure boundaries, and the bands grow at essentially equal rates in all directions on the {1212} slip planes. Dislocation velocity at room temperature was found to be proportional to stress to the 8.7th power. The velocity at a given stress decreases with a decrease in temperature, and also decreases as the initial dislocation density increases. This indicates that motion of {1212} 〈1213〉 dislocations is assisted by thermal activation and that forrest dislocations may be the most significant obstacles to their motion. Quantitative studies of the temperature and stress dependence are underway.

Item Type:Report or Paper (Technical Report)
Additional Information:This work was sponsored by the United States Atomic Energy Commission under Contract No. AT(04-3)-473. An extended abstract of a paper to be given at the Seattle-Harrison Colloquium on "Dislocation Dynamics", May 1-6, 1967 sponsored by Battelle Memorial Institute. Submitted to the U. S. Atomic Energy Commission under Contract # AT(04-3)-473. CALT-473-13.
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Atomic Energy CommissionAT(04-3)-473
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:54151
Deposited By: Tony Diaz
Deposited On:28 Jan 2015 06:41
Last Modified:28 Jan 2015 06:41

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