Experimental measurements of dislocation mobility and density and the strain rate sensitivity of the low stress have been made on 99.999 percent pure zinc orys-. 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 mobile dislocations is assocd.
The effect of the aluminum on the basal stress-strain behavior is explained in terms. of changes in non-basal dislocation density that determine. the separation distance of attractive and repulsive junctions between basal and nonbasal dislocations. The plastic shear strain rate is dependent on the average velocity of the moving dislocations and is given by. The technique can be used to determine the Burgers vector of the observed dislocations.
The orientation of the test specimens, which differed between tests on basal and non-basal slip systems, is described in Part III. The results of the testing program are presented in Part V together with observations on the dislocation density of the deformed samples and the effect of purity on the dislocation density.
ETCHING OF ZINC TO REVEAL DISLOCATIONS An etching procedure which reveals etch figures
The arrows indicate the basal plane track on this surface and etch pits can be seen lined up in the general direction of ~the {i212} second order pyramidal slip plane tracks as indicated by the dashed lines. Areas of general background pitting resulted in great interest in determining whether changes in dislocation density and configuration had actually occurred as a result of the basal deformation. further experiment was performed on the previously mentioned hemispherical crystal of 2 in. diameter. diameter £1a:t area was exposed on tihe [oool]. pole by cleaving the crystal. However, the quality of the etched surfaces was too poor to be useful for the study of dislocation mobility.
Sing1e orysta1s 0£ each of the four different pur- . soils were cultured in graphite by the Bridgeman technique. The preparation of the molds and .. the details of the growth procedure were described by Stofel {23). Compression test specimens in the :Corm of 1/2 in. cubes were machined with three different orientations of the basal slip plane with respect to the load axis.
Test samples were machined from sawn and split sections of the single crystals using a servomet El.eotric Spark Discharge Machine. Uncertainty in the crystallographic orientation of the test samples relative to the surfaces was due to machining.
27- TABLE I
EQUIPMENT AND TEST PROCEDURES
Full-scale sensitivity of the capacitance system is a function of the plate spacing, and the instrument sensitivity setting for a given scale range of the meter and plate area. Two micrometer heads are used to measure the initial spacing of the parallel plate capacitors and. The cylindrical rod above the test specimen is attached to the cross head of the Instron testing machine.
In this way, any interaction between the loaded parts of the system and the plate holding the load measuring probes is avoided. The cylindrical rod is guided in the fixture plate in such a way that the specimen can be rotated about the load axis, since the axis of the rod is exactly aligned with the load axis. Speoim<:m, after being centered, was given a slight bias by manual control of the Instron crosshead.
The required distance between plate and sample was set after the position of the probe corresponding to the zero plate separation was found. Figure-e-1-1-i-sJ~, schematic drawing of the dynamic test setup together with the capacitor plates used to measure the voltage in 45° samples oriented for basic sli~/~ The.
40 TPI
The threaded connection allows the plate spacing to be adjusted. mother sensitivity_ proximity meter range. shows details of cargo seats and alienation sleeve. Load cell calibration was performed with dead weights after the compression fixture was mounted. The condenser plate system is calibrated over all sensitivity scales of the proximity meter to be used during each test.
The scales used depended on the final desired stress value in a given test and the length of the sample in the direction of loading. The overall sensitivity of the strain measurement system was determined by the initial plate spacing, the plate area of the capacitance meter, and the sensitivity scale of the proximity meter. During the tests, changes in the strain rate were made by changing the crosshead speed of the Instron in the ratio of l/~/{af o.
The elastic spring constant of the clamp was calculated from the measured capacitance readings and the loads applied to the brass sample. The measured spring constant and the load cell spring constant were used to estimate the errors involved in assuming that the crosshead velocity ratios were the same.
DISCUSSION OF RESULTS
The differences between basal and nonbasal slip can be sub-. stood in the form of a fundamental relationship given by Eq. the dislocation mobility relation and the rate of dislocation multiplication with strain are known for each mode of deformation. Etsepi ts were observed on. lOlO) surfaces of the test specimens parallel to. This is inferred from the observation that in several pulse tests pile-ups observed on one of the (lOlO) surfaces of the test specimen were located on the same sliding plane as pile-ups observed on the other (10l0) surface with.in the limit of measurement accuracy.
The results of the variable strain rate tests undoubtedly involve both edge and screw dislocations with (1210] Burgers vectors. Figures 3i and J2 of the present results illustrate the nature of the basal dislocation distribution in work-hardened specimens. A striking feature of the work -hardened state is the occurrence of large numbers of dislocation stack-ups.
The validity of the strain rate sensitivity experiment for the determination of mobility exponents has been argued from these results. A qualitative picture of the stress difference 1"'-i; as a function of distance along a slip plane is given in Fig. The specific number of dislocations released in this way will depend on the details of i.
An alternative interpretation of the results in terms of the newly proposed model is proposed. The effect of the dislocation density of the forest on the initial current stress can be estimated from a given dislocation density. Thus, a large proportion of the increase in current over that of 99.999% pure crystals can be attributed to changes in forest density. produced with the addition of impurities.
This conclusion must be correct even though the disl.ooatlon velooitleB measured directly was 0£ .. the principal dislocations in a slip band and thus represented the maximum velocities, since the variable strain rate test depends on the average velocities in a specimen covered with intersecting slip bands like those seen in ~ig. The results are inconsistent with predictions using theoretical models that include thermally activated motion of a slip dislocation past forest dislocations or impurity atoms. Much of the strain sensitivity is attributed to changes in the number of moving dislocations, which accommodate a change in strain rate, rather than a change in dislocation velocity. "mo9-e,l.. c) . can also be applied to nonbe.sa,l slip in zino and o'opper and deformed aluminum crystals in slight slip, in which large changes of strain rate with very small changes in stress.
Velocities between 2 :x: lo-4 and 2 cm/sec were measured in the stress range :from 300 to 800 lb/in.2 • The results are inconsistent with theoretical models involving thermally activated events. The addition of 0.0025 and 0.02 percent aluminum 'to z:i.no produoes: a s:egregat:lon substructure• and increases. the density of non-basal dislocations.
