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The flow stress distribution below the surface of previously deformed large copper single crystals is obtained when these are sliced into smaller component crystals. It is found that a considerable flow stress gradient extends for a distance of about 2 mm below the surface, measured along the slip direction. Electron microscope studies show that there is an increase in the cell size of dislocation structures near the surface and a decrease in dislocation density.On the basis of flow stress distribution measurements the stress-strain curves of single crystals of the same orientation but different sizes are shown to be consistent with the complete absence of a stage II in an infinitely large crystal. It is further shown experimentally that for very large crystals the value of τ111 is reduced considerably. This leads to the conclusion that τ111 should not be related to a fundamental physical phenomenon such as the stacking-fault energy of a metal.

Thin copper single crystals with a common axial orientation but with two different surface orientations are deformed in tension. The surface orientations were chosen so that in the one case the maximum glide path length of edge dislocations is reduced-fey thinning the specimen by electrolytic jet polishing of the surface while in the other the glide path length of screw dislocations is reduced. The extent of strain (ε11) in Stage I of the stress-strain curve is found to depend strongly on the glide path length of edge dislocations, while it depends only slightly on the glide path length of screw dislocations. Thin crystals with edge dislocation glide path lengths of about 50 microns can sustain the Stage I mode of deformation at applied stress levels three to four times that of crystals of normal thicknesses, while the rate of hardening (θ1) shows no systematic variation with thickness. It is concluded that the extent of strain in Stage I depends on the rate of accumulation of a primary dislocation array giving rise to an internal stress, while the rate of hardening depends on the rate of increase in the density of forest dislocations.

Deformed single crystals of copper are sectioned on the primary slip plane. The sections are studied by transmission electron microscopy and the natures of dislocation dipoles are determined on the basis of the kinematical theory of electron diffraction. It is found that both vacancy and interstitial dipoles are present. Dislocation configurations associated with open dipoles are analysed. It is suggested that open dipoles which originate at sessile long jogs in a particular screw dislocation are ended-off by glissile long jogs in the same screw dislocation to form closed dipoles.

The formation of elongated dislocation loops in deformed copper single crystals has been studied by transmission electron microscopy. A mechanism by which elongated loops could be formed is considered in detail. The density of these loops was found to increase only slightly during stage I of the stress-strain curve, but during the transition from stage I to stage II the density increased from about 5 × 1011 loops/cm3 to about 1013 loops/cm3. A model is proposed for work-hardening in the easy glide and transition regions, in which the dislocation loops indirectly act as obstacles to moving screw dislocations.

The removal of a surface envelope by chemical polishing of an unloaded crystal, previously deformed plastically in tension, is shown to reduce the unloading yield point. In addition it is shown that the yield point changes continuously with stress and a maximum effect is obtained at a given stress which depends on the crystal size. Furthermore the value of the maximum is a function of crystal size. On the basis of these results it is argued that the unloading yield point can probably be attributed to the non-uniform flow stress distribution which exists near the surface in deformed crystals.

The sub-surface region of a copper single crystal which had been strain hardened in tension to the end of stage II was studied by transmission electron microscopy. Sections bounded by the original surface as well as sections further below the surface were investigated. It was found that the sub-surface dislocation structure was in agreement with flow stress distribution curves which indicate the existence of a soft surface - hard core arrangement in strain-hardened crystals.