The Mechanism of Work-hardening of Metals

Abstract

The most striking feature of the deformation of metals is the formation of slip lines. Recent investigations suggest that, when formed at low temperatures, each slip line is the result of a displacement of the material along a single lattice plane through a distance of about a thousand atomic diameters. Moreover, there is much evidence that the steps on the surface which appear as slip bands attain their full height in a small fraction of a second, though their length may thereafter increase slowly. At higher temperatures and at slow rates of strain the slip bands appear, under the electron microscope, as clusters of lines about a hundred atomic diameters apart. The origin of slip lines, the reason for this clustering and the cause of work-hardening are discussed. The two conceptions used in the discussion are the dislocation line and the vacant lattice site. Slip lines are believed to have their origin in a certain arrangement of dislocation lines of frequent occurrence in the interior of the crystal. These are known as Frank-Read sources; their relation to recent work on the growth of crystals is shown. Where a slip line terminates dislocations must remain in the crystal; to the internal strains round these is ascribed work-hardening, much as in Taylor's theory of 1934†. It is now, however, possible to explain what it is that stabilizes the dislocations and prevents them from moving back when the stress is released. Finally, vacant lattice sites are shown to be formed in a cold-worked material. If the temperature is high enough for them to diffuse, they soften the material round the slip band and allow the observed clusters to form. They also play a part in producing the observed “fragmentation” of cold-worked material.