CXVII. A theory of work-hardening of metal crystals

Summary A theory of work-hardening is put forward ; the salient points are as follows. There are two types of hardening ; the normal rapid hardening of cubic metals and the slower hardening characteristic of hexagonal crystals and cubic metals under conditions of easy glide. A quantitative theory of the rapid hardening is given. This does not occur unless dislocations are retained in the slip planes. The retained dislocations are supposed to occur in groups of about 1000 at the ends of the slip-bands. The strains round these groups are responsible for hardening as in Taylor's (1934) theory. The obstacles which stop slip are sessile dislocations, and the formation of sessile dislocations stabilizes the groups of dislocations. It is shown that the formation of slip lines must be a dynamic process, and on this assumption a parabolic stress-strain curve is deduced. Some discussion is given of the formation of vacancies by moving dislocations, and it is shown that this can account for the clustering of slip lines observed under the electron microscope. The theory agrees qualitatively with observations on the amount of retained energy in a cold-worked metal.