The flow of polycrystalline metals under simple shear

Abstract

The creep of three polycrystalline metals-commercial lead, pure lead and cadmium- has been investigated under conditions of simple shear, by a method in which a constant couple is applied to an annulus cut in a thicker disk of the metal. For comparison, normal tensile tests have been carried out on wires of the same metals. It has been found that for all three metals the shear strain under simple shear stress varies as the cube root of the time, there being no permanent creep even at temperatures at which, in tensile tests at constant stress, the creep is predominantly linear with time. The effect of varying stress and temperature has been investigated. The pure lead recrystallizes under conditions of strain which have been examined, the recrystallization being attended by accelerated flow. The method allows the creep under reversed stress to be examined. In reversal there is a contrast between lead, which shows shear strain linear with time, and cadmium which shows a variation according to t$^{\frac{1}{3}}$, as with direct stress. This difference of behaviour is explained in terms of the glide properties of cubic as contrasted with hexagonal metal single crystals. The effect of a second reversal of stress is consistent with this explanation. The cause of the elimination of permanent creep under the conditions of simple shear is discussed and a variety of experimental results explained in terms of intracrystalline and boundary effects, account being taken of the different crystal structure of cadmium and lead.