The flow of polycrystalline metals under simple shear. II

Abstract

The flow of lead of different purities under conditions of simple shear has been investigated over a wide range of strain, at different stresses, and mostly at a temperature in the neighbourhood of 27 °C. The phenomena have been studied both with unchanged direction of stress and with the direction of stress reversed during flow. At the beginning of forward creep the t 1/3 law is strictly obeyed, within a range of strain which is determined by the purity and grain size of the lead. The region within which the t 1/3 law holds, here called stage F I, is succeeded by stage F II, during which the strain increases according to a logarithmic law, the creep rate being proportional to the increase in strain. In stage F III, which begins at a strain of about 0.3, the creep is strictly linear until the strain approaches 2.0, when rupture starts. If the stress be reversed while stage F I is in progress a creep linear with time takes place, at a rate which is twice that of the forward creep at the moment of reversal, and this constant rate continues for a time equal to that of the forward creep. This initial stage R 0 is succeeded by a stage R I which is also governed by a t 1/3 law, the constant multiplying t 1/3 being proportional to that for stage F I at the same stress. This is followed by a stage R II in which the creep rate increases according to the same law which prevailed for stage F II, and this in turn is followed by a linear stage R III characterized by the same constant creep rate as F III. If the stress be reversed at a strain so large that the t 1/3 law has ceased to be valid, the reverse creep stages are markedly affected, which emphasizes the physical significance of the t 1/3 law. A law of corresponding times is enunciated, which connects the flow in stages R 0 and R I with that in stage F I. Photomicrographs and back-reflexion X-ray photographs have shown that the t 1/3 flow is accompanied by progressive slip in the grains and local rotation of the lattice, and that in the R 0 stage slip takes place on the same slip bands as were active in the F I stage. Recrystallization and grain growth occur during stages F II and R II. In stages F III and R III there is a balance between grain break-up and grain growth, and the slip direction shows a preference for the directions of principal stresses. By considering the variation of strain rate with stress and temperature in stage III, constants have been derived which indicate that the flow in this quasi- viscous stage resembles that of a single crystal far more than that of a polycrystalline metal. The general implications of the experimental findings are discussed.