The flow of polycrystalline lead under simple shear at higher temperatures

The flow of polycrystalline lead has been investigated in detail by the method of simple shear, as used by Andrade & Jolliffe, at temperatures from 27 to 80 °C and also at 150 °C. Pre­liminary work on the variation of stable grain size with temperatures has shown that at 150 °C there are very few grains to the thickness of the metal annulus subject to shear, which is why this temperature is distinguished from the others. The range of temperature of the detailed measurements appears to be one of particular significance. The t 1/2 formula followed, as strain increases, by a t 1/3 formula, which has been shown to represent in detail the transient flow at 27 °C, has proved to be valid at all the higher temperatures and the laws governing the variation of the constants of the formulae with stress and temperature have been determined. At temperatures approaching 80 °C the flow so expressed is accompanied by a small flow linear with time, the laws of which have been determined. This linear flow has been traced to grain boundary slip, which is not involved in the t 1/2 and t 1/3 flow. Particular attention has been paid to stage III of the flow, in which the metal has the properties of a non-Newtonian fluid. The constant rate of flow has been expressed in terms of a function of the velocity and temperature which is linear with stress. It has been shown that at a certain transition stress the slope of the line changes abruptly. This transition stress is a simple function of the temperature. Photomicrographs showing the deformation of the surface grains, which with the method employed, but not with the method of tensile stress, is held to be typical of the behaviour of internal grains, have thrown some light on the processes associated with the various stages of flow. An intermittent grain boundary migration has been established at higher temperatures. The results obtained by the method of simple shear are compared with the results of tests carried out by the usual method of tensile stress with rods or wires, from which they differ in certain significant respects, such as the conditions leading to recrystallization and the accompanying effects.