The flow of polycrystalline metals under simple shear. Ill

Abstract

The flow behaviour of poly crystalline lead under simple shear has been further investigated, the method involving an annulus cut in a disk being supplemented by a method that utilizes a rectangular plate cut with two rectangular grooves, which makes for easy photomicrography. It has been found that in a region of strain below that in which the t 1/3 law is valid a t 1/2 law holds precisely, from a strain of 0.01 upwards. The transition between the t 1/2 and t 1/3 flow is continuous and expressible in terms of the constants of the two régimes. When preceded by a t 1/2 régime the t 1/3 law holds with the time as measured from the first imposition of stress, an explanation of which is offered in terms of a first process involving the establishment of glide planes and a second process consisting in the establishment of glide lamellae on a fixed number of planes, optically visible as coarse slip bands. The flow behaviour of the metal at strains of less than 0.01, where the t 1/2 law does not prevail, and the magnitude of the immediate strain have received attention. The flow linear with time which ensues on a first reversal of stress has been further examined. It has been shown in particular that, although the flow is uniform, the metal is changing its properties with time, since when a re-reversal is effected at an interval after the reversal, the behaviour depends upon the interval. This throws some light on the mechanism of the linear flow. Repeated reversals of stress at intervals equal to the duration of first forward flow produce a succession of alternate linear and t 1/3 stages, with an increase of strain at each cycle. The final strain can attain a large value without recrystallization. Repeated reversals at intervals larger than the duration of the first forward flow produce a more complicated behaviour which leads to recrystallization at small strains. It appears from this and other experimental results that a t 1/3 forward flow followed by a reverse linear flow of approximately the same duration, in which the same slip bands have been shown to be involved, is a comparatively simple process which restores the metal to a state in some respects similar to that on first application of stress. Some attention has been paid to the effect of grain size and surface conditions. The general flow properties of polycrystalline lead under the simple mechanical conditions in question have been reviewed and a general explanation attempted of the behaviour under forward and reversed stress.