Plastic deformation of silver chloride. II. Photoelastic study of the internal stresses in glide packets

Abstract

The pattern of birefringence caused by glide in silver chloride is most clearly resolved when the glide planes are observed edge on. It is then possible to sketch the two orthogonal sets of curves giving the directions of polarization at each point of the pattern and, by means of a compensator, to find how the magnitude of the birefringence varies from point to point. The stress distribution can be deduced from the distribution of birefringence, provided that the important influence of the relative orientation of stress axes and crystal axes is taken into account. The state of stress in each plastically deformed grain can be resolved into two parts. (1) Each glide packet is subject to pure bending about two perpendicular axes lying in the glide plane (but with a neutral plane that is not always the middle plane of the packet). This gives stresses that vary discontinuously with a period equal to the spacing of the glide zones. (2) There is also a continuously varying stress distribution which changes only slightly in distances equal to the glide zone spacing. The curvature of the glide packets means either that all the dislocations in one glide zone have the same sign or that positive and negative dislocations are present there in unequal numbers. It is found that, throughout any one grain, the ‘ dislocation charge ’ on each zone (to use an obvious electrical analogy) has the same sign and is never more than about twice the average for the grain.