X-ray structure and elastic strains in copper

Abstract

An X-ray examination of copper has been undertaken whilst the metal has been actually under tensile stress, and the X-ray structure investigated at a systematic series of points on the load-extension curve and during unloading and reloading from selected points on the curve. It is shown that the permanent strain is associated with the breakdown of the grains into the crystallite formation and that this change is essentially irreversible. The elastic strain of the metal is accompanied by reversible changes in dimensions of the atomic lattice which take place without leaving in the lattice any permanent distortion, as shown by the observation that the X-ray diffraction rings, including rings of the diffuse type, contract and expand under the action of the applied stress without any change in the degree of diffusion. The lattice changes are distinguished in this way from certain lattice strains or lattice distortions permanently imposed on the lattice as a result of deformation by cold-work. Quantitative measurements are made on the elastic lattice strains exhibited by the (400) and (331) planes in a direction perpendicular to the axis of the applied stress, and these are compared with the equivalent external elastic constants. These measurements show that marked difference in rate of strain may take place in neighbouring grains subjected to the same external stress, and on this difference is based an explanation of the extensive breakdown of the grains into components of widely varying orientations which characterizes the structure of a polycrystalline metal after deformation beyond the yield point.