Observation of a dislocation source in ice by synchrotron radiation topography

Abstract

A fundamental requirement for the plastic deformation of a crystalline material is a mechanism for the multiplication of dislocations. Such a mechanism has been proposed by Frank and Read¹. Whenever a segment of dislocation lying in its glide plane terminates at a node or at a step taking it out of that plane, the forward motion of the segment on the glide plane causes the dislocation line to spiral around this point, with the result that dislocations can sweep across the plane many times over. There is much evidence for dislocation multiplication in crystals. Sometimes a source in a thin foil throws off a sequence of dislocations while being observed in the electron microscope^2,3. In other experiments on bulk specimens the dislocation loops or spirals produced from a source have been observed after the event by the technique of dislocation decoration⁴ or X-ray topography^5,6. We present here a sequence of topographs which show a complete source in an ice crystal. The topographs were obtained using the X-ray topography facilities at the Synchrotron Radiation Source Daresbury, UK⁷, and provide an exceptionally clear demonstration of the operation of this kind of Frank-Read source.