Electron microscopy; proceedings of the Electron microscopy; proceedings of the Stockholm Conference, September, 1956 electronmicrosco00euro Year: 1957 Direct Observation of Dislocations and Their Movement in Metal Foils P. B. HiRSCH, R. W. HoRNE, and M. J. Whelan Cavendish Laboratory, Cambridge In order to explain the low values of the shear stress required to start plastic flow in metal crystals, it is necessary to postulate the existence of a lattice imperfection, known as a dislocation. There is a considerable amount of indirect evidence from etch- ing and precipitation experiments (9, 13

Electron microscopy; proceedings of the Electron microscopy; proceedings of the Stockholm Conference, September, 1956 electronmicrosco00euro Year: 1957  Direct Observation of Dislocations and Their Movement in Metal Foils P. B. HiRSCH, R. W. HoRNE, and M. J. Whelan Cavendish Laboratory, Cambridge In order to explain the low values of the shear stress required to start plastic flow in metal crystals, it is necessary to postulate the existence of a lattice imperfection, known as a dislocation. There is a considerable amount of indirect evidence from etch- ing and precipitation experiments (9, 13 Stock Photo
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Electron microscopy; proceedings of the Electron microscopy; proceedings of the Stockholm Conference, September, 1956 electronmicrosco00euro Year: 1957 Direct Observation of Dislocations and Their Movement in Metal Foils P. B. HiRSCH, R. W. HoRNE, and M. J. Whelan Cavendish Laboratory, Cambridge In order to explain the low values of the shear stress required to start plastic flow in metal crystals, it is necessary to postulate the existence of a lattice imperfection, known as a dislocation. There is a considerable amount of indirect evidence from etch- ing and precipitation experiments (9, 13) that such imperfections exist in metal crystal, whilst for inor- ganic crystals such as AgBr and NaCl (1, 5) the dislocation networks may be decorated by various techniques and made visible under the optical micro- scope. However no similar direct observations have been made on metals, simply because it is not possible to examine interior structures with optical tech- niques. It occurred to the authors that much useful information on the arrangement and movement of dislocations in metals might possibly be obtained by examining thin foils directly in the electron microscope. Electron optical transmission experi- ments with gold foils had shown that the contrast was essentially due to Bragg scattering which is structure sensitive. It was therefore thought that dis- locations might be made visible by virtue of their strain fields. This paper is a short account of some observations on dislocation distributions and move- ment in aluminium foils. A fuller account is published elsewhere (8). Aluminium was used in this work on account of its transparency to electrons, and also because much is already known about the substructures formed by de- formation in this metal. Beaten foils /i thick, either cold worked or annealed at 350 C in vacuo, were etched in dilute hydrofluoric acid. The foils were examined directly in the Siemens and Halske 'Elmiskop 1' operating at 80 KV at an instrum