Moiré patterns on electron micrographs, and their application to the study of dislocations in metals

Abstract

The direct resolution of a crystal lattice has already been obtained in the electron microscope, with lattice spacings down to about 7 angstrom. Although a metal lattice has not so far been resolved in this way, a method has been developed to allow this to be carried out in an indirect manner, by means of moire patterns. In this way, indirect resolution of crystal planes of even less than 1 angstrom in spacing may be obtained, and the atomic array in metal lattices may be studied. The moire patterns are obtained on transmission electron micrographs from pairs of overlapping single-crystal films of metals, prepared in a suitable manner. The mode of formation of these moire patterns has been interpreted in terms of the kinematical theory of electron diffraction, and the relation between the patterns and the individual lattices has been considered. If a dislocation line passes through one of the metal crystals, it may be readily detected on the moire pattern. It is shown that both edge and screw dislocations in one of the crystals will give rise to terminating half-lines in the moire pattern: the number of these terminating half-lines has been deduced, for the f.c.c. system, for any general dislocation line. The results are completely consistent with the observations, and several examples of dislocations are illustrated in the paper. Partial dislocations and stacking faults are also observed, and a full interpretation of these is given. The moire patterns have also been used to record dislocation movements. The smallest periodicity which we have so far succeeded in resolving in the electron microscope is a moire pattern of spacing 5$\cdot $8 angstrom, obtained from overlapping nickel and gold crystals. The potentialities and limitations of the technique of moire patterns are discussed in relation to their application to various problems in the metal physics of thin films, with particular reference to the study of lattice imperfections and their effect on mechanical properties.