A study of the atomic structure of twin boundaries in body-centred cubic crystals by electron microscopy

Abstract

The atomic structure of coherent (112) twin boundaries has been studied by means of transmission electron microscopy with the intention of distinguishing between two models of the structure. In one model, which is the conventional one, the atomic configuration has mirror symmetry about the boundary plane. In the other model, which has been suggested by computer simulation studies, atoms on one side of the boundary are shifted from the mirror-symmetry positions in a direction parallel to the twinning shear. It is deduced that the two models can be distinguished from each other by examining the features of boundary images taken in some particular reflections. On the basis of observations in such reflections, it is concluded that the twin boundary in Fe-3·25 wt.% Si alloy is of the former type. The twin boundaries observed were always accompanied by twinning partials. No dissociation of partials was observed. This indicates that the difference in the boundary energy between the two structures is greater than 1·9 mJ/m².