MECHANISM OF GRAIN BOUNDARY MIGRATION

It is shown by electron transmission microscopy that a grain boundary has a step-like structure. In f.c.c. metals the steps are formed by the {111} crystal-planes of both grains. The electron microscopic observations show that grain boundary migration occurs in the following way: Under the influence of a driving force, atoms are emitted from the steps of the shrinking grain into the grain boundary while the steps of the growing grain absorb the same number of atoms out of the grain boundary. Therefore, the micromechanism of grain boundary migration is the absorption and emission of atoms leading to the movement of the steps. Due to this process, the surfaces of both grains move perpendicular to the grain boundary plane leading to the migration of the grain boundary. The steps are formed by a Frank spiral mechanism or at junction points of grain boundaries. The observed mechanism indicates that a grain boundary is formed by the stepped surface of both adjacent grains and atoms lying between these surfaces. © 1969.