Observation of grain boundaries

Abstract

SUMMARYThe observation of grain boundaries using the optical microscope, the electron microscope and the field‐ion microscope has considerably aided our understanding of grain boundary structure. The study of etch pits in the optical microscope has proved that low angle boundaries are composed of dislocations.A large number of grain boundary observations have been made by transmission electron microscopy. Here, various classes of boundary structural features have been characterized in grain boundaries; they can be classified into three categories: (1) features which are essentially independent of the true structure of the boundary (thickness fringes, moiré fringes); (2) accidental features which result from a perturbation of the boundary by phenomena originating in the adjacent grains (dislocations, ledges); (3) features of the true structure of the grain boundary (features of the equilibrium structure‐features of the perturbed structure). It must be appreciated that certain features of the structure are not resolved because of their poor contrast in the electron microscope image.The field‐ion microscope has been used for a relatively short time in studies of grain boundaries. Nevertheless it has yielded very interesting results. Unfortunately, at the present time, it seems that the positions of the atoms may not be determined sufficiently accurately in this instrument to allow a precise determination of the structure. Only in a limited number of cases has it been possible to account for the observed features on the basis of theoretical considerations.