The nature of dislocations in high-angle grain boundaries

Abstract

The width of dislocations in grain boundaries is calculated by minimizing the interfacial energy of the boundary and the strain field energy of the dislocations. The calculations indicate that models, assuming equilibrium grain boundaries to consist of characteristic low energy structures with, if necessary, superimposed' arrays of localized misfit dislocations are of physical significance only for those boundaries whose energy depends strongly on the orientation relationship. For all other boundaries, dislocation models, although geometrically correct, seem to be of limited physical significance. Localized misfit dislocations are found to be unstable in these boundaries in the sense that their cores spread over the entire boundary area. The same applies to extrinsic grain boundary dislocations. They are found to be localized only in boundaries whose energy depends strongly on the mis-orientation. Measurements of the widths of misfit dislocations and observations on the width of extrinsic grain boundary dislocations agree with the results of the calculations. External forces (externally applied mechanical forces, thermal stresses, chemical forces) may result in the localization of dislocations in grain boundaries. This effect may be important for grain boundary sliding, phase transformations and the generation of dislocations from grain boundaries.