A novel continuum theory of incoherent interfaces with triple junctions is applied to study three-dimensional coupled grain boundary (GB) motion in polycrystalline materials. The kinetic relations for grain dynamics, relative sliding and migration of the boundary, and junction evolution are developed. In doing so a vectorial form of the geometric coupling factor, which relates the tangential motion at the GB to the migration, is also obtained. Diffusion along the GBs and the junctions is

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... so as to prevent nucleation of voids and overlapping of material near the GBs. The coupled dynamics has been studied in detail for two bicrystalline and one tricrystalline arrangements. The first bicrystal consists of two rectangular grains separated by a GB, while the second is composed of a spherical grain embedded inside a larger grain. The tricrystal has an arbitrary shaped grain embedded inside a much larger bicrystal made of two rectangular grains. In all these cases, analytical solutions are obtained wherever possible while emphasizing the role of various kinetic coefficients during the coupled motion.