Kinematics of connected grain boundaries in 2D

The motion of connected boundary systems was investigated in Al and Zn tricrystals. It was found that triple junctions can exert a drag force on the adjoining boundaries owing to a lower mobility. This drag manifests itself by a deviation of the dihedral angles at the triple junction from the equilibrium angles due to grain boundary surface tensions. The strength of triple junction drag can be expressed in terms of a criterion Lambda = (m(tj) (.) a/m(b)), where mt(j) is the triple junction mobility, m(b) the boundary mobility, and a is the grain size. Therefore, the effect should be particularly pronounced in fine-grained and nanocrystalline materials. It is shown that the von Neumann-Mullins relation of 2D grain growth is modified by triple junction drag, such that there is no unique relationship between growth rate and the number of sides anymore. For very small A, the triple junctions control grain structure evolution during grain growth, which leads to polyhedral grain shapes with flat boundaries. The theoretical concept is supported by computer simulations of grain growth and in-situ annealing experiments in the SEM.