A model is developed for describing rapid penetration of a liquid phase along a grain boundary. It is based on the assumption of a highly faceted solid/liquid interface. Experiments showing the faceting of the solid/liquid interface in grain boundary penetration experiments are presented. The basic hypothesis of the model are an undersaturated solid and a positive spreading coefficient of the liquid phase along the grain boundary. The model explains the apparent concave shape of the tip of the groove and the reason why penetration also occurs if the liquid phase is pre-saturated with the material of the solid. Moreover it predicts a power law with an exponent close to unity for the time dependence of the depth of penetration of the liquid layer along the grain boundary.
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