ON STATIC AND DYNAMICAL YOUNGS CONDITION AT A TRIJUNCTION

Using a consistent purely kinetic model we show that Young's condition at a trijunction results from a chemical and not a mechanical equilib rium, as is often believed in the literature. In an out-of equilibrium situation this condition is altered in a way such that the triple poi nt acquires an effective mobility. The order of magnitude of the mobil ity is fixed by a velocity scale <(V)over tilde(att)> associated with chemical attachment at the interface, and not by the sound speed. Whil e for ordinary eutectics (V) over tilde(att) is large (except in rapid solidification experiments) in comparison to the growth speed, implyi ng a quasi-instantaneous mobility, finite mobility effects should show up for faceted eutectics as well as for eutectoid transformations.