NUCLEATION AHEAD OF THE ADVANCING INTERFACE IN DIRECTIONAL SOLIDIFICATION

During directional solidification of an alloy, it is possible to nucle ate the growing phase or a new phase at or ahead of the interface. Thi s is critical in the phase selection, in the columnar to equiaxed tran sition under casting, welding or rapid solidification conditions and t he formation of bands in peritectic systems. Following Hunt, an approp riate theoretical model is developed to determine the conditions under which nucleation can occur in the liquid close to a moving solid-liqu id interface for both, low and high interface velocities. At high grow th rates, non-equilibrium effects are shown to play an important role in predicting such transitions. (C) 1997 Elsevier Science S.A.