Nucleation ahead of the advancing interface in directional solidification

During directional solidification of an alloy, it is possible to nucleate the growing phase or a new phase at or ahead of the interface. This is critical in the phase selection, in the columnar to equiaxed transition under casting, welding or rapid solidification conditions and the formation of bands in peritectic systems. Following Hunt, an appropriate theoretical model is developed to determine the conditions under which nucleation can occur in the liquid close to a moving solid-liquid interface for both, low and high interface velocities. At high growth rates, non-equilibrium effects are shown to play an important role in predicting such transitions.