Design of processes involving directional solidification with an equilibrium two-phase zone

Directional crystallization of a binary melt under conditions permitting the formation of a thin metastable region ahead of the moving front is examined for the case when metastability nearly disappears completely due to the appearance of an elemental phase in this region. The corresponding boundary-value problems of heat and mass transfer can be formulated in the usual manner in this case, except for the replacement of the traditional condition for the mass balance of the impurity at the front by a new condition reflecting the quasi-equilibrium nature of the two-phase zone which is formed. The change in the mass-balance condition leads to important changes in the characteristics of the solidification process. These changes are examined in detail for two examples of unidimensional nonsteady crystallization corresponding to a constant front velocity and a self-similar solidification regime.