  A coarsened model for a binary system with limited miscibility of components
is proposed; the system is described in terms of structural states in small
parts of the material. The material is assumed to have two alternative types of
crystalline local arrangements associated with two components of the alloy.
Fluctuating characteristics of a cluster are the type and the space orientation
of its crystalline arrangement. There are two different phase transitions in
the model system, an orientation order-disorder transition representing
melting, and a phase transition between phases differing in concentration of
components. Depending on the parameters characterizing the interaction in the
system, this last transition may take place both in the crystalline and in the
amorphous (molten) phase. A special approximation is used to study the
thermodynamics of the system. The calculated phase diagram describes, at least
qualitatively, the most important features of a binary system.
