A computer the simulation of kinetics of precipitationof an ordered intermetallic phase from a disordered solid solution in a model binar two-dimensional alloy is considered. All diffusion processes, the concentrational delamination (clustering), ordering, antiphase domain boundary movement and coarsening, are described by the microscopic kinetic equations. The ordering transition of the first and second kinds are considered. It is shown that the conventional precipitation mechanism through nucleation and growth of an equilibrium ordered phase occurs only in a very narrow "strip" of the two-phase field in the phase diagram. In the remaining part, the decomposition always starts from a congruent ordering, which produces a transient nonstoichiometric ordered single-phase state with the same composition as the parent disordered phase and the same symmetry as the product intermetallic phase. Decomposition of the transient ordered phase occurs predominantly at the antiphase domain boundaries (APBs), which results in a two-phase morphology with layers of disordered films separating antiphase domains of the ordered phase. This decomposition is a result of a concentration instability on APBs, which is more substantial than the conventional homogeneous spinodal instability. Further morphological evolution after decomposition is controlled by coarsening, which reduces the order/disorder interfacial area. The predicted precipitation mechanism through a formation of a single-phase transient ordered state is general. It is expected in the most part of a two-phase field of a phase diagram of any alloy systems with intermetallic precipitates related to the parent one by ordering.
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