Composition of a solid phase growing under nonequilibrium conditions

The composition of a binary solid phase is predicted in a multiphase environment during phase formation by a steady-state process in linear growth geometry characterized by planar interfaces. In this situation, Gibbs free energy is also dissipated at the opposite interfaces of the forming phase which is not in thermodynamic equilibrium with its surroundings. Although the common tangent rule of an equilibrium situation cannot be applied to the steady-state process, the composition is derived here from the free energy curves of the equilibrium phase by a self-consistent, iterative solution of a new model that combines thermodynamic and diffusional considerations. Accurate composition—as proven experimentally—is predicted for the amorphous Al–Pt phase using a reasonable assumption for the ratio of chemical potential drops at the opposite interfaces of the forming phase. Although our numerical example is the amorphous Al–Pt system, the model can also be applied to other amorphous or solid solution phases which exist in a wide compositional range.