Ordering in binary transition metal alloys

We present the phenomenological thermodynamic modeling of binary alloys which demonstrate solubility of the components at high temperatures, and form intermediate phase near equiatomic composition at lower ones (the so-called sigma-phase). Besides, the regular solution miscibility gap takes place also. The nonequilibrium thermodynamic potential is written out as a sum of the free energy of regular solution and polynomial term of scalar order parameter φ, which describes the σ-phase ordering. There are four parameters in the model: the energy of regular solution mixing, the energy of σ-phase formation at zero temperature, and the widths of temperature and concentration intervals of σ-phase existence in the alloy with frozen-in random distribution of components. Up to now, both phase transitions which take place in a number of transition metals binary alloys (the σ-phase formation and miscibility in the regular solution) have been treated separately. In present work, the standard technique of phase diagram calculation allows us to analyze all possible phase diagrams which may arise in the alloy.