The phase diagram section A–BC (ordered compound)

Solutions between the A apex of the compositional triangle and a BC ordered compound have been investigated, from the statistical mechanics point of view, for comparison with the phase section from pure iron to NiAl in which a miscibility gap separates a disordered iron-rich from an ordered NiAl-rich phase. The “zeroth” approximation was used with allowance for next nearest neighbour interactions. It was predicted that the critical temperature for ordering (second order transformation) and the locus of equal equilibrium values of the long range order parameter fell linearly with additions of the A atoms. The critical temperature and composition for phase separation depend on the ratio of the BC exchange interaction energies for first and second neighbours as well as on the exchange interaction energies of the atom pairs AB and AC. Suitable values caused phase separation to occur at higher temperatures in the ordered solution than in the disordered. The theory accounts for the general properties of the iron-NiAl phase section.