Monte Carlo analysis of order-disorder reactions in binary alloys with hexagonal structure

The phase diagrams of binary alloys with hexagonal crystal structure have been calculated by means of Monte Carlo simulations taking into account anisotropic nearest neighbour interactions. The anisotropy is assumed between interactions within a basal plane (0 0 1) and those between such planes. This is an attempt to account for the experimentally observed deviation from the ideal hexagonal close packed c/a ratio. Various values of the anisotropy ratio have been assumed in order to produce a series of prototype diagrams. In the isotropic case, the structure difference between the hexagonal close packed and the face centered cubic structures disappears in a nearest neighbour approximation and the phase diagram is topologically the same for both structures. The ordered structures B19 and DO 19 are the hexagonal variants of the ordered FCC structures L1 0 and L1 2, respectively. When the interactions within the (0 0 1) planes are stronger than those between such planes, an A 2B phase appears at compositions between the ordered B19 and DO 19 phases. When they are weaker, two kinds of new phases (R n and T n with the limiting variants R 1 ≡ B19 and T 1 ≡ DO 19) appear between them. These new phases have not been observed so far in real systems. It is found that the phases R 2 and T 2 are stable at high temperatures and there should be a chance to observe at least these phases in real alloys. The corresponding phase diagrams are presented.