Configurational energies of different A7B types of the long-range order in Fe-Si alloys

In order to examine the existence of the long-range order of A₇B types in Fe-Si alloys with composition less than 12·5 at.% Si, the configurational energies of several superstructures in b.c.c. alloys with the cubic elementary cell having the lattice parameter 2a are calculated on the basis of a simple model with a pairwise interaction between atoms extending to the third neighbours. From the proposed model it follows that the superstructure of A₇B type suggested byFallot can be expected if the interaction parameter for the third neighboursv₃>0. On the other hand, the two-phase structure (DO₃ order and pure Fe) will be favoured by the alloy crystal ifv₃<0.The author is indebted to Dr. F.Kroupa and Dr. A.Gemperle for stimulating discussions.