Phase stability of the sigma phase in Fe-Cr based alloys

The FeCr sigma phase is a good example of a complex structure: it has 30 atoms in the unit cell and 5 inequivalent lattice sites, and it belongs to the class of tetrahedrally close packed structures, also known as Frank-Kaspar structures. So far, such structures have not been treated within a first-principles statistical thermodynamics framework. It will be shown that due to advances in algorithms and hardware important features of the phase stability of complex phases can be computed. The factors which affect the stability of the sigma phase have been studied using carefully selected supercells for electronic total energy calculations. Cluster variation calculations in the tetrahedron approximation were performed to evaluate the effect of partial disorder and of finite temperature. The preferred occupancy of the 5 lattice sites has been investigated and is compared with experimental determinations.