Analysis of Dislocation Loops in Superlattices

A detailed computer analysis of dislocation loop behavior in DO3 and B2 type superlattices has been performed. This analysis shows that DO3 superlattice dislocation nucleation most probably proceeds by the independent nucleation of four 1/2 a0 〈111〉 loops while the same process in B2 superlattices occurs by the independent nucleation of two such loops. Thus, superlattice dislocation loop nucleation requires a greater amount of energy in the DO3 case than does loop nucleation in a comparable B2 superlattice. These processes may be thermally activated so that at sufficiently low temperatures, e.g. 243 °K for Fe3Si (DO3), insufficient thermal energy is available and imperfect dislocations are nucleated. Thus, subsequent deformation in DO3 structures can only occur at stress levels commensurate with antiphase boundary production while deformation of B2 structures can occur at lower stresses since nucleation is much easier. — An analysis of equilibrium dislocation loop configurations in DO3 and B2 superlattices has also been carried out for various applied stresses. This analysis shows that the stress required to maintain a given radius of curvature in disordered, B2, and DO3 structures increases approximately in the ratio 1 : 2: 3. It has also been shown that the equilibrium spacing of a superdislocation loop pair increases with decreasing applied stress until, at some critical stress, the inner loop becomes unstable and collapses. The above processes have been shown to markedly effect the deformation behavior of ordered alloys.