THE DISSOCIATION OF A SCREW SUPERDISLOCATION IN THE L12 STRUCTURE

The transition by cross-slip from an infinite superdislocation fully dissociated in the primary octahedral slip plane to the Kear-Wilsdorf (KW) configuration, which results from dissociation in the cube cross-slip plane, is examined analytically in the approximation of linear anisotropic elasticity. In the absence of lattice friction, a twofold (or a manifold) configuration that straddles the cube and the octahedral planes is always unstable. Lattice friction may temporarily freeze twofold configurations, but there is no mechanical driving force that tends to favour a nonplanar configuration, irrespective of the application of an external stress. The preference for either of the two planar stable configurations, which is controlled by the antiphase-boundary (APB) energy ratio on these planes, z = gamma(o)/gamma(c), and by the deviation from isotropic elasticity, can be modified by an appropriate choice of the shear stresses resolved on the two planes under consideration. The role of the subdissociation of each superpartial into two Shockley partials is discussed qualitatively. Most L1(2) intermetallics whose elastic constants are documented belong to the category of alloys whose only favoured state is the KW configuration. However, uncertainties in the experimental determination of the APB ratio z are such that it should be envisaged that, in some L1(2) alloys, the screw configuration has access to two metastable states that are fully planar configurations located either in the cube or in the octahedral plane. These two states are separated by an energy barrier that can be thermally overcome. There is no example of an L1(2) alloy in which the planar octahedral configuration is the only stable one (z < 1/3(1/2)).