On the stability of the three-fold symmetrically dissociated screw dislocation in the B.C.C. lattice

Abstract

Contrary to the conclusions based on isotropic elasticity, anisotropic elasticity theory predicts that a screw dislocation in the b.c.c. lattice dissociated symmetrically into three 1/6 [111] partial dislocations on {110} planes is stable or metastable. A symmetric dissociation on {112} planes is unstable. For the highly anisotropic alkali metals the threefold symmetrically dissociated stable configuration on {110} has a lower interaction energy than the asymmetric dissociation considered by Sleeswyk (1963). This is consistent with the 3-fold symmetrically dissociated structure for sodium found by atomistic simulations. For the less anisotropic b.c.c. transition metals the sign of the energy difference is reversed, but the total fault energy for the symmetric structure is lower than for the Sleeswyk configuration by an amount estimated to be larger than the interaction energy difference. For an asymmetric configuration with the third partial off-centre, the interaction energy difference will be smaller and may be outweighed by a larger self energy of this partial. The symmetric configuration (dissociated on {110} planes) may then have a lower total energy than either of the asymmetrical configurations, which is consistent with the predictions of several atomistic simulations of the core structure of the screw dislocation for b.c.c. transition metals.