Atomistic computation of the image force on a dislocation in a bicrystal - II. Case of a large difference between the elastic moduli of the two half-crystals

The behaviour of a dislocation in the vicinity of an interface between two half-crystals having notably different elastic moduli has previously been investigated by a computer simulation by Beauchamp and Lepinoux, who considered two half-crystals with similar elastic coefficients. The two half-crystals have a bcc structure with the same orientation and lattice parameter and are welded along the (100) plane, parallel to the a(0)[001] screw dislocation. Two interatomic potentials representing alpha-iron have been used. Detailed exploration of the Peierls valleys within 4a(0) from the interface has been performed by applying an external stress which balances both the local lattice friction and the long-range image force. It is found that the mean stresses of each valley follow approximately the stress curve determined by Pacheco and Mura on the basis of the continuous Peierls dislocation core, perturbed by the interface. The simulation presents, however, a stress peak which is lower and more extended. Changes in the dislocation core have been observed as the dislocation approaches the interface; in the 'soft' medium the core tends to become slightly narrower, whereas it clearly widens in the 'hard' medium. This is because the effect of the applied stress is uniform throughout each half-crystal whereas the image force is stronger on the parts of the dislocation core close to the interface than on those farther away. Systematic variations in the Peierls valleys amplitudes have been found; when the interface is approached from the 'soft' medium side, the amplitude increases whereas it decreases when approaching from the 'hard' side. The latter decrease is such that Peierls valleys completely disappear near the interface and an unstable zone forms on the 'hard' side. It has been possible to keep dislocations there, but only under a dissociated form compound of two partials having b/2 screw components. The stacking fault created, which is not normally stable in the bcc structure, is of the 'unstable fault' type to which Rice attributes a key role in the dislocation nucleation at a crack tip.