½ screw dislocations and the nucleation of {112} twins in the b.c.c. lattice

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Abstract
It is shown that in the b.c.c. lattice the crystallography and the commonly observed low stacking-fault energy on {112} lead to the prediction that ½ screw dislocations will each dissociate into three 1/6 partials situated on {112} planes which intersect along the axis of the original screw dislocation. Calculation shows that an equilibrium configuration of the three partial screw dislocations possessing threefold symmetry is not stable. The stable equilibrium configuration resulting from the dissociation of the ½>111< screw dislocation under zero stress possesses twofold symmetry, with one partial to the intersection of {112} planes, and the two others at equal distances from it on different {112} planes. Under critical stress conditions the configuration can be transformed to a second configuration, which is stable under zero stress. Here again one of the partials is at the intersection of {112} planes, but now the others are at different distances from it, situated on parallel, neighbouring {112} planes. If the stress is raised to about twice the value at which the transformation between stable configurations occurred, the second stable configuration develops into a three-layer twin, with the three 1/6 partials at its tip. When the growing twin meets a crossing ½ screw dislocation a pole mechanism for growth may result, which causes a multi-layer twin lamella to be created. It is further shown that there are two types of ½ slip dislocations moving on {112} planes: one type which may easily cross-slip from one {112} plane to another, and another which can only move on a different {112} plane as a three-layer twin, at the critical twinning stress. Emissary dislocations produced by thick twin lamellae belong to the former category.