Origin of anomalous slip in bcc metals

The anomalous slip, first observed by Duesbery and Foxall in Nb (Phil. Mag. A 20:719, 1969), is the slip along the {110}<111> system with very low Schmid factor that dominates straining at low temperatures. Since then, this phenomenon has been observed in several transition bcc metals but not in all, notably not in alpha-Fe. In 1973, Matsui and Kimura proposed a “co-planar double slip” model to explain the anomalous slip when considering the glide of 1/2[111] and 1/2[-111] screw dislocations on the (0-11) plane. This model is based on the assumption that the two intersecting screw dislocations and the associated <100> screw junction form a dislocation network, which can only move on the (0-11) plane. We have made a series of molecular statics simulations employing bond order potentials to investigate the glide of these junctions in five non-magnetic bcc refractory metals (Ta, Nb, V, Mo, W) and alpha-Fe. The external load was applied as uniform uniaxial tension and compression in the [-238] direction for which the (-101) plane is the maximum resolved shear stress plane for the 1/2[111] dislocation. Two different behaviors of this network are observed depending on the material and the character of the applied load. In some materials, the three dislocations move on the low-stressed (0-11) plane, in agreement with the predictions of the model of Matsui and Kimura. However, in other materials, the two 1/2<111> dislocations break from the junction and move on their expected {110} planes. In the latter case, no anomalous slip occurs. These observations suggest that the anomalous slip is not generic for transition metals and its occurrence depends on details of bonding controlled by the electronic structure.