Origin of variable propensity for anomalous slip in body-centered cubic metals

Many transition metals crystalizing in the body-centered cubic (bcc) structure exhibit anomalous slip on low-stressed {110} planes at low homologous temperatures, which cannot be reconciled with the Schmid law. Specifically, for uniaxial loading in the center of the [001] - [011] - [(1) over bar 11] stereographic triangle, this is manifested by 1/2[111] and 1/2[1 (1) over bar(1) over bar] screw dislocations moving on low-stressed (0 (1) over bar1) planes. While the anomalous slip is often attributed to non-planar cores of 1/2 < 111 > screw dislocations or to the tendency for their networks to glide easily, it remains unclear why it dominates the plastic deformation in some bcc metals, whereas it is weak or even absent in others. Using molecular statics simulations at 0 K, we demonstrate that the anomalous slip in bcc metals is intimately linked with the stability of < 100 > screw junctions between two intersecting 1/2 < 111 > screw dislocations under stress (for example, 1/2[111] and 1/2[111] screws giving rise to the [100] junction). Our atomic-level studies show that in nearly all bcc metals of the 5th and 6th groups these junctions cannot be broken by the applied stress and the three dislocations can only move on the common {110} plane (in the above example on the (0 (1) over bar1) plane). On the other hand, these junctions are found to be unstable in alkali metals, tantalum, and iron, where the application of stress results in unzipping of the two dislocations and their further glide on the planes predicted for isolated dislocations. These results also suggest that the experimentally observed increased propensity for the anomalous slip in further stages of plastic deformation may be explained by reduced curvatures of 1/2 < 111 > screw dislocations in dense networks.