Computer modelling, involving many-body central-force potentials, was used to study the response of the core of ½[111] screw dislocations, in body-centered cubic transition metals, to externally applied stresses. The object was to identify the components of the applied stress tensor which played the principal role in the breakdown of the Schmid law, and to establish the dependence of the critical stress, needed for dislocation motion, upon the stress components. This aided the development of constitutive relations which reflected the non-Schmid character of plastic flow in body-centered cubic metals, and which were needed in order to apply continuum approaches to the plasticity of these materials. An investigation was first made of the effect of a pure shear stress which acted parallel to the Burgers vector. This involved the calculation of the critical resolved shear stress for various orientations of the maximum resolved shear stress plane. The results revealed a dependence upon the sense of shearing, a twinning/anti-twinning asymmetry and the overall quantitative deviation from the Schmid law. The effects of tensile and compressive stresses were then investigated for variously oriented tension and compression axes. These results demonstrated that shear stresses which were parallel to the Burgers vector were insufficient to explain the variation in the critical resolved shear stress with the orientation of the loading axis, and suggested that other components of the stress tensor affected the dislocation behaviour. The combined effect of shear stresses parallel to, and perpendicular to, the Burgers vector was investigated. The resultant dependence of the critical resolved shear stress upon the shear stress perpendicular to the Burgers vector explained the orientational dependence which was found in the tension and compression studies. The present calculations established that glide of the ½[111] screw dislocation in body-centered cubic metals depended upon shear stresses both parallel to, and perpendicular to, the Burgers that acted not only in the slip plane but also on all three {110} planes; and also on the {112} planes of the [111] zone.

Atomistic Study of Non-Schmid Effects in the Plastic Yielding of BCC Metals. K.Ito, V.Vitek: Philosophical Magazine A, 2001, 81[5], 1387-407