Atomistic study of non-Schmid effects in the plastic yielding of bcc metals

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Abstract
In this paper we investigate by computer modelling, using many-body central force potentials, the response of the core of ½[111] screw dislocations in bcc transition metals to externally applied stresses. The objective is to identify those components of the applied stress tensor which play the main role in the breakdown of the Schmid law and to establish the dependence of the critical stress needed for dislocation motion upon these stress components. This development lays the ground for constitutive relations that reflect correctly the non-Schmid character of plastic flow in bcc metals that are needed in continuum approaches to plasticity of these materials. First, we investigate the effect of pure shear stress acting parallel to the Burgers vector. This study involves calculation of the critical resolved shear stress (CRSS) for various orientations of the maximum resolved shear stress plane. The results show the dependence on the sense of shearing, exhibit the so-called twinning-antitwinning asymmetry and reveal quantitatively the overall deviation from the Schmid law. The next step is investigation of the effect of tensile and compressive stresses for a number of differently oriented tension or compression axes. These calculations demonstrate that shear stresses parallel to the Burgers vector are not sufficient to explain the variation in the CRSS with the orientation of the loading axis and suggest that other components of the stress tensor are affecting the dislocation behaviour. Finally, a combined effect of the shear stresses parallel and perpendicular to the Burgers vector is investigated. The resultant dependence of the CRSS on the shear stress perpendicular to the Burgers vector explains the orientation dependence found in the tension and compression studies. The present atomistic calculations establish that gliding of the ½[111] screw dislocation in bcc metals depends on shear stresses both parallel and perpendicular to the Burgers vector that act not only in the slip plane but all three {110} and also {112} planes of the [111] zone. The results of these calculations determine the functional dependence of the CRSS on these shear stresses.