A dislocation density based constitutive law for BCC materials in crystal plasticity FEM

We introduce a crystal plasticity constitutive model for BCC materials which is build on dislocation movement and uses dislocation density variables as internal state variables. Besides the statistically stored dislocations geometrically necessary dislocations are used to consider nonlocal effects as recently proposed by Ma, Roters and Raabe for the FCC crystal structure. In this paper the model will be adopted to the BCC crystal structure. Due to the special core structure of screw dislocations formed at low temperatures, the mechanical behavior of BCC crystals is controlled by the movement of screw dislocations rather than edge dislocations. For this reason, the Peierls mechanism has to be considered and several modifications have been introduced which include a new scaling relation between the mobile and immobile dislocations, and new flow rules for bulk and grain boundary elements. A pure Nb bicrystal is studied experimentally and numerically under channel die compression boundary conditions, to demonstrate the applicability of the new model variant.