A Cosserat-phase-field theory of crystal plasticity and grain boundary migration at finite deformation

In metallic polycrystals, an important descriptor of the underlying microstructure is the orientation of the crystal lattice of each grain. During thermomechanical processing, the microstructure can be significantly altered through deformation, nucleation of new subgrains and grain boundary migration. Cosserat crystal plasticity provides orientation as a degree of freedom and is therefore a natural choice for the development of a coupled framework to deal with concurrent viscoplasticity and grain growth. In order to take into account grain boundary motion, the Cosserat theory is adapted with inspiration from orientation phase-field models. This allows for the microstructure at a material point to evolve on the one hand due to deformation-induced lattice reorientation and on the other hand due to a sweeping grain boundary. With a proper separation of plastic evolution in the bulk of the grain and in the grain boundary, the model can successfully capture grain boundary migration due to lattice curvature and due to statistically stored dislocations.