  Plastic deformation in polycrystals is governed by the interplay between
intra-granular slip and grain boundary-mediated plasticity. However, while the
role played by bulk dislocations is relatively well-understood, the
contribution of grain boundaries (GBs) has only recently begun to be studied.
GB plasticity is known to play a key role along with bulk plasticity under a
wide range of conditions, such as dynamic recovery, superplasticity, severe
plastic deformation , etc., and developing models capable of simultaneously
capturing GB and bulk plasticity has become a topic of high relevance. In this
paper we develop a thermodynamically-consistent polycrystal plasticity model
capable of simulating a variety of grain boundary-mediated plastic processes in
conjunction with bulk dislocation slip. The model starts from the description
of a single crystal and creates lattice strain-free polycrystalline
configurations by using a specially-designed multiplicative decomposition
developed by the authors. This leads to the introduction of a particular class
of geometrically necessary dislocations (GND) that define fundamental GB
features such as misorientation and inclination. The evolution of the system is
based on an energy functional that uses a non-standard function of the GND
tensor to account for the grain boundary energy, as well as for the standard
elastic energy. Our implementation builds on smooth descriptions of GBs
inspired on diffuse-interface models of grain evolution for numerical
convenience. We demonstrate the generality and potential of the methodology by
simulating a wide variety of phenomena such as shear-induced GB sliding,
coupled GB motion, curvature-induced grain rotation and shrinkage, and
polygonization via dislocation sub-grain formation.
