  Grain structure plays a key role in the mechanical properties of alloy
materials. Engineering the grain structure requires a comprehensive
understanding of the evolution of grain boundaries (GBs) when a material is
subjected to various manufacturing processes. To this end, we present a
computationally efficient framework to describe the co-evolution of bulk
plasticity and GBs. We represent GBs as diffused geometrically necessary
dislocations, whose evolution describes GB plasticity. Under this
representation, the evolution of GBs and bulk plasticity can be described in
unison using the evolution equation for the plastic deformation gradient, an
equation central to classical crystal plasticity theories. In addition, we
outline a method to introduce a synthetic potential to drive migration of a
flat GB within our diffuse-interface framework. We validate the framework by
simulating the evolution of a triple junction, and demonstrate its
computational efficiency. We further leverage this framework to study the
migration of a flat GB driven by a potential difference, subject to different
sets of available slip systems and mechanical boundary conditions.
