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.

Discrete Dislocation Dynamics (DDD) is an established mesoscopic numerical method for simulating dislocation motion to determine the plastic behaviour of metals. However, DDD’s reliance on analytical expressions for internal stress fields limits its application to infinite isotropic media. The Discrete-Continuous Model (DCM), which couples DDD with a finite element elastic solver, has been proposed to handle more complex boundary conditions by regularising plastic strain using the eigenstrain formalism. However, this model still relies on analytical solutions for short-range dislocation interactions, making it unsuitable for anisotropic media. In this work, we present an improved DCM framework that replaces the FE solver with a Fast Fourier transform (FFT) solver for improved computational efficiency and full numerical calculation of stress fields, eliminating the need for analytical corrections. The proposed FFT solver employs a discrete theory of Green’s operators and uses a sharp eigenstrain field to describe dislocations. The solver mesh is aligned with the face-centred cubic (fcc) lattice of the DDD, forming an octahedral cell to address symmetry artefacts around { 111 } slip planes. Our FFT-based approach successfully maintains numerical stability by representing fcc dislocations as sharp fields without generating oscillations. This coupling allows the study of plasticity in anisotropic materials and interactions between dislocations and diffuse inclusions, such as precipitates, without short-range stress corrections.