During the thermomechanical treatment of a metal, its grain microstructure evolves significantly due to viscoplastic deformation and recrystallization. Severe plastic deformation can result in texture formation, fragmentation into sub-grains and localized deformation bands. The highly non-uniform deformed state determines the evolution throughout subsequent heat treatment, where grain boundaries migrate at elevated temperatures and new dislocation-free grains nucleate forming a recrystallized microstructure. Defects such as grain boundaries and second-phase particles are preferred nucleation sites. The modeling of the coupled evolution problem is mostly handled using separate specialized frameworks for mechanical deformation and for grain growth in a staggered scheme, where nucleation is incorporated as an intermediate step. The nuclei are planted in an ad-hoc manner based on trigger criteria such as critical dislocation density, stress or strain. In this work, we build on the previously developed unified thermomechanical framework [1], and show that it is naturally capable of dislocation induced spontaneous nucleation at grain boundaries. The model couples Cosserat crystal plasticity with a Henry-Mellenthin-Plapp orientation phase field [2], which captures both curvature and deformation driven grain boundary motion, and can account for inclination-dependent grain boundary energies. The model is implemented in 3D finite deformation framework, and its abilities are evaluated based on numerical examples using periodic bi- and polycrystals. Nucleation mechanisms such as strain-induced boundary migration, sub-grain coarsening and coalescence are reproduced.