Gradient plasticity provides an effective theoretical framework to interpret
heterogeneous and irreversible deformation processes on micron and submicron
scales. By incorporating internal length scales into a plasticity framework,
gradient plasticity gives access to size effects, strain heterogeneities at
interfaces, and characteristic lengths of strain localization. To relate the
magnitude of the internal length scale to parameters of the dislocation
microstructure of the material, 3D discrete dislocation dynamics (DDD)
simulations were performed for tricrystals of different dislocation source
lengths (100, 200, and 300 nm). Comparing the strain profiles deduced from DDD
with gradient plasticity predictions demonstrated that the internal length scale
depends on the flow-stress-controlling mechanism. Different dislocation
mechanisms produce different internal lengths. Furthermore, by comparing a
gradient plasticity framework with interfacial yielding to the simulations it
was found that, even though in the DDD simulations grain boundaries (GBs) were
physically impenetrable to dislocations, on the continuum scale the assumption
of plastically deformable GBs produces a better match of the DDD data than the
assumption of rigid GBs. The associated effective GB strength again depends on
the dislocation microstructure in the grain interior.