constitutive modeling of strain rate effects in nanocrystalline and ultrafine grained polycrystals

we present a variational two-phase constitutive model capable of capturing the enhanced rate sensitivity in nanocrystalline (nc) and ultrafine-grained (ufg) fcc metals. the nc/ufg-material consists of a grain interior phase and a grain boundary affected zone (gbaz). the behavior of the gbaz is described by a rate-dependent isotropic porous plasticity model, whereas a rate-independent crystal-plasticity model which accounts for the transition from partial dislocation to full dislocation mediated plasticity is employed for the grain interior. the scale bridging from a single grain to a polycrystal is done by a taylor-type homogenization. it is shown that the enhanced rate sensitivity caused by the grain size refinement is successfully captured by the proposed model.