Decreasing the grain size into nano-scale can effectively enhance the strength of material. However, the expense of superior strength is paid by substantial reduction of the ductility, which is considered as the ‘heel of Achilles’ for nanocrystalline metal. Recent experimental works have shown that materials with certain gradient microstructures can overcome the strength and formability trade-off dilemma, using specific processing such as severe plastic deformation and heterogeneous lamellar ecrystallization, SMAT etc. The grain size in such microstructures ranges from a hundred nanometers to tens of micron meters. Therefore, to study the macroscopic behavior of this type of ‘Multiscale’ microstructure, a dislocation-‐based multi-scale framework is employed, which includes a continuum dislocation dynamic (CDD) model, stress/strain gradient model and Visco-Plastic Self-‐Consistent (VPSC) model. The simulation results were compared with experimental data. The break-down of Hall-Petch as observed and a new interpretation of grain size effect with considering spatial gradient of grain size is proposed.
