ABSTRACT
Boundaries have been reported to significantly strengthen and toughen metallic materials. However, there has been a longstanding question on whether hetero-boundary-affected region (HBAR) exists, and how it might behave. The strain gradient near the boundaries developed during tensile testing cannot be fully preserved post-mortem because the dislocation pile-up configuration evolves during unloading to quantify the effect of boundary on the mechanical behaviour. Plastic strain in a metal is typically dominated by the nucleation and propagation of dislocations. The higher strain at the boundary indicates that dislocations are emitted from the boundaries as proposed by Li and Murr. It is found that a characteristic HBAR was formed near boundaries in hetero-structured laminates. The HBAR was defined as the region with strain gradient on one side of a boundary. There is consensus that the piling up of GNDs near the boundary produces strain gradient. Nanoindentation experiments were conducted using an MTS Nanoindenter XP equipped with a Berkovich pyramid indenter.
