Enhanced strain delocalization through formation of dispersive micro shear bands in laminated Ni

Abstract

Nanocrystalline metals usually exhibit limited ductility due to catastrophic shear fracture caused by the preferential formation of a few premature shear bands during tensile deformation. In this work, the tensile and fracture behavior of laminated Ni with varied degree of difference in grain size of hard and soft layers were investigated. Compared with monolithic nanocrystalline Ni, laminated Ni with a large difference in the grain size of hard and soft layers exhibited enhanced elongation to failure without sacrificing tensile strength, because the formation of dispersive micro shear bands could undertake large plastic strain and lead to strain delocalization. With decreasing the difference in the grain size of hard and soft layers and the corresponding resistance to propagation of micro shear bands, there is a transition of fracture mode from ductile necking fracture accompanied with dispersive micro shear bands to shear fracture dominated by a few macroscopic premature shear bands. A mechanical model based on the transition from grain interior dislocation emission to grain-boundary-mediated deformation within the soft layers was proposed and the critical grain size of the soft layers corresponding to the optimum synergy of high strength and ductile necking fracture was obtained.