Role of extrinsic and intrinsic toughening mechanisms in graphene nanosheets reinforced magnesium matrix layered composites

Constructing a layered structure is an effective approach to achieve a balance between strength and toughness. The toughening mechanisms of layered composites are generally considered to include reinforcement bridging, pullout, and crack deflection. However, the above toughening mechanisms are more applicable to ceramic matrix composites with predominantly cleavage fracture. For metal matrix composites with a larger fracture process zone, the energy consumption in crack propagation may be dominated by the plastic deformation at the crack tip. Therefore, elucidating the dislocation emission behavior and the interaction between graphene nanosheets (GNSs) and crack is the key to revealing the toughening mechanisms. In this work, the extrinsic and intrinsic toughening mechanisms for the layered GNS/Mg composites were investigated systematically. The three-dimensional reconstruction of the fracture morphology, combined with statistical analysis and molecular dynamic (MD) simulations, elucidates that macroscopic crack deflection in layered composites does not always result in a microscopically longer crack propagation path. The mitigation and redistribution of stress concentration and the promotion of dislocation emission at the crack tip induced by the GNS layers are proposed to be critical toughening mechanisms in the GNS/Mg layered composites.