Study On Microscopic Deformation And Fracture Behavior Of Graphene/Magnesium Layered Composites

One of the eternal pursuits of advanced metallic structural materials is to achieve light weight,high strength,and high toughness.Forming composite is an effective method to improve the strength of metals,which is often accompanied by a dramatic reduction in plasticity and toughness.The strength-toughness trade-off severely limits the applications of metal matrix composites.Fortunately,nature guides the direction.The subtle structures formed in millions of years of evolution show a promising way to balance strength and toughness by architectural design.Shell nacre,made of about95 vol.%calcium carbonate and 5 vol.%organic polymers,exhibits more than twice the strength and three thousand times higher toughness than its constituent materials,which derives from the special“brick-and-mortar”structure.Via mimicking the micro-nano layered structure of nacre,it is highly desirable for simultaneous improvement of strength and plasticity.As the lightest metal structural material,magnesium（Mg）is a prospective metal for aerospace,automotive manufacturing,biomedical and other fields due to its high specific stiffness/strength,excellent electrical/thermal conductivity,and good biocompatibility.However,the low-symmetry hexagonal close-packed structure of Mg results in low plasticity at room temperature.Although structural-functional integrated materials with high strength and specific functionality can be prepared by combining different materials,it usually tends to further aggravate the intrinsic brittleness of Mg.At this point,the simulation of micro-nano layered structure of nacre would be expected to resolve the contradiction mentioned above between strength and plasticity.This work treated graphene nanosheets（GNSs）as the nano layers and Mg as the micro layers to prepare GNS/Mg layered composites with micro-nano layered structure,which showed surprisingly ultra-high strengthening and toughening efficiencies.The corresponding strengthening and toughening mechanisms were clarified by microstructure observation and molecular dynamic（MD）simulations.The surface of GNSs was modified by the mixture of H₂SO₄ and HNO₃（3:1 vol ratio）to introduce functional groups to improve the dispersibility.Considering both surface energy and boiling point,acetone was chosen as the dispersant of GNSs.Uniformly deposited GNS/Mg layered units were obtained by spray deposition.After comparing extrusion,rolling,and forging processes,the repeat extrusion and forging process was proposed to obtain effective densification of the GNS/Mg layered composites,and the simultaneous improvement of strength and plasticity was achieved.The evolution of local strain in pure Mg and GNS/Mg layered composites was analyzed from both macro and micro perspectives by in-situ tensile under OM and SEM.From the macroscopic point of view,it is proposed that GNS layers are effective in relieving local stress concentration and preventing premature failure of the composites.Statistical analysis of local strain proved the larger uniform plastic deformation area of GNS/Mg layered composites.From the microscopic point of view,the GNS layers induced high local stress around the interfaces,which promoted the slip of<c+a>dislocations.The macroscopically uniform but microscopically inhomogeneous strain state introduced by GNS layers effectively improves the uniform deformation ability of Mg.The geometrically necessary dislocations（GNDs）densities were characterized by EBSD,which proves the higher content of GNS layers,the higher density of GNDs.The EBSD-based slip trace analysis showed a higher percent of<c+a>dislocation slips in GNS/Mg layered composites.Since the<c+a>cross-slip is proved to be the critical factor in the ductility of Mg.MD simulations were carried out to study the stress effects on the<c+a>cross-slip energy barrier.An analytical expression was proposed to describe the<c+a>cross-slip energy barrier under a complex stress state.GNS/Mg and GNS/GNS interfaces are pointed out as the weak parts.MD simulations prove the strengthening effect of Mg O films between GNS layers and the Mg matrix.Fractography analysis and AFM observation show the role of in-situ thinned GNSs on high-strength interfaces.The combination of the above two interfacial strengthening methods effectively delays the instability fracture of the composites.The interactions between crack tips of continuous GNS layers,discontinuous GNS layers with a small gap,and discontinuous GNS layers with a big gap were investigated by MD simulations.On the one hand,continuous GNS layers can directly interact with crack tip to relieve stress concentration and blunt crack tip.On the other hand,discontinuous GNS layers increase the driving force required for crack propagation by changing the stress distribution and blunt crack tip.The promoted dislocation emission from crack tip promotes the transition from brittle to ductile fracture.The toughening mechanisms were studied by statistical mechanical analysis and three-dimensional reconstruction of the fracture surface.It is proposed that macroscopic crack deflection in GNS/Mg layered composites does not increase the total path of crack propagation at the microscopic level.The toughening effect of crack deflection depends on the ratio of the cleavage surface energy to the GNS/Mg interface energy.The presence of a large number of dimples in GNS/Mg layered composites was confirmed by the observation of fracture surface.In addition,the micro cracks far from main crack could be a general toughening mechanism for layered composites.