Nanolaminated Graphene-aluminum Composites: An In Situ Study Using Micro-pillar Tensile Tests

The development of conventional metal matrix composites(MMCs)is guided by the design principle of homogeneous dispersion of reinforcements.Despite many enhanced properties such as strength,stiffness and wear resistance that may be achieved,however,such a strategy usually leads to a sacrifice in the ductility and toughness of the composite,which greatly limits the engineering application of these composites.Biological structural materials,represented by nacre,spider silk and bone,are naturally composite materials.Although constituted by materials that generally exhibit poor macroscale mechanical properties,such as soft proteins and brittle minerals,biological materials show simultaneous improvement in strength and toughness over the composites consisted of the same components.This remarkable mechanical property synergy in biological hard materials is considered to mainly result from their delicate microstructures.Therefore,designing artificial MMCs with a bioinspired microstructure is a potential way to achieve superior strength and toughness/ductility.Our research group developed graphene(reduced graphene oxide,RGO)-reinforced Al matrix composite with a bioinspired nanolaminated microstructure by using flake powder metallurgy for the first time.RGO-Al composites demonstrate the highly desirable combination of strength and ductility,with the loading/tensile direction parallel to the laminated structure.This significant strengthening and toughening effect were rationalized by the effective load transfer between RGO and the Al matrix as a result of the robust RGO/Al interface,and a crack deflection and bridging mechanism derived from the nanolaminated structure.However,the directional arrangement of RGO in the bulk nanolaminated RGO-Al composites naturally leads to the following questions:(1)is the tensile behavior of the RGO-Al composites orientation-dependent? And if this is true,(2)what is the mechanism that is responsible for the anisotropy? What's more,quantitative characterization of the RGO/Al interfacial adhesive strength and the dynamic failure behavior/mechanism of nanolaminated RGO-Al composites have not been studied yet.Micro-/nano-scaled characterization techniques developed in recent years have provided new ideas and new possibilities for the above problems.Based on the samples of nanolaminated RGO-Al composites prepared by our research group,RGO-Al composites micro tensile specimens with different laminate orientations was prepared on the bulk samples surface using focused ion beam,then used in situ uniaxial tensile test on composite micro-pillars conducted in a scanning electron microscope.Specifically,we studied the effects of different laminate orientation relative to the loading axis(isostrain and isostress conditions)on the deformation behaviors(tensile strength,ductility,graphene strengthening effect)of the composite and analyzed the orientation-dependent deformation mechanisms.These in situ measurements enabled direct observation of the deformation procedure and the exact failure mode,which underscored the importance of microstructural control in designing the mechanical properties of advanced MMCs.Our micro-tension test results revealed that the tensile strength and ductility of nanolaminated RGO-Al composites have a strong dependence on the orientation of graphene.We found a transition from a weak-andbrittle behavior in the isostress composite configuration to a strong-yetductile tensile response in the composite under isostrain condition,indicating that bioinspired nanolaminated RGO-Al composites has strong anisotropy.Further in situ dynamic microstructure analysis reveals that the anisotropy of the composite is derived from the orientation bearing behavior of graphene and the degree of toughening of the bioinspired nanolaminated structure.The anisotropic properties,dynamic failure behavior and RGO/Al interfacial adhesive strength of bioinspired nanolaminated RGO-Al composites were investigated by in situ tensile tests.The anisotropic mechanical properties and mechanisms,toughening mechanism of microstructure,strengthening mechanism of reinforcing phase and other related problems of the RGO-Al composites are systematically revealed.This study scientifically clarifies the importance of structural design and control in improving the comprehensive mechanical properties of MMCs,and the micro-/nano-mechanical characterization method also has important practical significance for other composite systems.