| Metal matrix composites?MMCs?are suited for many structural applications such as aerospace,transportation,information due to many improved mechanical properties such as specific strength,specific stiffness and wear resistance that are inaccessible to pure metals and alloys.With the development of modern industrial technology,higher requirements are placed on the properties of metal structural materials,which has prompted materials scientists to develop a new generation of MMCs.One possible route would be to incorporate more excellent reinforcements into metal matrix.Graphene and its derivations are considered to be an ideal reinforcement for high-performance MMCs due to its excellent intrinsic mechanical properties.In the pristine,single-crystalline form,graphene is reported to have extremely high strengths?130 GPa?and high Young's modulus?1 TPa?.Even if these nanocarbon materials contain a certain concentration of crystalline defects,their intrinsic properties still prevail over those of conventional fiber and particle reinforcements.Generally,graphene is randomly introduced into a metal matrix to form a graphene-metal composite.Compared with the metal matrix,the strength and modulus of these graphene-metal composites with the uniform and random distribution of graphene have been improved significantly,however,their plasticity and toughness are inevitably decreased,limiting the application of graphene-metal composites.Composite architecture is an effective way to solve the strength-toughness conflict of metal matrix composites.Considering the two-dimensional planar structure of graphene and the advantages of nanolaminated structure widely adopted by hard biological materials in strength-toughness synergy,to design bio-inspired nanolaminated graphene-metal composites not only can effectively exert the advantage of composite architecture,but also give full play to the excellent intrinsic mechanical properties of graphene.So,in this thesis,by using the defective graphene?Reduced graphene oxide,RGO?and pure Al powder as raw materials,bio-inspired nanolaminated graphene?reduced graphene oxide,RGO?-Al composites with different Al lamella thicknesses and RGO/Al interface spacings were fabricated by using self-assemble process of flake composite powders.Their microstructures and the intrinsic structure of the RGO/Al interface were characterized by SEM,FIB,TEM,STEM and EELS.The macroscopic and micro/nano-scaled mechanical properties of the composites were studied by the combination of macromechanics and micro/nano mechanics,and the correlation between micro/nano scaled and macro-scaled mechanical behavior was established.On this basis,by changing strain rate and the interface spacing of RGO/Al,the effect of interfacial structure on the mechanical behavior of nanolamianted RGO-Al composites was investigated by using micro-compression test.The main results are as follows:?1?Bio-inspired laminated RGO-Al composites,with the Al lamella thicknesses of1?m,500 nm and200 nm,were obtained by changing the ball milling time of Al powder for 1 h,2 h and 4 h,respectively,as the coverage area and average lateral size of RGO nanosheets at the lamination interface were kept unchanged.In addition,as the ball milling time of Al powder was 4 h,and the volume fraction of RGO nanosheets was 0.5 vol.%,RGO-Al composites with the same Al lamella thickness of 200 nm and the different RGO/Al interface spacing of221 nm and68 nm were obtained by changing the lateral size of RGO nanosheets.?2?High-resolution TEM?HRTEM?characterization performed at the RGO/Al interfaces of the RGO-Al composites with different Al-lamella-thicknesses and RGO/Al interface spacing shows that all the composite interfaces comprised a few RGO layers?<15 layers,about5 nm thick?sandwiched between amorphous alumina layers?2?5 nm thick?,grown on the surfaces of two adjacent Al lamellas.In addition,the RGO in the inter-lamella interface was mainly composed of sp2 hybridized carbon atoms,which accounted for 78.9±1.6%.Compared with RGO interior,the proportion of sp2 hybridized carbon atoms at the RGO/alumina interface was decreased,which was72.1±1.3%,indicating that some sp2 hybridized carbon atoms at the RGO/alumina interface were converted into sp3 hybridized carbon atoms.Therefore,RGO/alumina interface was likely to be bonded by carbon-oxygen bonds.?3?Uniaxial tensile tests were carried on RGO–Al laminated composites with Al lamella thicknesses varying from 1?m down to 200 nm.It was found that the strengthening effect of RGO reinforcement was only demonstrated in composite having200 nm Al lamella thickness,whose yield strength and tensile strength were 27%and16%higher than those of the corresponding Al matrix while possessing comparable uniform and total elongations.?4?Uniaxial compression tests were carried out on nanolaminated RGO-Al composite micro-pillars of various diameters.It was found that the optimal size of micropillar was between 2?m and 3.5?m when the micro-nanoscale mechanical method is used to study the strengthening and deformation behavior of nano-laminated graphene-aluminum composites with Al lamella thickness of 200 nm.?5?From bulk nanolaminated RGO-Al composite and pure Al samples with Al lamella thickness of 200 nm,we fabricated micro-pillars with the laminate orientation perpendicular to the pillar axis.Uniaxial compression tests were conducted on the pillars at various strain rates spanning from 1×10-4 to 5×10-1 s-1.Combined with site-specific microstructural analysis,the strain-rate dependent deformation mechanism of RGO-Al composite and pure Al pillars was investigated based on the strain rate sensitivity index and the corresponding activation volume.It was found that the strengthening and deformation behavior of the composite caused by the interactions between RGO/Al interfaces and dislocations were related to the strain rate.?6?Uniaxial compression tests were performed on RGO-Al nanolaminated composite micro-pillars with different RGO/Al interface spacings and laminate orientations.The strengthening and deformation mechanisms of these composite pillars were evaluated from single and cyclic loading-unloading cycle,as well as site-specific microstructural analyses.It was found that a three folds'reduction in the lateral size of the nanosheets?the spacing of RGO/Al interface?conferred an over 15%enhancement in the mechanical strength,and a dramatically different deformation mechanism. |
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