Investigation On Fabrication And Mechanical Properties Of Graphene/Al Composites With A Biomimetic Microstructure

Strength and toughenss are the two major properties for the materials used for structural applications.However,in most cases,the increment of strength is comprised by the loss of toughess.The conflict between strength and toughenss greatly inhabit the application of high strength/stiffness materials,as the low toughness would lead to sudden failure of materials with little presage.Therefore,how to break the “strength-toughness inharmony relation” and realize the simultaneous increment of both strength and toughenss is the key development trend for artificial structural materials.Under the principle of “Survival Of The Fittest”,biological species evolve over time to acquire particular characteristics to adapt to the surrounding environment.Constituted by materials available in nature that generally exhibit poor macro-scale mechanical properties,biological structural materials can achieve orders of magnitude increase in strength and toughness over their individual constituents.This remarkable property optimization in biological materials is mainly to the result of its deliberate microstructural design.Nanolaminated structure is widely adopted by hard biological materials,such as bone,nacre and biosilica,which are composites basically comprised of brittle minerals,and a small fraction of soft organic constituents.Deriving the advantages from the intricate design of the lamellar structure,these materials possess the combination of superior strength and toughness,which are mutually exclusive in most manmade structural materials.The strategies of biological materials in dealing with the strength-toughness conflict by nanolaminated structures provide a potential solution for advanced artificial materials used for structural applications.However,in materials such as metal matrix composites(MMCs),guided by the design principle of homogenous dispersion of reinforcements,few attempts in microstructure design have been carried out,and the composites usually make a compromise between strength and toughness,which greatly hinders their broad engineering applications in the fields of transportation,aerospace and military industries.The advantages of biological materials have inspired studies on developing MMCs with a nanolaminated structure.Recently,efforts have been made to fabricate carbon nanotube(CNT)/metal composites with sandwiched structure.Li et al fabricated CNT/Cu nanolaminated composites by rolling of alternating CNT films and Cu foils.Kang et al fabricated CNT/Cu nanolaminated composites containing up to two CNT layers by selective dip-coating.Using powder metallurgy process,Jiang et al developed bulk CNT/Al nanolaminated composites,where flake-shaped Al powders absorbed with CNT on their surface were densified via ball milling and subsequent deformation processing.Although the above-mentioned studies adopted different fabrication approaches,they all demonstrate the advantage of nanolaminated structure in strengthening and toughening the composites.While compared to the one-dimensional CNT,the two-dimensional geometry of graphene is intrinsically more compatible with the planar laminated structure,and is thus considered an ideal reinforcement in nanolaminated MMCs.However,until now,most of the studies on graphene/metal composite emphasize the homogeneous dispersion of graphene and the suppression of its agglomeration,and limited attempts in careful microstructure design for graphene/metal composites have been done.Co-milling of metal powders(such as Al powders)with graphene is an effective and widely adopted way to disperse graphene uniformly in the metal matrix.However,the high energy milling process would seriously damage the intergrity of graphene and promotes interfacial reactions,causing a marginal or even a negative strengthening effect of graphene in the metal matrix.Moreover,the distribution of graphene in co-milled composites is usually random,which is unfavorable for fully realizing its strengthening capability due to the strong anisotropy,and also cause a poor match between strength and ductility in the as-fabricated composites.By alternately evaporating metal thin films and tranferring monolayer or bi-layer graphene onto the metal-deposited substrate,Kim et al fabricated graphene/Cu and graphene/Ni nanolaminated composite films and demonstrated a significant strengthening effect of graphene in nanolayered composites by dislocation blockade mechanism.Nevertheless,the method is time-consuming and only applicable for thin film-typed samples.Therefore,the development of a strategy to synthesize bulk graphene/metal composites with bio-inspired nanolaminated structure is critical for further demonstrating its strengthening and toughening mechanisms as well as its potential for large scale applications.In this study,flake powder metallurgy,a bottom-up assembly process of composite flake powders,was used to prepare bulk graphene/Al composite with biomimetic nanolaminated structure.Tensile test reveals that graphene in the nanolaminated composites has remarkably higher strengthening and stiffening efficiencies than those of other reinforcements,and the composites maintain a similar or even slightly higher total elongation than the unreinforced Al matrix.The deformation behavior of the composites is interpreted in terms of a competition between dislocation accumulation and dynamic recovery at the graphene/Al interfaces,and the large elongation after peak stress is reached of the composites is attributed to a toughening effect from the laminated structure,as revealed by in situ tensile tests in a transmission electron microscope(TEM).This work highlights the importance of biomimetic structural control in the stiffening,strengthening and toughening of the composites,and sheds new light on the development of graphene-reinforced MMCs with the potential for large-scale applications.The main conclusions of the thesis were summarized below: 1.The graphene/Al composites with a biomimetic nanolaminated microstructure can be fabricated by a flake powder assembly process,which includes a formation of graphene/metal composite flake and the subsequent densification processes.Graphene oxide,a precursor of graphene,can be spontaneously and uniformly absorbed on the Al flakes through electrostatic interactions,which was well demonstrated by both control experiments and quantitative analysis.More importantly,the uniform adsorption mechanisms by electrostatic interactions can be applicable to other nanofiller/metal matrix systems,making it possible to extend the research filed of this work(graphene/Al composite)to more general metal matrix composites systems.2.The conflict between strength and ductility/toughness of metal matrix composites can be well avoided in the biomimetic graphene/Al composites,as revealed by the tensile test characterizations.By comparing the tensile behaviors between graphene/Al composites with biomimetic and regular(random)microstructure,we found that the biomimetic microstructure is vital to the achievement of high strength/high ductility mechanical properties.The toughening effect of biomimetic microstructure,spanning different length scales from nanometer to millimeter range,was clearly demonstrated by combing in-situ tensile/bending tests and finite element modelling(FEM)simulations.The crack bridging and deflection abilities of biomimetic microstructure,together with the large deformation zones in Al matrix ahead of the main crack,greatly delay or inhabit the initiation/propagation of the cracks in graphene/Al composites,leading to a comparable tensile elongation to failure for biomimetic graphene/Al composites compared with unreinforced Al matrix.3.The near nano-regime Al lamellar thickness(~200 nm)in biomimetic graphene/Al composites make the graphene/Al interface a relatively high volume fraction,and the interactions between the interface and dislocations propagated in Al matrix had a strong influence on the tensile behavior of the composites.By tracking dislocation density evolvements during tensile deformation and measuring the thermal activation parameters(strain rate sensitively and activation volumes),we found that the dislocation kinetics/deformation mechanisms are different between biomimetic graphene/Al composites and unreinforced Al matrix.The graphene/Al interface can be act as a dislocation sink during deformation,which will accelerate the dislocation annihilating rate and cause a reduced uniform tensile elongation for the biomimetic graphene/Al composites,and that is highly in accord with the tensile results.4.The interactions between graphene/metal interface and dislocations in matrix in our composite system inspires a very promising way of controlling and optimizing the mechanical properties of metal matrix composites by introducing heterogeneous element(s)/phase(s)(“nanofillers”)into the grain boundary regions to modulate dislocation activity and subsequently the plastic deformation mechanism.Importantly,the fabrication process of flake powder assembly in this work emphasis its versatile and efficient characteristics to introduce nanofillers in metal matrix uniformly,providing a very promising future in this field from a technical prospect.