| Metal matrix composites(MMCs)are widely used in aerospace,transportation,electronic packaging and other fields,because they have both the ductility of metal matrix and high modulus,high strength and excellent functional characteristics of“reinforcements”such as particles,whiskers,fibers,etc.The development of modern science and technology puts forward higher requirements for the comprehensive properties of MMCs,and one of the research directions to further improve the properties of MMCs is to seek for high-performance reinforcements.In recent years,carbon nanotubes(CNTs)with high tensile strength(?30 GPa),high elastic modulus(?1 TPa)and high thermal conductivity(?6000W·m-1K-1)along the axis have attracted a lot of research interests,and are considered to be the ideal reinforcements for the new generation of MMCs.CNTs can be divided into single-walled carbon nanotubes(SWCNTs)and multi-walled carbon nanotubes(MWCNTs).Compared with MWCNTs,SWCNTs have smaller diameter(0.4-2 nm),higher aspect ratio,and no interlayer sliding between adjacent carbon monolayers in the tube wall upon deformation.Therefore,as a discontinuous fiber reinforcement of MMCs,SWCNTs have more advantages.However,the small diameter and large specific surface area of SWCNTs make them very easy to agglomerate during the fabrication of the composite.In addition,when the strength of SWCNTs reinforced MMCs increases,the ductility and toughness decrease significantly,showing the typical contradiction of strength and ductility/toughness.How to realize the uniform dispersion of SWCNTs in the metal matrix and improve the match of strength and ductility/toughness of the composite by controlling the preparation process is the bottleneck problem of SWCNTs reinforced MMCs.Inspired by the“brick-and-mortar”nanolaminated architecture of the nacre and its high strength and toughness,in this thesis,SWCNTs and pure aluminum(Al)were chosen as the reinforcements and the matrix,respectively,to fabricate SWCNT-Al composites with different nanolaminated microstructures by controlling flake powder metallurgy process,in order to explore the relationship between the nanolaminated microstructure and the mechanical behaviors of the composites;Combined with the macro-tensile method along laminate orientation,compression tests of composite micro-pillars with different laminate orientations and various strain rates were conducted to study the strengthening and deformation mechanisms of nanolaminated SWCNT-Al composites;Through the in situ tensile test of composite micro-pillars with different laminate orientations,the SWCNT/Al interfacial mechanical properties and their impact on the mechanical behavior of the nanolaminated SWCNT-Al composite were explored.The main research ideas and results are as follows:(1)Through electrostatic adsorption process,SWCNTs were uniformly adsorbed on the surface of flake Al powders.Three kinds of 0.5 wt.%SWCNT-Al composites with different microstructural architectures were fabricated by adjusting the cold-welding time of the flake composite powders.Three distinct microstructural architectures are well ordered nanolaminated structure with relatively weak interfacial bonding,intermediate and partially disrupted laminated structure with strong interfacial bonding,and disordered microstructure.Macro-tensile tests revealed that the partial laminated composite possessed the highest yield and tensile strengths and strain hardening capacity with an almost identical uniform elongation among the three composites.Combined with microstructural characterization,the excellent mechanical properties of partial laminated composite were rationalized by effective load-sharing of SWCNTs,grain refinement of Al matrix,and enhanced interface-dislocation interactions.For the other two kinds of composites,the strength of the well ordered nanolaminated composite was basically the same as that of the corresponding Al matrix,because the SWCNT/Al interfacial bonding was relatively weak.While the composite with disordered structure had similar yield strength to the partial laminated composite,its strain hardening ability and tensile strength were much lower,because the SWCNTs at the grain boundaries were difficult to interact with dislocations.(2)0.5 wt.%and 0.2 wt.%SWCNT-Al composites with partially disrupted nanolaminated structure were fabricated.Microstructural characterization showed that with the increase of the content of SWCNTs,the grain refinement of Al matrix became more obvious.The macro-tensile test along the laminate orientation revealed that the yield strength,tensile strength and strain hardening ability of the composites were enhanced with the increase of the SWCNT content,while the uniform elongation of the composites remained unchanged as compared to unreinforced Al.By comparison with pure Al,the yield and tensile strengths of 0.5 wt.%SWCNT-Al composite increased by approximately 18.7%and25.5%,respectively.However,with increasing SWCNT content,the total elongation of the composites decreased significantly,and a large number of SWCNTs were found on the fracture dimple surface,which was attributed to the cavitation-dominated mechanism at the SWCNT/Al interfaces resulting in the earlier fracture of composites.Subsequent micro-pillar compression tests at various pillar orientations and strain rates,together with associated post-mortem site-specific microstructural analysis,revealed that this SWCNT-induced strengthening in the composites came from a combined effect from the load-sharing of the SWCNTs,the grain refinement in the Al matrix,and the pinning effect of SWCNT/Al interfaces on dislocations,in which the load-sharing of the SWCNTs played a predominant role.In addition,the strengthening effect caused by the pinning of SWCNT/Al interfaces on dislocations was comparable to the strengthening contribution resulted from grain refinement of the Al matrix.(3)Through in situ tensile test of the micro-pillars with various laminate orientation(the angle between the laminate orientation and the tensile direction is 90°,45°and 0°,respectively)under the scanning electron microscope(SEM),it was revealed that both the composite strength and deformation mode showed strong anisotropy.Combined with the microstructural characterization of the micro-pillars after tensile deformation,it was found that only the 0°composite micro-pillar having iso-strain configuration was strengthened as compared to the corresponding unreinforced matrix,the ductility was well maintained,and their fracture mechanism was shear fracture through multiple Al grains,which can be attributed to the effective load transfer across SWCNT/Al interfaces and the effect of the well-bonded SWCNT/Al interface on crack deflection.However,90°and 45°composite micro-pillars were not strengthened by comparison with the Al matrix,and their fracture mechanisms were transverse fracture and shear fracture along the SWCNT/Al interfaces,respectively.Due to the failure of both 90°and 45°composite micro-pillars along the SWCNT/Al interfaces,the SWCNT/Al interfacial normal and shear strengths were measured to be 254.9±9.3 MPa and 112.8±3.5 MPa,respectively.In conclusion,through the combination of macro-tensile test and micro-pillar compression/tensile test,the strengthening and deformation mechanisms of nanolaminated SWCNT-Al composite were explored in detail.In particular,in situ micro-pillar tensile test was used to quantitatively measure the interfacial strength of CNT/Al composite,which provided a new idea and method for evaluating the interfacial mechanical properties of discontinuous fiber reinforced MMCs,and also provided experimental basis for the architecture design and process optimization of advanced MMCs. |
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