Interface Tailoring And Mechanical Performance Of Carbon Nanotube Reinforced Aluminum Matrix Composites

Aluminum and its alloys are major candidates for lightweight structure design.However,the disadvantage of low modulus and stiffness limited its application in the fields of aerospace and national defense.The introduction of carbon nanotubes?CNTs?and fabrication of CNTs reinforced Al matrix composites could increase strength and modulus but decrease ductility.Thus,how to break the bottleneck of "strength/modulus-ductility/toughness conflict" has become a hot research topic in the field of metal matrix composites?MMCs?.Generally,the mechanical property of MMCs was determined by its interfaces.In CNT/Al composite,there exists two kinds of interfaces,namely CNT/Al composite interface and matrix grain boundaries.The former one determines the strengthening/stiffening effect of CNTs,while the latter one strongly influences the deformation behavior of Al matrix.However,compared with traditional MMCs,the introduction of CNTs and consequent grain refinement strongly increases the volume fraction of CNT/Al interface and grain boundary and makes the interfacial structure/property much more complex.These two facts make it harder to regulate and control the interfacial properties and restricted the full realization of CNT strengthening and Al matrix deformation,which is the key reason for "strength-ductility conflict" .To solve such problem,the flake-powder-metallurgy prepared CNTs reinforced Al-Mg alloy?CNT/Al-Mg?composite was set as the research object.Through controlling interfacial reaction and the existence form of interfacial native alumina,the structure and bonding state of CNT/Al interface were well tailored.Their effects on composite's mechanical performance and CNT load transfer effect were further investigated.In addition to this,bimodal structure design in Al-Mg matrix was further achieved through modifying the material processing route.The effects of grain size and spatial distribution on composite's mechanical performance and plastic deformation behavior were investigated.Relevant strengthening/toughening mechanisms in bimodal structure design were further discussed.The main findings of the present study are as follows:?1?Material preparation and Young's modulus measurement:With the raw materials of CNTs and Al-Mg alloy powders?AA5083?,the 1.5 wt.%CNT/Al-Mg composite with high densification and tight interfacial bonding was prepared by the flake powder metallurgy.The Young's modulus measurement accuracy and precision of tensile test,nanoindentation and resonance tests were comparatively investigated.The results revealed that,resonance test was the most feasible method for measuring the composite's Young's modulus owing to its feature of tiny load and non-destructive testing.?2?Relationship between interfacial reaction and Young's modulus:Through controlling annealing time,the CNT/Al interfacial reaction in 1.5 wt.%CNT/Al-Mg composite was well tailored.The evolution of interfacial structure and Al₄C₃ morphology with increasing interfacial reaction degree was investigated.The relationship between Young's modulus and interfacial reaction degree was quantitatively analyzed.The results revealed that,with increasing interfacial reaction degree,larger number of lath-like Al₄C₃ phases formed and serve as second-phase nano-dispersoids to strengthen/stiffen the composite.The strong volume expansion and stiffening effect of reaction-formed Al₄C₃ overwhelmed the modulus loss caused by CNT reaction,and led to the simultaneous increase of Young's modulus and interfacial reaction degree.Thus,CNT/Al interfacial reaction influences composite's Young's modulus through altering the phase composition of "CNT-Al₄C₃" reinforcements,which did not further release the strengthening potential of CNTs and was not the ideal method to improve CNT/Al composite's mechanical performance.?3?Relationship between interfacial bonding and overall mechanical performance:Owing to the reaction between the interfacial native oxide and Mg element during modified annealing process,the interfacial structure and bonding state in 1.5 wt.%CNT/Al-Mg composite was well tailored,and its effects on the composite's mechanical properties and the CNT load transfer effect were investigated.It is found that the reaction between Mg element and the native oxide destroys the continuity of the interfacial oxide layer,leading to direct CNT-Al contact and establishment of tight mechanical interfacial bonding.As the result,the CNT load transfer effect was improved without forming hydrolysable carbide nor causing CNT damage,and the composite's strength/modulus and ductility were simultaneously increased.Thus,achieving mechanical CNT/Al interfacial bonding through modifying annealing process and designing matrix alloy should be an ideal strategy to improve the mechanical performance of CNT/Al composite.?4?Matrix structure design and plastic deformation behavior:By modifying the powder mixing and densification process,the 1.5 wt.%CNT/Al-Mg composite with lamellar coarse grain/ultrafine grain bimodal structure was prepared.The effect of grain size and spatial distribution on mechanical properties and plastic deformation behavior was investigated.It was found that the high fraction continuous CNT/Al-Mg?UFG?hard phase strongly constrained the plastic deformation of the coarse-grained?CG?soft phase,which activated multi-slip dislocation motion and gradual release of dislocation storage ability in coarse grains.Under such condition,the accumulation of intragranular dislocations and strain hardening ability were enhanced,leading to the avoidance of local strain concentration and better "modulus-strength-ductility" balance.In summary,the present study has conducted systematical investigation into tailoring the property of composite interface and designing the structure of Al matrix in CNT/Al composite.Consequent effects on mechanical properties and plastic deformation behavior were revealed,trying to provide both practical approach and theoretical guidance for solving the problem of "strength-ductility conflict" in CNT/Al composite.