Study On Microstructure And Thermal Deformation Behavior Of CNTs/Al-Cu Composites

Owing to excellent properties including light weight,high modulus,high thermal conductivity,low expansion,heat and wear resistance,noise reduction with damping and radiation shielding,aluminum matrix composites have been widely used in the fields of aerospace,electronic packaging,transportation,electronics,energy and environment.Owing to unique structure,superior mechanical properties,stable chemical properties,extraordinary thermal properties and unique conductive properties,carbon nanotubes?CNTs?are considered as ideal reinforcements for Al matrix composites.However,previous stuedies have found that CNTs are not uniformly dispersed in Al matrix,resulting in the strength and toughness of the composites can not be greatly improved,and there are some difficulties in plastic processing.The current research on CNTs/Al composites mainly focuses on the preparation and mechanical properties,while few studies have been done on the evolution of microstructure,phase interface structure characteristics,structure regulation,strengthening mechanism,physical properties and corrosion resistance.In this paper,CNTs/Al-Cu composites were fabricated by spark plasma sintering and flake powder metallurgy.The effects of sintering temperature,extrusion deformation,CNTs content,microstructure and interface structure,solid solution and aging heat treatment on the properties of the composites were systematically studied.Thermal deformation behavior of the composites was studied,and the thermal processing maps and constitutive equations were constructed.The main research contents and results are obtained as follows:?1?The effects of sintering temperature,extrusion deformation,content of CNTs,and heat treatment on the mechanical properties,physical properties and corrosion resistance of CNTs/Al-Cu composites were systematically studied.By means of transmission electron microscopy?TEM?,electron backscatter diffraction?EBSD?and X-ray diffraction?XRD?,the reasons for the change of the performance were discussed combined with the microstructure evolution.The results show that after spark plasma sintering at 500?,550? and 600?,the tensile strength,the elongation,the corrosion resistance,electrical conductivity and thermal conductivity of CNTs/Al-Cu composites increase with the increasement of sintering temperature.The extrusion process can improve the density,tensile strength,elongation,electrical conductivity,thermal conductivity and corrosion resistance of CNTs/Al-Cu composites.Increasing the content of CNTs and solution aging can improve tensile strength and hardness of the composites.CNTs addition can refine the grain size,increase the nucleation position and shorten the peak aging time.However,due to the fomatiion of Al₄C₃ by the reaction of CNTs and Al and the formation of nano-lamellar structure in matrix,the increase of CNTs content reduces the conductivity and thermal conductivity of the composites.?2?The interface state between the reinforcing phases and the metal matrix of CNTs/Al-Cu composites were observed by high-resolution transmission electron microscopy?HRTEM?.The results show that the interfacial structure of the composites can be controlled by adjusting sintering temperature.When sintering temperature is 500C,CNTs and Al matrix are mechanically bonded directly;when sintering temperature increases to 550 C,amorphous layer appears at the interface;when sintering temperature is 600 C,Al₄C₃ phase forms at the interface.The formation of amorphous layer and Al₄C₃ phase is helpful to enhance the bonding force between CNTs and Al matri.The strong interfacial bonding can make the load to be transferred from the Al matrix to CNTs efficiently and improve the mechanical properties of materials.The fractured CNTs caused by ball milling can react with Al to form single crystal Al₄C₃phase easily during high temperature sintering process.The interface between Al₄C₃ and Al matrix is coherent or semi coherent.There are two kinds of orientation relationships between Al₄C₃ and the matrix,which including Al?111?//Al₄C₃?001?and Al?200?//Al₄C₃?001?.?3?CNTs/Al-Cu composites with various types of nano-strengthening phases?CNTs,Al₄C₃,Al₂O₃?were successfully prepared by flake powder metallurgy.The matrix has bionic nano-lamellar structure.The nano-strengthening phases distribute uniformly in the matrix and the interfaces between the phases and matrix bond well.Several strengthening mechanisms exist in CNTs/Al-Cu composites,such as dislocation strengthening,fine grain strengthening,second phase strengthening and load transfer.These strengthening mechanisms have synergistic effects in composites.Compared with the spark plasma sintering method,the deformation storage energy of CNTS/Al-Cu composites prepared by flake powder metallurgy method is higher,which promotes the formation of more Al₄C₃ phases.?4?The structure stabilities,mechanical properties,thermodynamic properties of strengthening phases and interfacial stability between the phases and Al matrix of CNTs/Al-Cu composites were determined by the first-principles method.It is found that compared with Al₂Cu,Al₄C₃ has higher stiffness and hardness,stronger stability,shear resistance and thermal conductivity,but worse plasticity and conductivity.With the increase of setting pressure in the calculation model,the volume modulus,shear modulus,Young's modulus,Poisson's ratio,hardness,Debye temperature,melting point and thermal conductivity of Al₄C₃ and Al₂Cu increase.?5?The thermal deformation behaviors of sintered CNTs/Al-Cu composites,extruded Al-Cu alloy and extruded CNTs/Al-Cu composites fabricated by FPM were studied through isothermal compression tests.The relationship among the flow stress,strain rate and deformation temperature during hot deformation was discussed.The constitutive equations were deduced and the processing maps were established.The effects of extrusion process and CNTs on the thermal deformation behavior of the material were investigated.The results indicate that the stress-strain curves of the Al-Cu alloy and the composites show obvious dynamic softening characteristcis.The flow stress increases with increasing strain rate at a specific deformation temperature,while it decreases with increasing temperature at a specific strain rate.The calculation results show that the addition of CNTs increases the activation energy of hot deformation and reduces the flow instability zone of the alloy.When the strain is 0.6,the suitable processing domains of sintered CNTs/Al-Cu composites are 375-425? at 0.4-10 s^-1 as well as 525-550? at 0.02-10 s^-1.The suitable processing areas for extruded Al-Cu alloys are 400-440? at 4.5-10 s^-1,530-550? at 0.05-0.6 s^-11 and 300-350? at0.01-0.13 s^-1.The suitable processing areas for extruded CNTs/Al-Cu composites are400-440? at 4.5-10 s^-1,540-550? at 0.001-0.6 s^-1 and 300-330? at 0.03-0.82 s^-1.