Microstructure and Mechanical Properties of Carbon Nanotubes Reinforced AZ91D Composites

Author:Liang Jun Hao

Supervisor:wei jian feng li he jun


Degree Year:2018





With the development of lightweight in the automotive and aerospace fields,higher demands have been presented on the lightweight and high strength of magnesium matrix composites.Carbon nanotubes(CNTs)demonstrate unusually superior mechanical properties,high aspect ratio and stable chemical properties,and may become an ideal reinforcing material for new nanocomposites.The appearance of CNTs provides a new opportunity for the development of magnesium matrix composites.Carbon nanotubes reinforced magnesium matrix composites have the advantages of light weight,high specific strength,high specific stiffness,low thermal expansion coefficient,excellent damping performance and good dimensional stability,which are great potential structural and functional integration of materials and have the potential to have a tremendous impact on various sectors such as aerospace,military industry,automobile and electronic packaging.However,it is very difficult to disperse as carbon nanotubes have high surface free energy and large ratio of length to diameter.The objectives of this study were to characterize and compare mechanical properties of carbon nanotube reinforced AZ91 D composites(CNTs/AZ91 D composites)by improving the dispersion of carbon nanotubes.The magnesium matrix composites reinforced with CNTs,Ni-CNTs and Co-CNTs were prepared.In addition,the morphology and microstructure of the composites were characterized by XRD,Raman,TEM,HRTEM and EDX.The mechanical properties were tested by a universal mechanical testing machine.The influence of process parameters on the microstructure and mechanical properties of the composites was analyzed,and the enhancement mechanism of the composites was revealed.The as-cast CNTs/AZ91 D composites were successfully prepared by a novel compound process including friction stir process and liquid-state ultrasonic process.The dispersion of carbon nanotubes was improved by sonication and the effects of ultrasonic parameters(ultrasonic power,ultrasonic time and ultrasonic temperature)on the microstructure and mechanical properties of as-cast CNTs/AZ91 D composites were discussed.Based on the thermodynamic conditions of the enhancement being trapped or pushed by the solid-liquid interface,the influence of the content of CNTs on the CNTs distribution of as-cast CNTs/AZ91 D composite were analyzed.The as-cast 1.0 wt.% CNTs/AZ91 D composites had tensile strength of 247 MPa,increased 32.8% compared to AZ91 D alloy.The change of the fracture morphology between the matrix and the composite was observed and the microscopic fracture mechanism of the composite was discussed.The as-cast CNTs/AZ91 D composites were extruded and deformed by liquid-solid extrusion process.The extruded CNTs/AZ91 D composites with a tensile strength up to 389 MPa were obtained,which increased by 25.5% compared with the matrix.The effects of extrusion temperature and extrusion speed on the microstructure and mechanical properties of extruded CNTs/AZ91 D composites were investigated.The effects of CNTs content on the morphology and size of β-Mg17Al12 phase in the composites were discussed.Al2 Mg C2 phase was found at the interface between the carbon nanotubes and the AZ91 D matrix and the growth mode of Al2 Mg C2 phase was analyzed.The enhancement mechanism of CNTs on Mg-based composites was revealed.The droplets of micron AZ91 D magnesium alloy were deposited by the metal micro-droplet deposition manufacturing technique,the static contact angle of magnesium alloy droplet on carbon substrate and nickel substrate was studied.Continuous Ni nanoparticles were deposited on the surfaces of carbon nanotubes(CNTs)by electroless plating.The wettability was improved between the carbon nanotubes and the magnesium alloy matrix.Ni-CNTs/AZ91 D composites were prepared and the effects of Ni-CNTs content on the microstructure and mechanical properties of the Ni-CNTs/AZ91 D composites were discussed.The formula of predicting elastic modulus and yield strength of composite material were proposed.The theoretical predicted compared well with the experimental results.The reinforcement content and the relationship between the enhancement mechanism of composites were quantitative analyzed.The effect of orientation of carbon nanotubes on the microstructure and properties of magnesium matrix composites was explored.Uniform nano-Co particles were electroless plating on the surface of CNTs.The saturation magnetization of magnetic modification CNTs was 2.81 emu/g and the coercivity was as high as 1258.2Oe,which was more than 4 times of the coercivity of cobalt powder(308Oe).Co-CNTs/AZ91 D composites were prepared successfully by ultrasound/magnetic compound field.For the first time,the orientation of carbon nanotubes in metal matrix under magnetic induction was achieved.The compressive properties of the composites parallel to and perpendicular to the direction of the magnetic field were tested respectively.The anisotropy of compressive properties of composites was found.The influence of the magnetic field strength and the heating temperature on the orientation of the composites were studied.The morphologies of the compression fracture surfaces of Co-CNTs/AZ91 D composites were discussed.The formation mechanism of CNTs "chain" clusters under magnetic field was analyzed.The above research can provide technical guarantee for lightweight and high-strength structural materials.It has important theoretical guiding significance for the development and application of magnesium matrix composites.