Study On Fabrication And Physical Properties Of CNTs/Magnesium Matrix Composites

The research progress of carbon nanotubes(CNTs) modified magnesium matrix composites has been reviewed in this thesis. The fabrication, interface behavior, hydrogen storage, mechanical and thermodynamics properties and CNTs reinforcing mechanisms of Mg-based composites have been studied. The main results are listed as followings:Quasi-straight carbon nanotubes and carbon nanotubes encapsulating metal particles have been produced by using anode arc plasma method. The interface bonding between CNTs and magnesium matrix was investigated. The good combination between CNTs and magnesium matrix composites has been achieved by plating nickel on CNTs via an electroless method. The plating nickel consisting of Ni, Ni3P nanocrystals is compact and homogeneous coating. The compatibility of CNTs and magnesium matrix composites has been improved due to the presence of nickel coating containing hexagonal close-packed structure and its lattice constants are close to that of bulk magnesium.The Mg-based hydrogen storage composites such as Mg-Ni, Mg-Ni-CNTs, Mg-Ni-CNTs-TiO2 have been prepared by mechanical alloying technique. The largest hydrogen absorption capacity of Mg-Ni binary composites can reach 5.419%. However, the hydrogen releasing temperature was high and the kinetic properties of the alloys was poor, and which limiting the applications of composites. Hydrogen storage of the Mg-Ni-CNTs is much higher than that of the Mg-Ni, and the temperature of hydrogen release was lower. When the CNTs was 10%, it can absorb 7.2%hydrogen in 1 min and desorb 6.2%hydrogen within 60-100 s. Ternary alloys showed excellent hydrogen storage properties due to Ni and CNTs play an import role as catalyst. Compared with Mg-Ni-CNTs, Mg-Ni-CNTs-TiO2 alloy has not significant improvement in hydrogen absorption and desorption. However, the using life was prolonged and the fabrication cost was decreased significantly due to the addition of TiO2 decreasing the forming enthalpy of hydrogenation and increasing the activation energy.Four kinds of composites including CNTs/Mg, CNTs/SiCp/Mg, CNTs/AZ91D, CNTs/SiCp/AZ91D were successfully fabricated using stirring of liquid slurry methods. With increasing the content of CNTs, both the elastic modulus and microhardness of the composites increased, but the tensile strength and elongation rate firstly increased and then decreased. When the volume fraction of SiCp is within 0-3%, the elastic modulus, microhardness and tensile strength of composites increased with increasing SiC content, but the elongation rate decreased continuously. For the CNTs/Mg, CNTs/2vol%SiCp/Mg composite, when the volume fraction of CNTs is 1.1%, the elastic modulus, and microhardness of composite improved 28.4% and 51.8%,10.2%and 25%,67%and 80%respectively, elongation rate of CNTs/Mg is improved 60.6%, but elongation rate of CNTs/2vol%SiCp/Mg is reduced comparing with Mg matrix. For CNTs/AZ91D and CNTs/3.0vol%SiCp/AZ91D composite, when the volume fraction of CNTs is 1.0%, the elastic modulus, microhardness, tensile strength and elongation rate of composite improved 23.8%and 29.8%,7.8%and9.3%,39.4%and 46.76%,34%and 14.6%respectively than those of the corresponding matrix alloys. For Mg matrix and AZ91D matrix composite, the volume fraction of CNTs could not exceed 1.1%and 1.0%respectively, otherwise, their tensile strength and elongation of composite could largely reduced and the increasing amplitude of the microhardness and elastic modulus also decreased due to agglomeration of the CNTs. The reinforcement of CNTs and SiCp could clearly refine the primary grains of composite. The fracture way of the CNTs/SiCp/Mg was transgranular and intercrystalline fracture, and the later was not dominated. The fracture way of AZ91D/CNTs/SiCp is a mixed fracture, which contains cleavage, intercrystalline and gliding fracture. The formation of MgSi2 is due to the reaction between SiCp and Mg matrix.The average coefficients of thermal expansion of the CNTs/Mg and CNTs-SiCp /AZ91D composites are half of their matrix's coefficients, and the coefficient of SiCp/AZ91D and CNTs/AZ91D composites are reduced by 18%and 36.5% respectively comparing with their own matrix's coefficients. The thermal conductivities of the SiCp/AZ91D and CNTs/SiCp/AZ91D composites are higher than those of their matrix. The warm deep-drawing process of the CNTs/AZ91D magnesium composite sheet was simulated by finite element analysis at the temperature from 25℃to 300℃and different blank holder force conditions. It is found that the CNTs/AZ91D magnesium composite sheet with a thickness of lmm and diameter of 160mm has poor drawing performance under 100℃.With the increase of processing temperature, the deep drawing performance is improved. The composite has the best drawing performance at 250℃. In addition, the blank holder force should be selected from 5kN to 8kN. The calculated results are according with the experimental results.Through establishing the shear-lag model of CNTs/Mg composites, the reinforcing mechanisms of composites have been analyzed. The yield strength values of each reinforcing mechanism were estimated by using four kinds of reinforcing mechanisms. The results indicates that the reinforcing mechanism of stress transfer has the largest reinforcing role for the magnesium composites, followed by the enhanced dislocation and enhanced grain refinement, the impact of the enhanced residual thermal stress is the smallest. When the content of CNTs is lower than 1.1%, the calculation results are consistent with the experimental results. The stress field, saturated stress, stress transfer efficiency and effective length of CNTs for CNTs/Mg composites were analyzed based on the shear-lag model. The interfacial region monotonically decreased the stress transfer efficiency and the CNTs saturated stress, but increased the CNTs effective length. The aspect ratio of CNTs has obvious influence on the stress transfer efficiency and effective length when the aspect ratio is small.The local stress fields of CNTs reinforced magnesium matrix composites, CNTs/Mg and CNTs/AZ91D, were simulated using the finite element method. The results suggest that the laws of stress-strain distribution in the CNTs/Mg and CNTs/AZ91D were similar generally. With identical pulling force, the axial distortions in the composites are lower than that that in Mg and AZ91D matrix material. At the end faces of CNTs, the phenomenon of stress concentration can been observed in some areas. The results illuminated that the composite is strengthened by the reinforced phase CNTs. The fractures of composites were start from the end interfaces of CNTs, and the fracture mechanism is the interface fracture. The simulation results are consistent with the experimental results.