Research About Mechanical Properties Of CNTs Enforced Magnesium Matrix Composite Materials

The research progress of magnesium matrix composites has been reviewed in this thesis.The fabrication, mechanical properties and reinforcing mechanisms of Mg-based composites have been studied. The main results are listed as followings:An improvement of the compatibility between the CNTs and Mg matrix has been achieved by a electroless nickel plating process on the surface of CNTs. The nickel-plated CNTs and SiCp,having the excellent strength and dispersion in melt, have been used as the reinforcement, as well as the pure Mg and AZ91D have been used as the matrix.The melt stirring method has been adopted to prepare four composites, CNTs/Mg,CNTs/SiCp/Mg,CNTs/AZ91D,CNTs/SiCp/AZ91D.The influence of the key process parameters on mechanical properties of compositess has been analyzed to determine the optimum parameters during fabrication of the Mg compositeIn this study, the elastic modulus, hardness, tensile strength and elongation of the four prepared composites have been measured and the relationship between the performances of composites and the reinforcement content has been investigatd.Result indicate, when the volume fraction of CNTs is 1.1%,the elastic modulus and microhardness of composites improved 28.4%and 51.8%,10.2% and 25%,67% and 80% respectively, elongation rate of CNTs/Mg is improved 60.6%, but compared with Mg matrix,elongation rate of CNTs/2vol%SiCp/Mg is reduced. 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%and 9.3%,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 formation of Mg2Si and Mg2Ni is due to the reaction between SiCp,CNTs plated nickel and Mg matrix,the content of Mg2Si and Mg2Ni is very little.Finally, the yield strength increment formulae dealing with the stress transfer, dislocation, thermal residual stress and grain refinement have been used to estimate the yield strength of composites reinforced by the various strengthening 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 volume fraction of CNTs is lower than 1.1%,the calculation results are consistent with the experimental results.