Fabrication And Properties Of Mg Matrix Composites Reinforced By Carbon Nanotubes In Situ Synthesized At Low Temperatures

Carbon nanotubes (CNTs), having a high specific strength and high specificmodulus, low density, and excellent mechanical properties, are considered to be oneof the ideal reinforcement of metal matrix composites. CNTs should be uniformlydispersed in Mg matrix and have intact interface with Mg matrix in order to play therole of CNTs as reinforcements. However, it is difficult to meet the aboverequirements using traditional methods, which restricts the development of CNTsreinforced metal matrix composites directly.Firstly, CNTs were in situ synthesized in Mg powder by chemical vapordeposition method in this paper, obtaining CNTs/Mg composite powder. Then thecomposite powder was used to fabricate CNTs/Mg composites by powder metallurgyafter mechanically milled. The method of "in situ synthesis-short time ballmilling-powder metallurgy" is applied to the magnesium matrix composites for thefirst time.The catalyst precursor was prepared by deposition precipitation, and then Co/Mgcatalyst was obtained after calcination and reduction of the precursor. The influencingfactors such as catalyst content, the type of carbon source, synthesis temperature,synthesis time and the proportion of carrier gas for the synthesis of CNTs wereinvestigated. SEM was used to analyze the dispersion of CNTs in the magnesiummatrix, and TEM and Raman was used to analyze morphology and structure of in situsynthesized CNTs, while XRD was used to analyze the oxidation of the magnesiummatrix. The results show that using a chemical vapor deposition method can in situsynthesize CNTs with a high degree of graphitization, and uniform dispersion in themagnesium matrix.Mechanical milling of CNTs/Mg composite powder was conducted and theinfluences of milling speed, milling time on the microstructure and performance of thecomposites were studied. It is found that short time ball milling could prevent theCNTs from damage and destruction, and embed the CNTs within the matrix. Afterball milling, the metallographic structure became dense, and the mirco hardness andtensile properties increased. Finally, CNTs/Mg composites were fabricated by powder metallurgy method.The impact of CNTs content on the composite structure, hardness, tensile properties,thermal expansion properties and electrical properties were investigated. Theevolution of CNTs, oxidation of magnesium matrix as well as the interface bindingbetween the reinforcement and the matrix in the preparation process were discussed.The results indicated that with the increase of the contents of CNTs, the hardness andtensile strength of the composites increased, and reached the peaks at the CNTscontent of2.4wt.%. The tensile strength of2.4wt.%CNTs/Mg composite reached289MPa, which is an increase of47%compared to that of commercial pure Mg. Thehardness is72.8HV, increased30%than pure magnesium, but the elongationdecreased, which shows the typical features of brittle fracture. In the entire process,oxidation degree of the magnesium matrix was low, and it is advantageous for theperformance of the composite. CNTs maintained a complete graphite layer structure,and interface reaction with the matrix did not occur, that is to say, CNTs embeddingin magnesium played the role of bearing stress to strengthen the matrix.