Microstructure Refinement Mechanism Of AZ91 Magnesium Matrix Composite Reinforced By Pretreated CNTs

The study of high performance magnesium matrix composites reinforced by carbon nanotube is a hot field in automotive lightweight materials.It is one of the important ways to achieve energy saving and emission reduction.Therefore,the study has important research significance and application value.As a reinforcing phase,the pretreated carbon nanotube?CNTs after high temperature graphitization treatment,MgO-coated CNTs?are expected to improve the mechanical properties of magnesium alloy.CNTs can be dispersed uniformly in the magnesium matrix by using the mechanical ball milling.In order to solve the dispersion problem of CNTs,it's necessary to determine the optimized parameters of ball milling process.In this paper,the ball milling dispersing process of carbon nanotube and aluminum powder?CNTs/Al?was investigated.The effect of ball milling speed,ball milling time and CNTs content on the pre-dispersion of CNTs was analyzed.The dispersion of CNTs in Al powder and the structural damage of CNTs were characterized by scanning electron microscopy?SEM?and Raman spectroscopy?Raman Spectrometer?,respectively.Considering the dispersion of CNTs in Al powder and the degree of structural damage of CNTs,the optimized parameters of ball milling process were developed.Then,by using optimized parameters of ball milling process,the uniform dispersion of pretreated CNTs?Graphitizable CNTs and MgO@CNTs?in the Al powder were achieved.The mixture was compressed into CNTs intermediate blocks in a pressure.The two AZ91 magnesium matrix composite reinforced by pretreated CNTs?Graphitizable CNTs and MgO@CNTs?with the same reinforcements mass fraction?0.5 wt.%?were successfully prepared through stirring cast process.The microstructure of the cast composite was studied by using metallographic and image analysis techniques.In addition,identification of each phase was also approached by X-ray diffraction?XRD?.Meanwhile,the microhardness of the composite was also studied using Vickers microhardness tester.The microstructure refinement mechanism of AZ91 magnesium matrix composite reinforced by pretreated CNTs was also studied.The study provides some guidance for the development of high performance magnesium matrix composites reinforced by CNTs.The main results are as following:?1?The effect of ball milling speed,ball milling time and CNTs content on the pre-dispersion of CNTs were studied.The optimized parameters of ball milling process?ball milling speed of 300 r/min,ball milling time of 2 h and CNTs content of5 wt.%?were obtained.Using the optimal processing parameters of ball milling,CNTs can be dispersed uniformly in Al powder particles without obvious agglomeration and structural damage of CNTs.?2?AZ91 magnesium matrix composite reinforced by pretreated CNTs were successfully prepared by stirring cast process.The secondary dendrite arm spacing?SDAS?in magnesium alloys was analyzed.The SDAS value of AZ91 alloy is 5.54?m.The pretreated CNTs?MgO@CNTs and Graphitizable CNTs?could effectively promote the decrease of the SDAS value.?3?The grain size distribution and average grain size of composite were studied.The grain size distribution of AZ91 magnesium alloy is less than 75?m.At the same time,the grain size distribution is uneven.The pretreated CNTs?MgO@CNTs and Graphitizable CNTs?can refine grain and increase the uniformity of grain size distribution.?4?The morphology of?-Mg₁₇Al₁₂ phase was analyzed by SEM.The addition of pretreated CNTs?MgO@CNTs and Graphitizable CNTs?can dissolve the continuous reticular eutectic?-Mg₁₇Al₁₂ phase,which leads to the formation of isolated and discontinuous short rod like second?-Mg₁₇Al₁₂ phase distributed at the grain boundary.?5?The effect of pretreated CNTs?MgO@CNTs and Graphitizable CNTs?on the microhardness of as cast AZ91 magnesium alloy was studied.The pretreated CNTs produces the increase in the microhardness of as cast AZ91 magnesium alloy.The increase in microhardness of as cast AZ91 composites mainly results from the refinement of the grain and the?-Mg₁₇Al₁₂ phase.?6?The microstructure refinement mechanism of AZ91 magnesium matrix composite reinforced by pretreated CNTs was revealed by analyzing the microstructure of composite,which is mainly related to the cooling rate and pin effect of the CNTs in the grain boundary of the composite.