| Magnesium is widely distributed in nature and abundant in reserves.Magnesium alloys have wide spread applications in automobile industry,electronics industry,sports equipment and medical equipment,etc.,because of their low density and high specific strength.However,the low absolute strength and modulus of magnesium alloy are the main factors restricting its development,so it has a great significance to develop magnesium matrix composites.In this paper,the precursor of composite was prepared by electroless nickel plating,ultrasonic vibration,ball milling and dispersing,and mechanical stirring.Then,AZ31 magnesium matrix composites reinforced by CNTs,GNPs and CNTs combined with GNPs were prepared by vacuum hot press sintering.Finally,AZ31 magnesium matrix composites were processed by hot extrusion.The mechanical properties of sintered and extruded composites were tested at room temperature.Optical metallographic microscopy(OM),X-ray diffraction(XRD),scanning electron microscopy(SEM)and transmission electron microscopy(TEM)were used to study the microstructure,fracture morphology,surface morphology and wear surface morphology of composites under different conditions.The dispersion and morphology characteristics of the reinforcement in the matrix were analyzed by the Raman spectroscopy(RM)and TEM.The effect of the amount of reinforcement on microstructure and mechanical properties of sintered composites,and the effects of hot extrusion on microstructure,mechanical properties at room temperature and wear resistance of composites were studied.The results show that the preparation process of composite by the preparation of precursor and vacuum hot press sintering combined with hot extrusion can successfully add CNTs and GNPs into AZ31,and the CNTs and GNPs can maintain their own morphological characteristics and present a good dispersion state.When CNTs are used as a single reinforcement,the mechanical properties are significantly improved with the addition of CNTs.When the addition amount of CNTs is 1.0 wt.%,the microhardness and tensile strength of CNTs/AZ31 composite reach peak values,and a better elongation could be obtained also.When GNPs are used as a single reinforcement,the mechanical properties of the composite are also significantly improved with the increase of the amount of GNPs:when the amount of GNPs is 0.50wt.%,the microhardness,compressive strength and ductility of GNPs/AZ31 composite reach the peak.When CNTs and GNPs are used as reinforcement at the same time,with the change of the proportion of CNTs and GNPs,the mechanical properties of the composite vary:when the reinforcement is added at 1.0wt.%and the proportion of CNTs to GNPs is 1:2,the composite presents the best mechanical properties.The wear resistance of AZ31 can be improved by the addition of GNPs.The friction coefficient of 0.50wt.%(GNPs+Ni)/AZ31 is reduced by 44.68%and 27.78%,respectively,compared with AZ31 and 0.50wt.%GNPs/AZ31.The wear rate of 0.50wt.%(GNPs+Ni)/AZ31 is reduced by 66.46%and 40.22%,respectively,compared with AZ31 and 0.50wt.%GNPs/AZ31.The wear behavior of(GNPs+Ni)/AZ31 is typical abrasive wear.The reinforcement mechanism of AZ31 composites reinforced by nano-carbon materials mainly includes two types.The first type is direct reinforcement,which includes two mechanisms:(1)Load transfer enhancement:nano-carbon materials are uniformly dispersed in the composite,which can bear the additional load and improve the mechanical properties of the composite;(2)Thermal mismatch enhancement,which is caused by the large difference in the coefficient of thermal expansion between Nano-carbon materials and AZ31 magnesium alloys.The second type is indirect reinforcement,which includes two mechanisms:(1)Grain refinement enhancement.Nano-carbon materials are dispersed near the grain boundary,which can prevent grain growth during hot press sintering and hot extrusion,thus refining grain structure;(2)Dislocation reinforcement:the presence of nano-carbon materials increases the dislocation density of the composites,which can promote the mechanical properties of the composites indirectly.The theoretical yield strength values of the raw AZ31 and AZ31 reinforced by nano-carbon materials after hot extrution are estimated with four kinds of strengthening mechanism formulas.The results show that:in the former,thermal mismatch strengthening has the greatest effect on magnesium matrix composites,followed by dislocation strengthening and load transfer strengthening,the least effect is grain refinement strengthening;in the latter,thermal mismatch strengthening and grain refinement strengthening have the greatest and the least effect on magnesium matrix composites also,the order of dislocation strengthening and load transfer strengthening has changed. |
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