| In this paper,MgO coated carbon nanotubes(MgO coated CNTs,MgO@CNTs)were prepared by chemical co-deposition,and the microstructures of original CNTs and pretreated CNTs(MgO@CNTs)were studied.Two kinds of reinforcements(CNTs,MgO@CNTs)with the same content(1 wt.%)were added to the pure magnesium matrix to prepare carbon nanotube-reinforced magnesium-based composite materials(Mg-CNTs,Mg-MgO@CNTs)by powder metallurgy.The effects of the microstructure(grain size,distribution,etc.)and mechanical properties(yield strength,microhardness)of the two composite materials were studied,and the preliminary exploration of the hierarchical structure and microstructural characteristics of fine and coarse crystals caused by discontinuous reinforcement,as well as the influence of modified carbon nanotubes(MgO@CNTs)and magnesium matrix interface regulation.At the same time,the first-principles calculation was used to calculate and analyze the separation work of magnesium-based composite materials.The main conclusions of this article were summarized as follows:(1)The chemical co-deposition method realized the coating of MgO nanoparticles on the surface of carbon nanotubes.The coating mechanism was that a large amount of carboxyl functional groups were introduced on the surface of the carbon nanotubes by means of a strong mixed acid.Then added Mg2+(magnesium chloride),a complex reaction occured and Mg2+was adsorbed to the surface of the carbon nanotubes.After added ammonia water,magnesium hydroxide precipitate produced on the surface of the carbon nanotubes and covered on the surface of the carbon nanotubes.Finally,coated magnesium oxide carbon nanotubes were obtained by sintering at high temperature.XRD characterization found that MgO@CNTs was smoother than the original CNTs,it indicated that the order degree of MgO@CNTs was greatly improved and the surface roughness of CNTs was improved,which was beneficial to improve the quality of interface bonding.(2)Both carbon nanotubes can effectively refine the grain structure of pure magnesium matrix.The average grain size of 1 wt.%Mg-CNTs composite in the sintered state was 84.84±5.34 μm,and the extruded state was 34.76 ± 2.31 μm.The average grain size of 1 wt.%Mg-MgO@CNTs composite in the sintered state was 59.29 ± 4.19 μm,and the extruded state was 19.79±1.89 μm.Compared with the pure magnesium powder size of 150 μm in the matrix,the composite materials had obvious refining effect,but the refining effect of MgO@CNTs was more obvious.The reasons for obvious refining effect were that the carbon nanotubes coated with MgO can provide more heterogeneous nucleation points during the dynamic recrystallization process of the matrix,form more small grains and refine the grains.At the same time,the addition of MgO@CNTs significantly improved the pore defects of the matrix and increased the density,compactness and hardness of the composite material.(3)Initially explored the microstructure of the two composite materials,and found that there were coarse-grained region(white region)and fine-grained region(black region)in two kinds of structure.And combined with the X-ray energy spectrum scanning,which found that the coarse-grained region of the CNTs/Mg composite was mainly composed of pure magnesium,and the fine-grained region was mainly composed of a mixture of carbon nanotubes and pure magnesium interwoven and distributed together,and the distribution of coarse and fine grain regions were not uniform.These characteristics were characterized as hierarchical structure.It was calculated that the proportion of fine-grained regions in the hierarchical structure of Mg-CNTs composite was 18.7%,and the proportion of fine-grained regions in the hierarchical structure of Mg-MgO@CNTs composite was 59.6%,which further illustrated the refinement ability of MgO@CNTs was stronger.(4)It was found that the hardness of the coarse and fine grain regions in the hierarchical structure of the two composite materials was quite different.In the sintered coarse-grained region of the Mg-CNTs composite,the hardness value was 57.91 ± 3.21 HV,and in the fine-grained region was 86.73 ± 4.98 HV.In the Mg-MgO@CNTs composite,the sintered coarse-grained region had a hardness of 72.94±4.32 HV,and the fine-grained region had a hardness of 90.11±5.82 HV.By comparison,the hardness value of the sintered coarse-grained region of the composite material with MgO@CNTs increased by 25.9%compared to the sintered state of the CNTs,and the hardness value of the fine-grained region increased by 3.9%.In addition,Mg-CNTs and Mg-MgO@CNTs composites had a similar trend of hardness value in the extruded state between coarse and fine grain regions as in the sintered state.The tensile test results showed that the yield strength of the Mg-CNTs and Mg-MgO@CNTs composites were 78.5±2.3 MPa and 202.1±3.4 MPa,respectively,which were 36.8%higher than those of the matrix(57.4±2.5 MPa)251.9%.The reasons for the improvement of the yield strength of the composite material were that the effects of fine-grain strengthening and stress transfer enable carbon nanotubes with super-strong mechanical properties to be exerted,and the mechanical properties were significantly improved.(5)Magnesium oxide can effectively improve the interface bonding between carbon nanotubes and magnesium matrix.Through transmission electron microscopy characterization analysis,it was concluded that the coated MgO carbon nanotubes embedded in the magnesium matrix and the magnesium matrix form a semi-coherent interface with a mismatch degree of 12.8%.At the same time,it was found that carbon nanotubes were mainly distributed in the fine-grained region,grain boundaries and inside the grains.Combined with the first-principles calculation results of the Mg/MgO/CNTs interface,it was found that the presence of oxygen atoms between the carbon tube and the magnesium matrix increased the interface separation work from 0.93 J/m2 to 4.70 J/m2,indicating the presence of oxygen make the interface tighter.Which was consistent with the experimental results that MgO effectively improved the carbon nanotubes/magnesium interface combination. |
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