Fabrication,Processing And Mechanical Behavior Of CNT/7055Al Composites

Carbon nanotube reinforced aluminum matrix composites(CNT/Al)have the advantages of high strength,high stiffness,low density,etc.,and they have broad application prospects in aerospace,transportation and other fields.In the past two decades,although many studies on CNT/Al composites have been reported,there are still some problems in preparation,processing and mechanical behavior of CNT/Al composites.Although the high-energy ball milling(HEBM)process could efficiently disperse CNTs,the damage and dispersion of CNTs in HEBM still lack a unified and in-depth understanding.The problems of low room temperature ductility and difficulty in high temperature deformation caused by the addition of CNTs are the bottleneck restricting the wide application of CNT/AI composites in engineering,but there are no effective improvement methods so far.In addition,most studies on the mechanical behavior of CNT/Al composites focus on static mechanical properties.There are few investigations on dynamic mechanical properties.The high-temperature deformation behavior of CNT/Al composites is also poorly understood.Aiming at the above problems,the following investigations have been carried out.To obtain high performance CNT/Al composites,the ultra-high strength 7055Al alloy was used as the matrix alloy.The damage and dispersion mechanisms of CNTs in the HEBM process were systematically studied.Two HEBM powder morphology evolution modes(HEBM-1 and HEBM-2)were designed by adjusting the process control agent.In HEBM-l,the powders were flattened,cold-welded and fractured.In HEBM-2,cold-welding was suppressed.and the powders were flattened,and maintained flaky shape up to fracture.It was found that the HEBM-1 rather than HEBM-2 evolution could well disperse CNTs.Repetitive powder fracture and cold-welding processes were considered as the main pattern for dispersing CNTs in HEBM-1.The HEBM-2 evolution failed to disperse CNTs,because the thickness of the flaky Al powders exceeded the critical value,and the agglomeration of CNTs embedded in the surface of the flaky Al powders.It is found that the product of ID/IG and FWHM(D)rather than the ID/IG can more accurately describe the CNT structure damage.The results showed that the damage of CNT was serious in the stages of powder flattening and fracture rather than the powder cold welding stage.On this basis,the CNT/7055Al composites with different CNT contents were prepared and their mcrostructure and mechanical behavior were analyzed in detail.It was found that the CNT/7055Al composites presented a mixed structure containing coarse and ultrafine grains.CNTs were mainly distributed in the ultrafine grain zones,and most of CNTs were aligned along the extrusion direction.The CNT-Al interface was clean and free of reaction products.The tensile strength of 3 vol.%CNT/7055Al composite reached 816 MPa,but the elongation was only 0.5%.Fine grain strengthening and Orowan strengthening were the main strengthening mechanisms of the CNT/7055Al composites.Due to the presence of coarse grain strips,the composites had serious anisotropy.For improving the strength and ductility of the CNT/7055Al composites,the texture components of the composites were optimized by hot-extrusion followed by hot-rolling process.The microstructure,texture components and mechanical properties of the composites before and after hot-rolling were studied.The results showed that the coarse grain strips were almost eliminated after hot-rolling,and the composites were composed of uniform ultrafine grains.The CNT structure was not damaged during hot-rolling.The composites had a fiber texture including Copper{112}<111>and{123}<111>components before rolling.After rolling,the composites had a plate texture including {011}<111>,{113}<332>and {215}<342>components.With the increase of rolling reduction,the texture intensity of the composites increased.After rolling,the elongation of the composites was improved by twofold with slightly decreased tensile strength.This was due to:(a)large numbers of grains with favorable orientations in the plate texture reduced the threshold of deformation;(b)the increased dislocation density after hot-rolling could promote the deformation;(c)the increased proportion of low angle grain boundaries after hot-rolling contributed to dislocation slip;and(d)the fine and densely distributed precipitates increased the dislocation storage capacity of ultrafine grains.In addition,after rolling,the anisotropy of composites was obviously weakened due to the elimination of coarse grain strips.In order to improve the hot deformation ability of the composites,the superplastic deformation behavior of CNT/7055Al composites was investigated.The optimal deformation parameters of the composites were obtained,and the role of CNTs in superplastic deformation was analyzed.The results showed that by optimizing the number of the second phases,the T6 treated composite exhibited higher elongation than the extruded composite.The maximum elongations of the T6 treated and extruded composites were obtained at 400? and 5 s-1,and the values were 153%and 125%,respectively.The deformation activation energy and m value of the T6 treated composite were 1 71 kJ/mol and 0.2,respectively.Lattice diffusion-controlled dislocation climbing was the main deformation mechanism.The superplastic deformation behavior of the composites with different CNT contents was also studied.It was indicated that the maximum elongations of 2 vol.%and 3 vol.%composites were obtained under the conditions of 400?,3 s-1 and 400?,5 s-1 with the values of 11 8%and 108%,respectively.The large aspect ratio of CNTs and the strong CNT-Al interface bonding greatly inhibited grain boundary sliding,which was the main reason for the lower superplastic elongation of the CNT/7055Al composite.To evaluate the service performance of the CNT/7055Al composites,the high cycle fatigue behavior of the composites under tension-tension and tension-compression conditions were studied.The results indicated that in the tension-tension fatigue,the load transfer effect of CNTs inhibited the crack initiation in the ultrafine grains,making the fatigue strength of the composite better than that of the matrix alloy.In the tension-compression fatigue,due to the fracture of the matrix near the CNT-Al interface,CNTs failed to exhibit reinforcing effects,resulting in the fatigue strength of the composite equivalent to that of the matrix alloy.The fatigue damage mechanism of the composites was analyzed,and the role of CNTs in fatigue deformation was also discussed.The results demonstrated that the strain localization of the coarse grains was the main mechanism of fatigue damage.In tension-tension fatigue,the damage only occurred in coarse grains.In tension-compression fatigue,the damage were both detected in the coarse and ultrafine grains due to the large stress amplitude.In the tension-tension fatigue and tension-compression fatigue,the deformation was mainly concentrated in the coarse grains.The dislocation densities in the coarse and ultrafine grains were both low under high stress condition.Under the low stress condition.dislocation cell,dislocation tangles and other substructures appeared in the coarse grains,but no obvious dislocation patterns were observed in the ultrafine grains.