Fabrication, Microstructure And Properties Of Carbon Nanotube Reinforced Aluminum Matrix Composites Prepared By Mechanical Method

CNT/Al composites were fabricated by two mechanical dispersion methods:high energy ball milling (HEM) and friction stir processing (FSP), respectively. The length, structure damage, distribution of the CNTs, and CNT-Al interface reaction as well as tensile properties and thermal physical properties were investigated using OM, SEM, TEM, XRD, raman spectrum, HV hardness, tensile test and thermal physical tests, respectively. The CNT length evolution and strengthening mechanism of CNT/Al composites were also discussed by modelling calculations.For the CNT/Al-4.9Mg composites fabricated by HEM method, the main factors influencing on the CNT distributions are rotation rate and ball milling time. At300rpm, the deformation extent of the Al powders was small and the surfaces of the Al powders were not large enough for CNTs to disperse. At450rpm, the cold-welding was severe and many CNT clusters were cold-welded into the matrix. The deformation under400rpm was relatively large and the cold-welding was not that severe, both of which were beneficial to CNT distribution. As the ball milling time increasing, CNTs got shorter and the distribution got better. After8h milling time, the CNTs were completely dispersed into the Al matrix, and the CNT length reduced to about250nm. Hot pressing temperature has significant effect on the interface reaction. CNTs were severely damaged at853K, with many Al4C3formed in the composites. As the hot pressing temperature decreased to753K, the reaction of CNT-Al was significantly restrained and only a little fine Al4C3(<50nm) could be found.CNT-Al interfaces in the corresponding composites were clean and smooth, and the tube structures of the CNTs were also well retained. As a result, the tensile strengths of the composites were greatly increased compared with the matrix alloy. For the composites fabricated under optimized parameters, CNTs were uniformly dispersed in the matrix and aligned along the direction of material flow during deformation. The dispersed CNTs resulted in refined grain size. The1.5and3vol.%CNTs resulted in300and200nm grain sizes, respectively. The densities of the composites were linearly reduced as the CNT concentration increasing, the moduli and tensile strengths of the composites were also significantly increased.3vol.%CNT/Al-4.9Mg composite had modulus of83GPa and tensile strength of601MPa. But the extremely fine grain size and the contamination introduced during ball milling resulted in low elongation,3vol.%CNT composite got a elongation of only3%.CNT/2009Al composites were fabricated by multi-pass FSP. Increasing FSP passes resulted in more uniform distribution of CNTs. After3passes, CNTs were completely dispersed into the matrix. CNT length was reduced as increasing FSP pass, and length of only250nm was retained after5passes FSP. However, the diameters of the CNTs changed little, even after5passes FSP. The uniformly dispersed CNTs could pin the grain boundaries and led to refined grain size. Grain size of800nm was obtained after3passes. The composites under3-4passes showed better strength-ductility compared with other passes due to the uniformly dispersed CNTs and relatively long CNTs. For the4passes FSP composites, CNTs were uniformly dispersed. The structure damages of CNTs were not severe, the CNT-Al interfaces were well bonded and only a few Al4C3were found at some CNT tips. The CNT adding led to significant increase on yield strength (YS). The YS of the composites were increased26and43%, compared with the matrix alloy. But the increase on ultimate tensile strength (UTS) was not obvious, and only about11%increases were obtained for UTS. And the UTS were even reduced as increasing the CNT concentration from1.5to4.5vol.%, mainly due to the randomly orientation of the CNTs. Therefore, the FSP composites were hot-rolled for CNT aligning. It was observed that CNTs were aligned along the rolling direction after hot-rolling. As a result, the YS, UTS, elongation and moduli were greatly increased compared with those of the FSP composites. And the FSP-rolled CNT/Al composites showed much better strength-ductility than the CNT/Al composites fabricated by other methods. Especially,3vol.%CNT/2009A1shows UTS of600MPa and elongation of larger than10%.A model on CNT length evolution was proposed. The model indicated that the reciprocal of CNT length had a linear relationship with the number of mechanical processing cycles, which could describe the CNT length evolution for CNT-Metal composites fabrication and CNT cutting under mechanical processing. A strengthening model considering grain refinement, Orowan strengthening, load transfer mechanism and interaction strengthening was proposed to predict the YS of the CNT reinforced metal matrix composites. The model calculation indicated that the strength increase due to grain refinement and load transfer mechanisms were increased as the CNT concentration increasing. The strength increase due to Orowan strengthening mechanism was relatively large at coarse grain sizes. The interaction strengthening could be considered as part of matrix strength increase due to grain refinement and Orowan mechanism magnified by load transfer mechanism. Especially, the strengthening of residual stress due to CNT incorporation was found for CNT/Al-4.9Mg composites fabricated by high energy ball milling followed by hot-extrusion. A universal strength model is proposed by considering the factor of CNT clustering.The elevated temperature strength of the CNT/Al composites were also increased compared with those of the matrix alloy. The strengthening effect was more pronounced for the aligned CNT composites than the composites with randomly oriented CNTs. But the strengthening effect was reduced as temperature increasing, especially as the temperature higher than473K. CNTs resulted in lower coefficient of thermal expansion (CTE). Aligning the CNTs and restraining the interface reaction had higher effeciency to reduce CTE. The CTE changing was in accordance with the Schapery model. Thermal conductivity of the composites was not enhanced by CNT incorporation, which could be the result of the thermal barrier coatings at the interface and the dislocation density increasing due to CNT adding.