【作者】 刘振宇；

【导师】 马宗义； 肖伯律； 董闯；

【作者基本信息】 大连理工大学 ， 材料加工工程， 2014， 博士

【摘要】 本文分别利用两种机械法分散(高能球磨HEM,搅拌摩擦加工FSP)方式制备了CNT/Al复合材料。采用光学显微镜、扫描电镜、透射电镜、拉曼光谱：X射线衍射、显微硬度、拉伸实验机、热物理分析仪等测试手段对两种制备方法所涉及到的CNT长度变化、损伤、分散均匀性和CNT-A1界面反应以及拉伸性能和热物理性能等进行了系统地研究与分析,并结合模型计算就两种机械法分散工艺中的CNT长度演化以及复合材料的强化机制进行了深入的探讨。对于球磨工艺制备CNT/Al-4.9Mg复合材料来说,影响CNT分散的主要因素是球磨转速和时间,300rpm时A1粉末的变形程度小,比表面积不足以容纳CNT；450rpm时A1粉末的变形程度大,但粉末之间的冷焊严重,CNT来不及分散就被冷焊到基体中；400rpm时Al粉的变形较大,而冷焊也不严重,有利于获得均匀分散的CNT。在400rpm下,随着球磨时间延长,CNT长度变短,但分散性逐渐提高,经过8h球磨后CNT已经完全分散到Al基体中了,同时CNT的长度也从原始的数微米减小到250nm。热压温度对CNT/Al-4.9Mg复合材料的界面反应有重要的影响,853K热压时,CNT-A1界面反应严重,CNT损伤明显；而当热压温度减小到753K时,界面反应受到明显抑制,在基体中只观察到尺寸为数十纳米的细小Al4C3反应产物：CNT-A1界面干净、平直,CNT管状结构保持良好,复合材料的强度也得到明显提高。对于优化参数下制备的复合材料中CNT在基体中分布均匀、且沿着热变形方向定向分布。均匀分布的CNT使复合材料的晶粒明显细化,1.5和3vol.%CNT的复合材料的晶粒仅有300和200nm。复合材料的密度随CNT含量的增加而线性减小,而弹性模量和力学强度则大幅度提升。3vo1.%CNT/Al-4.9Mg复合材料的抗拉强度达到601MPa,弹性模量提高到83GPa,但过于细小的晶粒和HEM引入的杂质使复合材料延伸率较低,仅有3%。通过多道次FSP制备了CNT/2009A1复合材料。FSP道次增加,CNT的团聚含量减小,CNT分布均匀性提高。超过3道次后CNT完全分散到基体中。FSP道次增加,CNT的长度减小。当道次增加到5时,CNT的长度仅有250nm,但CNT的直径在整个FSP过程中变化很小。CNT分布在基体中可以有效地钉轧晶界,使得复合材料的晶粒尺寸随着FSP道次的增加而减小,超过3道次后晶粒尺寸达到稳定,约为800nm。3-4道次时,CNT既能均匀分布,又能保持一定的长度,且晶粒也比较细小,所以复合材料的强一塑性配合比其它道次的复合材料要好。利用4道次FSP制备了1.5和4.5vol.%CNT/2009A1复合材料,结果发现CNT在经过4道次FSP后均匀随机的分散在基体中,部分还有弯曲。CNT受到的损伤较小,管状结构保持良好且CNT-A1界面结合良好,仅在少量CNT的管端观察到有A14C3的生成。CNT的加入使复合材料的屈服强度大幅度提高,1.5和4.5vol.%CNT复合材料的屈服强度相对于基体分别提高了26和43%；但抗拉强度只提升了约11%,且CNT含量从1.5提高到4.5vo1.%时,复合材料的抗拉强度还出现下降,主要是CNT的随机取向造成的CNT的不同时损伤。为此,采用后续热轧制的方式使CNT定向分布,结果发现热轧制后CNT实现很明显的定向分布,CNT仍保持其管状结构且界面干净平直,复合材料的强度、塑性和弹性模量相比于FSP复合材料得到大幅度的提升,3vol.%CNT/2009A1复合材料在强度达到600MPa的同时仍能保持10%以上的延伸率,相比于其它方法制备的CNT/A1复合材料具有更明显的强一塑性优势。建立并验证了CNT在机械作用下的损伤模型。模型分析表明,CNT的平均长度的倒数与机械作用周次呈线性关系。该模型可以描述机械法分散CNT-Metal复合材料体系和单纯机械切断CNT过程中的CNT长度变化。建立并验证了CNT/Al复合材料的强化模型,结果表明其强化是一个多种强化机制综合作用的结果,主要包括：细晶强化、Orowan强化、载荷传递强化以及交互作用强化。细晶强化在CNT体积分数高时作用明显,Orowan强化在晶粒较粗大时起作用,载荷传递强化在CNT体积分数较高时作用明显,而交互作用强化可以看成是细晶强化和Orowan强化引起的基体强度增加值通过载荷传递机制而产生的放大部分,它始终都发挥一定的作用。特别的,对于球磨法分散的CNT/Al-4.9Mg复合材料,还发现CNT加入引起的基体残余应力所造成的强化。引入CNT团聚因素,扩展了CNT增强金属基复合材料的强度模型,使其可以应用于更广泛的情况。CNT的加入使复合材料的高温强度相比于基体合金得到提高,且定向排布的CNT复合材料比随机取向复合材料的强化效果更明显。但这种强化效果随着温度的上升而逐渐下降,在温度达到473K时,复合材料的强度尤其是4.5vol.%CNT/2009A1复合材料急剧下降。CNT的加入引起复合材料热膨胀系数的下降,且抑制CNT-A1界面反应和使CNT定向更有利于得到较低的热膨胀系数。CNT引起的热膨胀系数下降符合Schapery模型。CNT的加入并未能提升复合材料的热导率,可能的原因是界面热障层以及CNT的加入引起的位错密度的增加。更多还原

【Abstract】 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.更多还原