【作者】 曹玮；

【导师】 张荻； 范同祥；

【作者基本信息】 上海交通大学 ， 材料学， 2009， 博士

【摘要】 随着能源紧缺及环境污染问题的日益突出,汽车工业对高效率,低排放和轻量化的要求逐渐提高。镁合金的开发和应用顺应了汽车节能和环保的趋势,越来越引起人们的关注,但是镁合金的耐热性能较差限制了其在汽车工业上的广泛应用。本文的研究工作主要是针对上述问题进行,运用材料复合化思路和原位制备方法,探索研制耐热性能改善,弹性模量提高,强度提高和阻尼性能优良的结构功能一体化的原位合成镁基复合材料。相对于其它金属基复合材料(如铝基,钛基,铁基和铜基等)的研究发展水平,原位合成镁基复合材料的研究处于初步阶段。因此,研究镁基复合材料的原位反应体系,制备工艺,微观结构和力学性能,阻尼性能以及耐热性能等之间的内在关系,对研制和开发综合性能优越的镁基复合材料及其在汽车工业上的应用具有重要的指导意义。本文首先对原位反应的热力学研究表明,Mg-Al-Ti-C体系可以发生原位反应生成TiC增强颗粒。利用Miedema模型结合Wilson方程,计算并预测了常用合金元素或稀土元素会对Mg-Al-Ti-C体系的原位反应的影响规律。根据计算结果,选用汽车工业上常用的AZ91D作为基体合金,合金中的Zn和Mn等元素,有利于原位反应产物TiC的生成,同时有利于抑制TiAl3中间相的形成。对原位反应的实验研究表明,原位反应形成TiC增强镁基复合材料的过程为:Al先通过Al-Ti反应形成TiAl3金属间化合物,同时释放出大量热量,进一步引发了TiAl3和C以及Ti和C之间反应形成热力学上更稳定的TiC。预制块中的Al不但能够参与原位反应,降低原位反应的温度,使TiC颗粒尺寸减小,同时Al也可以作为稀释剂,有利于TiC颗粒在Mg熔体中的扩散和均匀分布。原位制备的TiC颗粒增强的镁基复合材料,颗粒细小且分布均匀,颗粒附近存在高密度的位错,颗粒与镁基体界面清洁,结合紧密,这些均对提高镁基复合材料的力学性能有利。对TiC/AZ91D镁基复合材料的室温力学性能研究表明,原位合成的TiC增强体的引入,使复合材料的室温拉伸强度和压缩强度均有了提高。与基体合金相比,TiC颗粒增强镁基复合材料的室温拉伸强度提高了约20-26%,弹性模量提高了3-11%。复合材料的强化机理主要归结为细晶强化和位错强化的综合作用。复合材料的失效主要是由颗粒与基体界面处的脱粘和基体的断裂引起。镁合金作为汽车零件主要受螺栓等压应力作用,常见使用温度在25-200°C之间,对以上工况条件下AZ91D镁合金和TiC/AZ91D镁基复合材料的压缩实验研究表明,AZ91D镁合金断裂区域和安全区域的分界线Z=8×1014s-1。TiC/AZ91D镁基复合材料的分界线Z值经计算为Z=5×1013s-1。与AZ91D镁合金相比,TiC/AZ91D镁基复合材料需要更低的Z值,即更高的温度和更低的应变速率以避免材料发生断裂。通过研究镁合金常见变形温度(250-400°C)和常见应变速率(10-3-8s-1)下TiC/AZ91D镁基复合材料的压缩变形行为,构建出TiC/AZ91D镁基复合材料的热加工图。热加工图能预测的加工性能良好区域和加工非稳区,为判定材料的最佳热加工条件和失稳区域,制定合理的热加工工艺提供理论和实践依据。本文进一步研究了原位颗粒增强镁基复合材料的阻尼性能。复合材料的阻尼性能优于基体合金,并且复合材料的阻尼性能随增强体体积分数的增加而提高。在室温时,增强颗粒附近存在高密度的位错,有利于复合材料的室温阻尼性能提高。高温时,增强体与镁基体界面强度随温度的升高而降低,在一定应力作用下,通过晶界滑移,界面滑移等方式消耗振动能量,从而提高高温阻尼性能。最后,本文建立了螺栓载荷保持(Bolt Load Retention,BLR)评价系统,模拟镁合金及复合材料作为汽车零件在螺栓预紧力作用下高温长时间服役的实际工况。AZ91D镁合金及TiC/AZ91D镁基复合材料的BLR行为测试结果表明,随着温度和载荷的增加,镁合金及其复合材料的螺栓剩余载荷比例减少,但是预载荷的增加仍然有利于残余载荷的增大。在本文测试范围内,镁基复合材料在不同温度和载荷下的BLR性能均优于镁基体合金,且随温度提高镁基复合材料BLR性能有较大改善。通过研究归纳的剩余载荷比例的等值线可以用来表示温度和初始载荷对剩余载荷百分数的综合影响。等值线图的建立为镁合金零件的工程设计提供理论依据。更多还原

【Abstract】 Due to energy shortage and environmental pollution, magnesium alloys are attracting more and more attention in automobile industry for losing the weight of a vehicle,reducing the exhaust emissions and raising fuel efficiency. However, the poor thermal resistance properties of magnesium alloys restrict development and application of magnesium alloys in automobile industry.This research is trying to solve the above problem. The research work aims to synthesize magnesium matrix composites by using in situ technique and obtain higher strength, better thermal resistance properties and better damping capacities than those of magnesium alloys. Compared with ex situ technique, in situ technique exhibits the finer reinforcement,the better distribution and the cleaner reinforcement-matrix interface than ex situ one. So in situ magnesium matrix composites have great potential to obtain excellent mechanical properties and functional properties and extend the application of magnesium alloys in automobile industry.In situ synthesis method has been extensively studied for aluminum, titanium, iron and copper matrix composites. However, this technique is still relatively new for in situ magnesium matrix composites. Therefore it is of great significance to study in situ reaction, fabrication, microstructure, mechanical properties, damping capacities and thermal resistance properties of in situ magnesium matrix composites.In this study, the feasibility of Mg-Al-Ti-C in situ reaction was firstly confirmed thermodynamically. According to the calculation using an extended Miedema model and the Wilson equation, the influences of alloying elements on the free energy of TiAl3 and TiC formation were calculated and the effect of the alloying additions on in situ reaction to synthesize TiC/Mg composites was evaluated. The calculation results show elements Zn and Mn in AZ91D alloys were beneficial to promote TiC formation, as well as hinder the brittle TiAl3 phase formation.Experimental research showed molten magnesium alloy was infiltrated into Al-Ti-C preforms, simultaneously the in situ reaction happened and TiC particles were formed in the liquid of magnesium alloy. Ti reacted with Al to form TiAl3 in the initial stage, and then C reacted with TiAl3 to form TiC. Al in the preforms serves not only as a reactant and participates in the in situ reaction to decrease reaction temperature and TiC particle size, but also as a diluent to to facilitate the diffusion and distribution of TiC particles. The as-cast microstructure of the in situ composites revealed the uniform distribution of TiC particulates with spherical sizes. Microstructural analysis showed high dislocation density around the reinforcements and good and clean interface between TiC particles and matrix.Introducing TiC particles into magnesium matrix improved the mechanical properties of matrix material. The tensile strength of magnesium matrix composites were 20-26% higher than those of magnesium alloy and elastic module were 3-11% higher than magnesium alloy. The strengthen effect of in situ magnesium matrix composites was explained by refine grain strengthen and dislocation strengthen mechanism. The fracture analysis of TiC/AZ91D composites revealed the failure of composites was caused by interfacial debonding and the fracture of matrix.The compressive deformation behaviors of AZ91D alloys and TiC/AZ91D composites were studied in temperature and strain rate range of 25-200°C and 10-3-1s-1. The border of early fracture is close to Z=8×1014s-1 in AZ91D alloys and Z=5×1013s-1 in TiC/AZ91D composites. So this value may represent the limit of hot deformation conditions because a premature fracture occurs for Z>8×1014s-1 in AZ91D alloys and Z>5×1013s-1 in TiC/AZ91D composites. Compared with AZ91D alloy, TiC/AZ91D composites need higher temperature and lower strain rate (lower Z value) to avoid premature fracture.The compressive deformation behavior of TiC/AZ91D composites has also been investigated at strain rates 10-3-8s-1and hot working temperatures 250-400°C. The processing map has been adopted to correlate processing parameters, predict safe regions and instable regions, obtain safe processing conditions and optimize hot forming process of TiC/AZ91D composites.Damping capacities of in situ TiC reinforced magnesium matrix composites with different reinforcement percentage were investigated. Experimental results show TiC reinforced magnesium matrix composites possessed better damping capacities than those of non-reinforced magnesium alloy. Compared to magnesium matrix alloy, an increase in damping capacities of TiC/AZ91D composites was attributed to the dislocation damping mechanism at room temperature. At elevated temperatures, the matrix became relatively soft with respect to ceramic reinforcement and interface sliding between the reinforcement and matrix occurred. Interface damping became a new contributor to the increase of damping capacity.A new bolt load retention evaluation system was set up to simulate automobiles parts served at high temperature and preloads and evaluate the thermal resistance properties of Mg alloys and composites. The effect of the variables such as temperature, preload and different samples on the feasibility of BLR evaluation systems was systematic investigated. BLR behaviors of AZ91D alloys and TiC/AZ91D composites at different temperature and preloads were tested. Results showed that the increases in temperature and preload led to a decrease in BLR as a fraction of preload. Higher preloads led to a higher remaining clamp loads, although a fraction of preloads was lower for higher preloads. The fraction of remaining load to preload of composites was found to be higher than that of AZ91D alloy, which indicated that TiC/AZ91D composites can improve BLR behaviors of magnesium matrix. The combined influence of temperature and preload level can be described by a series of“contour”maps that relate the fraction of remaining load to preload and temperature. This diagram is useful for engineering design.更多还原