【作者】 汪峰涛；

【导师】 吴玉程；

【作者基本信息】 合肥工业大学 ， 材料学， 2009， 博士

【摘要】 W-Cu复合材料独特的性能使其被广泛用作电接触器、真空断路器、热沉材料等功能和结构器件。由于W、Cu之间较大的性能差异,一直以来W-Cu复合材料的制备工艺都是该领域的研究热点。目前,传统工艺存在致密化程度低、微观组织不均匀或成分受限制等一系列问题,使得W-Cu复合材料无法发挥更大的潜力。现代电子信息业和国防工业高尖端领域的快速发展对W-Cu复合材料提出了新的发展方向和要求:(1)探索适用于工业化生产的材料制取工艺;(2)通过进一步提高致密度和微结构均匀性得到更高性能的W-Cu复合材料;(3)开发满足高科技要求的新型高性能W-Cu复合材料;(4)拓展W-Cu复合材料的应用领域。本论文针对以上方面提出以机械合金化技术为基础结合常压烧结或热压的方法制备高性能细晶W-Cu复合材料、W-Cu/AlN复合材料和W-Cu梯度功能材料,并对其工艺,优化设计及性能等方面进行探索和研究。为今后高性能W-Cu复合材料的实际生产和应用领域的拓展提供理论依据和数据支撑。首先,对不同成分W-Cu纳米晶复合粉体的机械合金化过程进行了研究,通过对球磨和退火热处理过程中复合粉体的内部相转变和成分分布,以及晶粒尺寸、晶格常数、微观应变和形貌特点等方面的分析,探讨了机械合金化制备W-Cu纳米晶粉体的工艺特点、粉体的特征和热稳定性。W-15Cu、W-20Cu和W-30Cu复合粉末在分别球磨30h、40h和60h后均形成了常温下稳定的W(Cu)过饱和固溶体。在W-15Cu系统中:随着球磨的进行,复合粉体中W晶粒尺寸逐渐减小;复合粉末的粒度在球磨过程中呈现先增大后减小的趋势,球磨30h粉末颗粒呈多面体状,表面平滑,平均粒度4μm左右。W(Cu)过饱和固溶体的形成机制为球磨初期,粉体呈现Cu包裹W的包覆结构形态;继续球磨,包覆结构的复合颗粒进一步细化,包覆层间距大大减小,形成W、Cu均匀弥散分布的复合组织;球磨30h,形成了组织和成分均一的W(Cu)过饱和固溶体。W-15Cu纳米晶复合粉末在退火热处理过程中晶格畸变程度降低、内应力释放,粉体结构有序度明显提高;球磨过程中形成的W(Cu)过饱和固溶体在500℃退火时开始脱溶。其次,采用机械合金化技术制备的纳米晶W-Cu复合粉体,分别通过常压液相烧结或热压烧结的方法,制备出W-15Cu、W-20Cu和W-30Cu,以及W-Cu/x%AlN(x=0.25,0.5,1.0,2.0,质量分数)复合材料并对其密度、热导率、电阻率、电导率等物理性能,以及硬度、抗弯强度等力学性能和显微结构进行测试和观察,探讨了高能球磨及烧结工艺参数对W-Cu复合材料组织结构和性能的影响,以及高能球磨后粉体的烧结致密化机理。机械合金化技术从细化晶粒、提高粉体组成的均匀度和形成W(Cu)固溶体等多方面改善了W-Cu复合粉体的烧结性能,加强了W-Cu之间的相互作用,增大了W-Cu之间的接触机会,从而大大提高了W-Cu复合材料的致密度和组织的均匀性,是获得近全致密W-Cu复合材料最佳制备工艺之一。常压烧结优化工艺参数为:成型压力350MPa,烧结温度为1200℃,保温90min。W-15Cu、W-20Cu、W-30Cu复合材料的致密度分别为98.42%,99.10%,99.34%。采用真空热压烧结工艺在相对较低的温度1050℃,25MPa压力下烧结90min制备出组织结构更加均匀细小的W-Cu复合材料,三种成分的W-Cu复合材料的致密度分别达到:97.87%,98.29%,98.94%。少量纳米AlN颗粒(≤1wt%)的加入对W-Cu复合材料的致密度影响并不大,在1wt%加入量时,致密度仍接近98%;纳米AlN颗粒均匀弥散分布于基体中Cu相中,提高基体材料中Cu的硬度;但是随着AlN纳米颗粒的含量增加,基体晶界上的增强相颗粒分布过多,影响烧结过程中相邻W颗粒间结合和材料的致密化,而致使材料的抗弯强度有所下降,但对导热性能的提高有一定的帮助。最后,采用有限元方法(Finite Element Method),针对W-Cu梯度功能材料在制备过程中产生的残余热应力进行了数值模拟分析。在综合分析了残余热应力大小和梯度层中应力分布状态等因素的基础上,确定了三层结构的W-Cu梯度材料,P=2.4时,过渡层Cu含量为33vol.%时的三层W-Cu梯度功能材料W-20%Cu/W-33%Cu/W-50%Cu具有较好的热应力缓和效果;四层结构的W-Cu梯度材料,在P=1.4时,模拟计算得出的W-20%Cu/W-29.1%Cu/ W-39.2%Cu/W-50%Cu四层均厚的W-Cu复合材料的等效应力具有最小值。并基于上述成分结构研究了W-Cu梯度复合材料的制备工艺,提出先冷压后低温热压烧结的制备工艺较好的保证了梯度材料具有较高的致密度,同时保持了单层的原始设计成分。所制备W-Cu三层和四层梯度复合材料的层界面结合良好,没有明显的裂纹等缺陷;热导率分别达到198 W·m-1K-1和202 W·m-1K-1,获得了较高的导热性能;两种结构的FGM样品经800℃温差的热震试验后,界面处没有发现裂纹和开裂现象,表现出良好的抗热震性能;热疲劳试验结果表明,三层结构和四层结构的梯度材料在分别经过86和143次热循环后首次出现裂纹,热疲劳裂纹总是最先出现在梯度材料的两端,与热应力模拟结果一致。更多还原

【Abstract】 W-Cu composite possesses unique properties of superior thermal and electronic managements, high microwave absorption capacity and etc. These features make the W-Cu composite a very attractive material which has been widely used as heavy-duty electronic contactors, circuit breakers and thermal management devices, and so on. However, due to their relatively large difference in properties between W and Cu, the preparation process of this composite requires higher requirement. W-Cu composite prepared by the conventional methods has drawbacks such as low densification, microstructure inhomogeneity and/or limited composition variation which hinders its further applications. With the rapid development of the electronic information industry and the high-tech fields of defense industry, there are several new directions of development and requirements on high performance W-Cu composites, such as （1） to explore the preparation process which can be applied to industrial production; （2） to further improve the densification and microstructure uniformity of W-Cu composites with a higher performance; （3） to develop new high-performance W-Cu composites to meet the requirements of the high-tech fields; and （4） to expand the applications of W-Cu composites. In this paper, W-Cu, W-Cu/AlN composites and W-Cu functionally graded materials with high performance were fabricated by combining mechanical alloying technique and pressureless sintering or hot-pressing technique. And the technic parameters of process, optimal design and properties of those composites were investigated and analyzed. The results of those studies would provide a theoretical basis and data base for the production and application of high-performance W-Cu composites.The mechanical alloying process of nanocrystalline W-Cu composite powder with different contents was investigated. The process characteristics, structural evolution and thermal stability of the MA W-Cu nanocrystalline powder were investigated in detail by analysing phase transformation, composition distribution, grain size, lattice parameter, microstrain, and morphology changes of the composite powder during mechanical alloying and subsequent annealing. The results showed that room temperature stable supersaturated W（Cu） solid solution formed after 30h, 40h and 60h milling of W-15Cu, W-20Cu and W-30Cu composite powder. During mechanical alloying of W-15Cu powder, the grain size decreased with increasing milling time, and the particle size first increased then decreased with increasing milling time. After milling for 30h, particles were polyhedral in shape with smooth surface, and average particle size was about 4μm. At the initial stage, the structure of W-15Cu powder was the ring-like composite layer generated by the Cu particles enwrapping around the W particles. In the middle stage, the composite particles of circle-like layers became finer and the space between the layers greatly reduced. The mixed microstructure of homogeneously dispersed W and Cu was formed. After milling for 30h, the powder was composed of homogeneous W（Cu） solid solution phase. Annealing of as-milled W-15Cu powder reduces the lattice deformation and internal strain, and incresased the lattice ordering degree. Meanwhile, Cu precipitated from the supersaturated W（Cu） solid solution at the temperature of 500℃.Liquid phase sintering, or hot pressing was used to consolidate MA nanocrystalline W-Cu powders to bulk compacts of W-15Cu, W-20Cu, W-30Cu, and bulk composites of W-Cu/AlN with different contents of AlN （0.25, 0.5, 1.0, 2.0wt.%）. The microstructure, physical properties such as density, thermal conductivity, electrical resistivity and conductivity, mechanical properties such as hardness, bending strength and etc. of these materials were characterized and investigated. The effects of process parameters of high energy ball milling and sintering on the microstructure and properties of W-Cu composite, and the sintering densification mechanism of MA powders were studied. The results showed that mechanical alloying technique promoted the sintering of W-Cu composite powder, strengthened the interaction and increased the contacts between W and Cu by grain refining, enhancing the microstructure homogeneity of powder and the formation of W（Cu） solid solution. Thus, the relative density and microstructure homogeneity of W-Cu composites were effectively enhanced. Which means mechanical alloying is one of the optimal preparation processes to obtain near full dense W-Cu composites. Considering all factors, the optimal sintering process parameters were as follows: forming pressure was 350MPa, sintering temperature and time were 1200℃and 90min. The relative densities of W-15Cu, W-20Cu and W-30Cu composites were 98.42%, 99.10% and 99.34%, respectively by using the above-mentioned parameters. Finer structure bulk W-Cu composites were successfully synthesized by the means of hot pressed vacuum sintering at 1200℃for 90min under the pressure of 25MPa. The relative densities of W-Cu composites with three different compositions were 97.87%, 98.29%, and 98.94%, respectively. Addition of small amount of nano AlN particles had less negative effect on degradation of relative density. The relative density of W-Cu composites remained at about 98% when the addition of AlN was 1wt%. Nano AlN particles were even distributed in Cu phase of the matrix. The hardness of Cu in the matrix would increase due to the reinforcement effect brought about by grain refining and dispersion strengthening. The increasing strengthening particles distributed at grain boundaries with the increasing of AlN content, would lead to the decrease of bending strength of composites by affecting the combination between adjacent particles and densification of materials during sintering process. However, the thermal conductivity of composites increased with increasing AlN content.The residual thermal stress of W-Cu functionally graded materials arising from the fabrication process was analyzed using finite element method （FEM）. Based on the calculation results of the residual thermal stress and stress state in graded layers, W-Cu functionally graded materials with three layers and four layers structure were designed. W-20%Cu/W-33%Cu/W-50%Cu FGM （P=2.4） and W-20%Cu/W-29.1%Cu/W-39.2%Cu/W-50%Cu FGM （P=1.4） had the minimal equivalent thermal stresses and the thermal stress were reduced by 52% and 68% respectively comparing with the non-FGM. Based on the optimization results, two kinds of W-Cu FGMs with high density and pretty microstructure were prepared by hot-pressing sintering. The thermal conductivities of W-Cu FGMs with three layers and four layers structure were 198 W·m^-1K^-1 and 202 W·m^-1K^-1 respectively. After the thermal shock test with 800℃temperature sdifference, no cracks were found at the interface of two kinds of FGM samples. The two kinds of FGM survived up to 86 and 143 thermal cycle tests respectively. Cracks were found at the interface at both ends of FGMs, which was consistent with the previous calculation results of the residual thermal stress. These results indicated the two kinds of FGMs had excellent heat resistance and exhibited good property of reducing thermal stress.更多还原