【作者】 肖逸锋；

【导师】 贺跃辉；

【作者基本信息】 中南大学 ， 材料科学与工程， 2008， 博士

【摘要】 传统均匀硬质合金材料由于其各部分的成分和组织均匀,其硬度和韧性之间存在着尖锐的矛盾,制约了其应用领域的进一步扩大,难以满足现代社会发展对硬质合金提出的“双高”（高硬度,高韧性）要求,所以,研发新型功能梯度硬质合金材料以满足工具不同的部位具有不同的使用功能要求显得尤为重要。为此,本文在国家自然科学基金的资助下,与国有大型硬质合金公司合作,利用有限元分析软件（Marc）、扫描电镜（SEM）、X射线衍射仪（XRD）、能谱仪（EDS）、电子探针（EPMA）和图像分析软件（Q550）等现代材料分析与测试手段,以残余热应力为对象进行梯度结构设计,渗碳处理制备梯度硬质合金,对缺碳硬质合金的组织和性能、渗碳处理过程中的组织转变、梯度形成机理、工艺参数对梯度硬质合金组织和性能的影响、以及梯度硬质合金的裂纹扩展、断裂韧性和高温强度进行了系统研究,得到以下主要结论:1)提出以质量百分含量作为模型的基本参数,建立了适合梯度硬质合金成分分布函数、弹性系数、热膨胀系数和热导率模型,预测值与试验结果吻合很好;用MARC有限元软件对YG6梯度硬质合金的残余应力进行数值模拟计算,计算结果与XRD实测值基本一致;YG6梯度硬质合金的梯度结构最优化设计的微观结构为:梯度层厚度为半径的20%～30%,梯度分布指数p的取值在1.5～2.5之间,钴含量峰值在12%～16%之间。2)碳含量影响缺碳合金中η相的类型、含量、分布和WC的形状。Co₃W₃C相出现在缺碳程度相对较小的合金中,其含量随碳含量的增大而增大;与之相反,Co₆W₆C相出现在缺碳程度相对严重的合金中,其含量随碳含量增大而减少;合金中η相总量随碳含量增大而线性减少:随着缺碳程度的增加,合金中η相的分散均匀性变差,并且η相趋于形成大块状;WC大多呈现多角特征。3)气压烧结缺碳硬质合金的组织和性能优于真空烧结缺碳硬质合金的组织和性能;气压烧结后期有效地控制渗碳气氛能够直接烧结渗碳制备梯度硬质合金;部分Ni替代Co作为粘结相的WC-6（xNiyCo）缺碳硬质合金的组织和性能接近于相同缺碳量的WC-6Co缺碳合金的组织和性能,以此作为渗碳前驱体可制备模具或轴承用无磁或低磁梯度硬质合金。4)采用缺碳硬质合金,成功地研究出一种制备高性能梯度硬质合金的简单渗碳万法。梯度组织的特征是合金表层和次表层的η相已经完全消失,为正常的WC+γ两相组织,并在合金的表层形成了一个贫钴区,次表层形成了一个富钴区;合金的芯部依然是没有多大变化的含η相组织。梯度组织的形成主要受碳扩散、碳和η相反应、W原子向合金表面迁移、液相压力差和WC晶粒长大导致的毛细管力等影响。5)梯度结构的形成导致合金的硬度和断裂韧性在截面上自外至内呈连续梯度变化;梯度结构的形成有利于提高合金的强度和断裂韧性。梯度硬质合金通过钴含量的梯度分布、引入表面压应力、促使裂纹偏转和桥接等方式增韧,获得了较好的综合性能。6)梯度硬质合金的高温强度较传统均匀硬质合金和缺碳硬质合金要显著提高。随温度升高,宏观断口表面趋于平整,穿晶断裂比例下降,沿晶断裂的比例增加。梯度硬质合金的高温强度缓慢下降是由于合金中形成了钴相梯度分布结构,表层钴含量低使得钴的高温软化和氧化造成的强度降低减小、表层中钴固溶了较多的W带来的强化作用及温度升高使得残余拉应力降低等共同作用的结果。更多还原

【Abstract】 Conventional homogeneous cemented carbide （CHCC） with homogeneous composition and microstructure in bulk has sharp contradiction between hardness and toughness, which restricts further application areas of it. It is difficult to meet the development of modern social on the double high require, namely high hardness and high toughness. So it seems particularly important to develop a new-type of functionally gradient cemented carbide （FGCC） materials to enable tools with different functions at different positions in bulk. For this reason, supported by the National Natural Science Foundation of China and cooperates with the large-scale state-owned cemented carbide enterprises, with modern material analysis and test methods, such as the finite element analysis software Marc, SEM, XRD, EDS, EPMA and picture analysis software Q550, etc, this paper carried out systematic researches on the gradient structure design base on the aim of thermal residual stress, the microstructures and performances of carbon-deficient cemented carbides （CDCC） and the structural transformation, effects of process parameter on microstructures and performance during preparing of gradient cemented carbides by carburizing treatment, the crack propagation, fracture toughness and high temperature strength of the gradient cemented carbides, getting the following conclusions.1) Putting forward the model suiting for gradient cemented carbides, such as the composition distributing function, the elastic coefficient, the hot expansion coefficient and heat transfer coefficient model, basing on the basic parameter with the mass percent of model. The results of the new established models are very well identical with the test results. The residual stress of YG6 gradient cemented carbides calculated by numerical simulation with the MARC finite element software is basically unanimous with the XRD test values. The best optimization design microstructure of YG6 gradient cemented carbides is the thickness of gradient layer should be of 20% to 30% radius, the value of the gradient distribution exponent p should be between 1.5 to 2.5 and the best cobalt content peak value should be between 12% to 16%.2) In CDCC, carbon content affects the type, amount and distribution ofη phase, and WC shape of alloys, etc. Co₃W₃C phase appears in alloys with relative lower degree of carbon-deficient and whose amount increase with increasing carbon content, whereas Co₆W₆C phase varies reversely. The total amount ofηphase decreases with increasing carbon content. With the increase of degree of carbon-deficient, the homogeneity ofηphase distribution decreases,ηphase tends to form clump shape and WC mostly maintains multangular character.3) The structure and properties of CDCC by Sintering-HIP are better than that of by Vacuum Sintering. The valid controlution of carburization atmosphere during the later stage of Sintering-HIP can directly prepare gradient cemented carbides. The WC-6（xNiyCo） CDCC with binder of xNiyCo alloyed powder, namely some Ni takes place of Co, can get near microstructures and performances of WC-6Co CDCC with same carbon content, which has application prospects in preparing no magnetism or low magnetism gradient cemented carbides for die industry or bearing.4) High performance gradient cemented carbide was successfully be prepared by a simple carburizing treatment on CDCC. The characteristic of the gradient microstructure is thatηphases in surface layer and sublayer have already totally disappeared, where consists of normal WC +γtwo phases. A cobalt-deficient area and a cobalt-rich area was formed on the surface layer and sublayer of the alloy respectively, however, the core area of the alloy has few change includingηphase. The forming of the cobalt gradient is influenced mainly by the carbon diffusion, reaction between carbon andηphase, outward migration of W atoms, pressure difference caused by liquid phase and the capillary force caused by grain grows of WC crystalline, etc.5) The hardness and the fracture toughness take on a continuous gradient change along the gradient direction on the cross section of alloys for the forming of the gradient structure. The gradient cemented carbides have better comprehensive performance, toughened by the gradient distribution of cobalt content, forming compressive stress at surface, inducing crack deflection and crack bridge, etc.6) The high temperature strength of gradient cemented carbides is higher than that of CHCC and CDCC. With the increasing of test temperature, the macroscopical fracture surface tends to smooth and the transcrystalline fracture proportion decreases while the intercrystalline fracture proportion increases. The slow decrease of high temperature strength of the gradient cemented carbides is the combined actions of the forming of cobalt gradient distribution, which weakens the soften and oxidation of metal cobalt at elevated temperature for lower cobalt content on surface layer, the strengthening for more tungsten dissolved in cobalt at surface layer and the relaxation of thermal residual tensile stress with the increasing of test temperature, etc.更多还原