【作者】 黄自谦；

【导师】 黄伯云； 贺跃辉；

【作者基本信息】 中南大学 ， 材料学， 2008， 博士

【摘要】 本研究主要采用有限元方法分析了梯度结构硬质合金在制备过程中形成的宏观热应力,同时对于均质硬质合金内部微观热应力采用有限元方法中的单胞模型进行分析,并与Eshelby理论模型进行比较。为了验证模型和分析结果的正确性,利用SEM、XRD、EDAX等现代材料分析与测试手段,对梯度结构硬质合金的成分、组织结构与性能之间的关系进行了必要研究,得到以下结论:（1）通过渗碳制备了具有三层结构的梯度硬质合金,外层贫钴,中间层富钴,芯部为渗碳前的名义钴含量,该合金在硬度和韧性方面均优于常规的均质硬质合金。（2）通过定义弹性约束因子,运用修正的混合律,建立了梯度硬质合金的弹性本构关系,即得到了简化的体积比相关的复合材料杨氏模量表达式,同时采用简单的混合律得到两相复合材料热膨胀系数和泊松比表达式,该模型可以通过有限元软件MSC.Marc2005实现,计算表明:渗碳后硬质合金在温度从800℃下降到室温时,材料表面产生了压应力,在富钴区产生了拉应力,要使表面存在500-700MPa的压应力以阻止热裂纹产生,合金内钴的名义含量应在15wt.%以上。（3）采用材料常数随温度变化的弹塑性本构关系有限元单胞模型分析了均质硬质合金内部残余热应力。计算表明,增强相钴相内最大拉应力为1400MPa,基体碳化钨相内最大压应力为1064MPa;当钴相为非理想的椭球形状（这里为圆柱形）时,钴相内部有较大的等效应力（565MPa）,且在应力集中处有塑性流动。（4）通过引入塑性约束因子,建立两相梯度复合材料弹塑性变形的本构关系,较之以前研究者们建立的塑性本构,该塑性本构模型的屈服准则是温度相关和体积比相关的,并可以通过有限元软件MSC.Marc2005实现,更符合客观实际,计算表明:相对于完全弹性,渗碳后硬质合金引入塑性变形时,最大轴向应力为750MPa,而最大等效应力为600MPa,下降了70%。（5）对于脱氮后得到的梯度硬质合金,采用同样方法,建立了弹塑性本构关系,分析制备时产生的宏观残余热应力,计算得出:表面两相区存在拉应力,向内逐渐过度到压应力,两相区最大拉应力为140MPa,而芯部最大压应力为120MPa。（6）采用有限元方法分析了均质基体和梯度基体单层和多层复合涂层内部宏观残余热应力。TiC/Al₂O₃/TiN复合涂层温度从800℃下降到0℃时,在TiN涂层内以拉应力为主,在硬质合金基体内以压应力为主。另外,TiC涂层内,表现为拉应力,Al₂O₃涂层为一应力低谷,夹在两个高峰之间。（7）采用XRD方法测试了渗碳后梯度硬质合金表面宏观残余应力,与有限元计算有较好的符合。更多还原

【Abstract】 In the present study, macrostress in functionally graded cemented carbides was investigated by finite element method, yet microstress in homogeneous cemented carbides was analyzed by finite element unit model and compared to the results of Eshelby theoretical model. To prove the accuracy of the models, the relationship between properties and microstructure for graded cemented carbides were studied by using SEM, XRD and EDAX, following conclusions are obtained:（1） Graded cemented carbides have been developed by pre-sintering carbon-deficient compacts and subsequently carburizing. The three-layered structure of the cemented carbides was formed after the carburization, the outer layer which is WC rich, the middle layer which is Co-rich and the inner layer which is as-sintered microstructure. The graded structure has significant advance compared to conventional cemented carbides.（2） The elastic constitutive relations of graded cemented carbides were developed by the definition of elastic constraint factor and modified mixed law, namely the formation of Young’s modulus for composite materials according to simple volume fraction and the thermal expansion coefficient and passion ratio were obtained by simple mixed law. The model can be achieved by the finite element software of MSC. Marc2005. The calculated results show when the temperature drops from initial stress-free temperature of 800°C to room temperature, compressive stress generates in the surface zone and tensile stress in the Co-rich zone, and the nominal cobalt content should be above 15wt.% in order to obtain compressive stress of 500-700MPa to prevent thermal cracking occurrences.（3） The microstress in homogenous cemented carbides was ananlyzed by finite element unit cell model which their material parameters are varied with temperature. The calculated results show that the maximum tensile stress in the reinforcement phase Co is 1400 MPa, the maximum compressive stress in matrix WC is 1064 MPa; When the Co phase is non-ideal ellipsoidal shape （here is cylindrical shape）, there is large equivalent stress （565 MPa） and plastic flow generates in stress concentration place.（4） The constitutive relation of elastoplastic deformation of functionally graded cemented carbides is developed by introducing plastic constraint factor. Compared to other researchers not long ago, the constitutive relation is temperature-dependent, volume fraction dependent and could be achieved in the finite element sofeware of MSC.Marc2005, having more objective rationality. The calculated results show in elastoplastic deformation （compared to elastic-only）, the maximum axial stress is 750MPa and the maximum equivalent stress is 600MPa, namely decreases by 70%.（5） The elastoplastic constitutive relation for cemented carbides after denitriding was developed also by constraint factors. The calculated results show tensile stresses generates in the surface two-phase zone and gradually transfer to compressive stress in the inner layer; the maximum hydrostatic tensile stress in the two-phase zone is 140MPa and the hydrostatic compressive stress in the centre is 120MPa.（6） The thermal residual macrostress for monolayer and multilayer of homogenous and graded cemented carbides were investigated also by finite element method. When the temperature drops from 800℃to 0℃for TiC/Al₂O₃/TiN composite coating with cemented carbide substrate, there is tensile stress in TiN coating. There is compressive stress in cemented carbide substrate. Furthermore there is tensile stress in TiC coating and the stress in Al₂O₃ coating is smaller than other coating layers.（7） The macrostress in the surface zone of cemented carbides after carburing was measured by XRD method and the results have a good agreement with the calculated results.更多还原