柱状晶Cu-Al-Mn形状记忆合金超弹性各向异性参数研究及数值计算

参考单层板的偏轴工程弹性常数与正轴工程弹性常数的转换关系,进行参数推导,实现了对合金任意角度的超弹性各向异性相关参数的计算。采用有限元用户子程序对合金任意角度下的超弹性应力-应变曲线进行数值模拟。模拟计算结果与实验结果吻合较好,可为柱状晶类组织合金的各向异性性能的模拟计算提供相应指导和参考。     

U-Notched Dionysos-Pentelicon Marble Beams in Three Point Bending: The Effect of Nonlinearity, Anisotropy and Microstructure

The failure of notched beams made from Dionysos-Pentelicon marble is studied in the present work, both experimentally and numerically. A series of properly designed three point bending tests were carried out using specimens with notches of various depths. The strain variation at strategic points of the specimens, as well as the deflection of the lower fibers and the notch mouth opening displacement were recorded during the tests as functions of the applied load. The experimental results were compared with the respective ones obtained from a linear elastic and isotropic numerical solution based on the Indirect Boundary Element Method. The agreement between them was found to be satisfactory for most quantities compared. Some discrepancies observed are critically discussed. It is indicated that ignoring the nonlinearity of the stress-strain relation as well as the anisotropy and microstructure of this type of marble may lead to fallacious conclusions. This revised version was published online in July 2006 with corrections to the Cover Date.     

热处理对真空压铸NZ30K镁合金微观组织及力学性能的影响

使用光学显微镜(OM)、扫描电镜(SEM)、能谱分析(EDS)、硬度测试和拉伸性能测试等方法,研究了热处理对真空压铸NZ30K镁合金微观组织及力学性能的影响。结果表明：铸态合金的宏观组织分为表层区和心部区,表层区组织由细小α-Mg等轴晶和分布在晶界的Mg₁₂Nd组成,心部区组织则由细小α-Mg等轴晶、粗大预结晶组织(ESCs)和分布在晶界的离异共晶Mg₁₂Nd组成。在固溶处理过程中心部区晶粒的长大比表层区更为显著,晶界迁移速率与晶粒尺寸不均匀呈正相关性,满足晶粒长大模型v=M₀ exp (-Q/RT) A (1/D₁-1/D₂)。合金的优化热处理工艺为540℃×6 h+200℃×8 h。与铸态合金(UTS=186.0±1.5 MPa,YS=131±2.5 MPa,EL=6.6±0.4%)相比,峰值时效态合金的抗拉强度和屈服强度分别提高到了223.6±4.1 MPa和172.8±2.9 MPa,但延伸率降低到了4.2±0.3%。其强度的提高主要得益于时效析出的片状纳米β"相能够有效地阻碍位错在基面上的滑移。铸态和热处理态合金的表层区断裂模式均为韧性断裂,而心部区的断裂模式在铸态下为准解理断裂、在固溶态下为解理断裂、在峰值时效态下为准解理断裂。     

基于数值模拟的K418B高温合金精密铸件组织与性能研究

目的 探究高温合金调压铸造的充型凝固过程,研究调压铸造工艺对铸件组织缺陷和力学性能的影响规律,并验证数值模拟对实际生产指导的可靠性。方法 以某精密构件为研究对象,借助ProCAST数值模拟软件模拟了铸件的调压铸造充型凝固过程并对组织缺陷的形成进行了预测。对成形铸件的特征关键部位进行了取样,通过金相显微镜和扫描电子显微镜对铸态试样的微观组织进行了观察,借助准静态万能拉伸试验机测试了特征试样的室温和高温(750 ℃)拉伸性能,并对断口形貌进行了观察和分析。结果 数值模拟结果表明,金属液充型平稳,凝固过程基本符合自上而下的顺序凝固,铸件缺陷较少,缩孔体积分数仅为0.22%。实验结果表明,铸件的铸态组织为典型的树枝晶组织,晶粒尺寸细小均匀;二次枝晶间距较小,组织致密,缩松缩孔缺陷较少,这与数值模拟的结果吻合较好;铸件的平均抗拉强度超过900 MPa,最大伸长率为15%,该铸件具备较好的综合力学性能。结论 通过数值模拟方法指导铸造生产具有一定的可靠性,同时,通过调压铸造工艺可以生产出具有较好组织和力学性能的高温合金薄壁铸件。     

马氏体不锈钢的微观组织各向异性对微区应力和氢分布的影响

利用电子背散射衍射(EBSD)实验分析了马氏体组织的微区弹性刚度分布,并在此基础上建立应力-氢交互作用的耦合有限元模型,研究马氏体组织的各向异性对微区应力和氢分布的影响。结果表明:相邻Block板条束之间的取向差互成60°,在同一受力方向上不同Block板条束具有不同的弹性刚度,从而引发组织间的微区应力和氢呈不均匀分布,Block板条束是表征微区应力的组织单元。Block板条束弹性刚度梯度和组织尺寸决定了组织间的应力集中,而应力集中又影响了氢的分布。弹性刚度梯度高和板条束尺寸大的Block组织单元应力集中较为严重,并富集高浓度的氢,最终引发氢致开裂。上述模拟结果与氢脆断口的微观断裂形貌和氢脆裂纹的EBSD分析结果相符。     

K2O对LZAS微晶玻璃微观结构及热膨胀性能的影响

采用DTA、XRD、SEM、TEC(热膨胀系数)等分析手段研究了加入K2O前后LZAS系统微晶玻璃的微观结构和热膨胀性能.结果表明,650℃晶化时,试样析出γⅡ-LZS和方石英晶体;725℃时,γⅡ-LZS逐渐转变为γ0-LZS晶体,并且出现β-石英固溶体;800℃时,β-石英固溶体转变为β-锂辉石固溶体,晶粒尺寸逐渐长大.制得微晶玻璃的热膨胀系数在(50～130)×10-7℃-1(20～500℃)之间,其大小取决于晶相的种类和含量.并且K2O的加入降低了玻璃的转变温度、粘度,抑制了玻璃的析晶倾向,增大了微晶玻璃的热膨胀系数.     

固溶及时效热处理对K465合金组织的影响

研究了K465合金热处理后的显微组织和力学性能。结果表明,合金热处理后,枝晶干处的γ'相呈立方状、体积分数为62%,同时还析出了细小点状碳化物,铸态MC碳化物转变为M6C。高温时效热处理结果表明,合金经1 000℃/500 h和1 100℃/100 h后,析出富集W和Mo的针状和棒状二次析出相;1 000℃条件下,随着时间的延长,二次析出相的数量逐渐增多;温度的升高可促进二次相的析出。     

基于DEM的回转筛分数值模拟和效果研究

目的 在粮食储藏和加工过程中,平面回转筛是重要的粮食筛分清理设备。研究各项筛分处理参数对回转筛筛分性能的影响,并寻找回转筛的最优筛分参数。方法 基于离散元法（DEM）对回转筛在不同参数条件下的回转筛分过程进行数值模拟,以筛分流量、回转速度、回转半径和筛面倾角为4项变量依次进行单因素分析,得出各参数对筛分性能指标（筛分效率、除杂效率和物料流动速度）的影响规律,再以此选取各因素的较优水平范围进行多因素正交试验,并采用矩阵分析方法对试验结果进行权重排序。结果 得到了综合各项性能评价指标较优的参数组合：筛分流量为3.0 t/h,回转速度为465 r/min,回转半径为24 mm,筛面倾角为9°。结论 研究结果表明,寻求更优的筛分效率必然会降低除杂效率和物料流动速度,即物料在筛面上停留时间越长就越能增大其通过筛孔机会,同时杂质通过筛孔的概率也在增大。     

Cohesive Crack Model with Rate-Dependent Opening and Viscoelasticity: II. Numerical Algorithm, Behavior and Size Effect

In the preceding companion paper (Bažant and Li, 1995), the solution of an aging viscoelastic law was structure containing a cohesive crack with a rate-dependent stress-displacement softening law was reduced to a system of one-dimensional integro-differential equations involving compliance functions for points on the crack faces and the load point. An effective numerical algorithm for solving these equations, which dramatically reduces the computer time compared to the general two-dimensional finite element solution, is presented. The behavior of the model for various loading conditions is studied. It is shown that the model can closely reproduce the available experimental data from fracture tests with different loading rates spanning several orders of magnitude, and tests with sudden changes of the loading rate. Influence of the loading rate on the size effect and brittleness is also analyzed and is shown to agree with experiments. This revised version was published online in July 2006 with corrections to the Cover Date.