多晶Cu在双向加载下的后继屈服与塑性流动分析

采用晶体塑性理论并结合多晶集合体模型来研究多晶Cu的塑性变形,用双向加载方式模拟材料的双向应力状态和分段加载路径,得到了材料的初始屈服而及在预剪切和预拉伸2种情况下的后继屈服面.通过对后继屈服面形状及其演化趋势的研究,探讨了用晶体塑性理论分析多品材料塑性流动规律的方法.结果表明:后继屈服面的形状和是否出现尖角与屈服点的定义有关,同时还与π平面上的预加载方向有关;通过对多晶集合体代表性单元的塑性流动方向与后继屈服面法向的差异进行统计分析,发现塑性流动的正交性不仅与屈服定义相关,也与预加载方向有关.     

对称拉压循环下金属屈服硬化的各向异性与细观变形转动的不均匀性

考虑金属材料晶体细观塑性变形流动的各向异性性质以及晶体滑移的非线性运动硬化,以Voronoi多晶集合体作为材料的代表性单元(RVE),用晶体塑性模型描述金属材料的细观本构关系,对多晶纯Cu进行了晶粒尺度的对称应变循环细观分析.证实了本文模型和方法可以用于描述多晶金属材料不同应变幅的循环滞回特性,并可以用于估计金属材料在循环加载过程中后继屈服面形状和曲率与预加载方向相关的变化.分析发现:在对称应变循环中,随循环数增加,多晶材料宏观拉伸方向的内部细观应变分量的统计变异系数(COV)不断增加,材料内部晶格的不均匀细观转动也越来越显著.这些结果表明,循环过程中应变分布差异和晶格转动差异逐渐增大,从而导致材料内部微结构越来越不均匀.     

Cu极薄带轧制中滑移与变形的晶体塑性有限元模拟

为了定量描述晶粒取向和结构对极薄带轧制微观塑性变形非均匀性的影响,采用晶体塑性有限元方法(CPFEM)和Voronoi图的多晶模型,考虑试样尺寸、晶粒尺寸、晶体取向及其分布,模拟了不同厚度Cu极薄带在相同压下率条件下的滑移与变形行为,得到了介观尺度上Cu极薄带的微观应力-应变和启动滑移系分布.模拟获得的应力-应变曲线和实验测得的曲线基本一致,验证了晶体塑性有限元模型的准确性.通过对40%压下率Cu极薄带轧制变形的研究表明,无论是在晶粒内部还是在晶粒间,材料内部的变形都非常不均匀,这种不均匀性主要是由初始晶粒取向和结构不同、近邻晶粒取向差以及变形时滑移系的运动特性和晶粒旋转不同引起的.滑移系首先在自由表面和晶界处被激活,而后引起晶粒内部滑移系的启动与运动.     

基于三阶贝塞尔曲线的屈服轨迹描述及演化模型的研究

为描述材料在塑性成形中的复杂硬化行为,提出了一种基于三次贝塞尔曲线的屈服轨迹离散插值型描述方法,以及后继屈服轨迹演化模型。针对Khan A S等采用Al6061-T6511铝合金进行拉伸-扭转复合预加载后,采用10~(-5)量级的屈服应变阈值时,二维应力空间屈服轨迹表现出极其强烈的大小、中心点和形状变化的实验结果,构造了相应的离散插值型屈服轨迹数值模型,通过拟合实验结果得到了各预应变条件下的曲线参数;提出了描述后继屈服轨迹大小、中心点和形状变化的拟合模型以描述材料的各向同性强化、随动强化和畸变强化行为。与采用应力不变量的建模方法相比,这种屈服模型是依据屈服轨迹的几何形状建立的,具有较大的灵活性。     

材料微结构三维建模及其晶体塑性有限元模拟

采用Voronoi图,生成具有随机晶粒形状的三维多晶集合体模型,并赋予每个晶粒相应的取向。基于率相关晶体塑性理论,开发了用户材料子程序,并将其嵌入有限元软件中模拟面心立方多晶集合体在单轴单向拉伸过程中的应力-应变响应,分析了网格细化及晶粒取向对模拟结果的影响。研究表明,随网格细化应力值有所降低,但变化不大,为保证结果的可靠度,平均每个晶粒离散的单元数目在5个以上;随多晶集合体中晶粒数目的增加,由于取向的随机性产生的应力-应变的差异逐渐减小,模拟时多晶集合体中晶粒的数目大于50个;模型较好的反映了材料的真应力随应变速率增加而增大的规律,且模拟结果和实验结果吻合良好,说明该模型具有较高的可靠性。     

材料微结构三维建模及其晶体塑性有限元模拟

采用Voronoi图,生成具有随机晶粒形状的三维多晶集合体模型,并赋予每个晶粒相应的取向。基于率相关晶体塑性理论,开发了用户材料子程序,并将其嵌入有限元软件中模拟面心立方多晶集合体在单轴单向拉伸过程中的应力-应变响应,分析了网格细化及晶粒取向对模拟结果的影响。研究表明,随网格细化应力值有所降低,但变化不大,为保证结果的可靠度,平均每个晶粒离散的单元数目在5个以上;随多晶集合体中晶粒数目的增加,由于取向的随机性产生的应力-应变的差异逐渐减小,模拟时多晶集合体中晶粒的数目大于50个;模型较好的反映了材料的真应力随应变速率增加而增大的规律,且模拟结果和实验结果吻合良好,说明该模型具有较高的可靠性。     

细观非均质性对多晶材料塑性行为的影响

构建晶粒取向随机分布的多晶集合体,采用晶体塑性本构模型,对多晶材料塑性宏观响应与细观结构的关系进行研究．结果表明：多晶材料细观非均质性对材料的宏观力学行为有重要影响;各晶粒之间的相互约束使得等向强化的晶体本构关系在对多晶材料描述时可以模拟Bauschinger效应;分析Bauschinger效应的机理需综合考虑晶粒之间约束．晶粒的弹性各向异性及不同取向的硬化能力等因素．     

ZEK100-O镁合金薄板拉压非对称各向异性屈服模型研究

为研究ZEK100-O镁合金薄板室温下的拉压强度差效应及比例加载过程中的各向异性演化,采用CPB06ex2屈服准则,建立等效塑性应变表达式,并利用MATLAB中的优化算法对ZEK100-O镁合金的屈服模型参数进行标定。获得了屈服模型各向异性参数随累积塑性应变的变化规律,归纳出各参数关于累积塑性应变的演化方程。为验证屈服模型的有效性,编写了ABAQUS子程序VUMAT并进行了数值模拟。采用非关联流动法则,预测了ZEK100-O镁合金薄板在准静态、比例加载路径下的变形行为。将模型预测的不同方向的拉伸/压缩屈服应力及厚向异性系数r值与实验结果进行对比,结果表明,CPB06ex2屈服准则能够较准确地描述ZEK100-O镁合金的拉压不对称性。     

磁控溅射Cu膜屈服强度的有限元计算

采用离子辅助轰击共溅射设备，在Si基体的(111)晶面上制得了所需的铜膜。采用纳米压入实验，获得不同退火温度下Cu膜的弹性模量和硬度。再在纳米压入实验的基础上，结合有限元模型计算不同退火温度下磁控溅射得到的Cu膜屈服强度。发现Cu膜的屈服强度远高于整体Cu材料的屈服强度，并且退火温度对薄膜的屈服强度影响很大。通过XRD测量发现其主要原因是退火改变了晶粒尺寸和多晶Cu膜的晶粒取向分布，而导致Cu膜屈服强度的降低。     

基于多晶体塑性模型的纯铜轧制有限元模拟

以晶体塑性有限元理论为基础,在有限元软件ABAQUS中通过Voronoi图的多晶模型对不同初始取向的两种多晶纯铜试样轧制后的变形进行了分析.结果表明,轧制后多晶纯铜试样中晶粒自身的变形与其内部的应力、应变都不均匀,且与其取向、大小、形状及晶粒间的相互作用相关.     

The Growth Habit of Electrodeposited Copper

By plating under base reproduction conditions in an acidified copper sulphate solution, the growth processes occurring at polycrystalline cathodes have been studied and compared with those at single crystal cathodes. It has been found that the surface topography of the deposit formed on any one grain of the polycrystalline aggregate is identical to that of a deposit, grown under similar conditions, on a single crystal of the same orientation. The growth habit is in each case related to the atomic configuration of the initial cathode surface. The thickness of the deposit formed on electropolished polycrystalline strip has been found to vary from grain to grain. This may be explained in terms of electrochemical anisotropy. It has been found that the growth habit of thick deposits (10 microns) formed on deeply etched surfaces is similar to that on electropolished surfaces. Mechanical polishing of the basis metal causes the development of a polycrystalline, textured deposit, although some base influence can be detected even through very thick deposits (40 microns). Under certain conditions, deposits of the base-reproduction type can be obtained from cyanide copper solutions. Such deposits develop well-defined surface structures, with large, regular features and highly reflecting facets.     

Inhibition of copper sulphidation by boron implantation

Pure polycrystalline copper and copper implanted with boron ions at a dose of 3.0 × 10¹⁷ ions cm^?2 at 25–150 keV have been exposed to hydrogen sulphide in moist air. The rate of growth of a sulphide film on the boron-implanted copper is lower than on pure copper by at least a factor of four after 18 h under the exposure conditions ([H₂S] = 3.0 ppm, T = 22.5°C, RH = 85%). The resistance of the implanted copper to sulphide corrosion is ascribed to inhibition of copper diffusion through the surface oxide layer.     

孪晶对多晶铜疲劳行为的影响

对含有高密度孪晶的多晶铜进行了塑性应变幅控制下的疲劳实验.结果表明,塑性应变幅小于8.14×10-4时,孪晶对 疲劳行为的影响不大;塑性应变幅大于8.14×10-4时,孪晶的约束作用、孪晶界与位错的反应及孪晶中位错的特殊组态,使多晶 铜的循环饱和应力提高,硬化曲线中应力饱和平台区延长.     

多晶与单晶铜塑性变形行为的相关性

利用TEM分析拉伸变形多晶铜中不同取向晶粒的位错组织, 研究了多晶与单晶形变行为的相关性. 结果发现, 多晶铜中诸晶粒的形变显微组织可分为3种不同类型, 而且组织类型与其相应晶粒的晶体学取向存在密切的相关性; 根据多晶中不同类型组织的体积含量, 结合相应取向单晶的应力应变曲线, 计算出了多晶的应力应变曲线, 其结果与实测曲线相似.     

Anisotropy of Wetting and Spreading in Binary Cu-Pb Metallic System: Experimental Facts and MD Modeling

Contact angle for millimeter-size drops of lead on {100} and {110} surfaces of monocrystalline copper and on polycrystalline copper was determined by means of dispensed drop technique at 450 °C under He-H₂ atmosphere. It was demonstrated that the wetting anisotropy (a difference between contact angles on differently oriented substrates) is not exceed a few degrees. Spreading kinetics was found to be different for the first and second drops deposited on each substrate. This result was interpreted as an effect of a lead precursor film formation on the substrate surface. Molecular dynamics simulations of the lead drop spreading over {111}, {100}, and {110} surfaces of monocrystalline copper confirm the weak anisotropy of equilibrium contact angle and a formation of lead precursor film on copper surface in front of wetting line.     

Modeling void growth in polycrystalline materials

Most structural materials are polycrystalline aggregates whose constituent crystals are irregular in shape, have anisotropic mechanical properties and contain a variety of defects, resulting in very complicated damage evolution. Failure models of these materials remain empirically calibrated due to the lack of a thorough understanding of the controlling processes at the scale of the materials’ heterogeneity, i.e. the mesoscale. This paper describes a novel formulation for a quantitative, microstructure-sensitive three-dimensional mesoscale prediction of ductile damage of polycrystalline materials, in the important void growth phase of the process. Specifically, we have extended a formulation based on fast Fourier transforms to compute growth of intergranular voids in porous polycrystalline materials. In this way, two widely used micromechanical formulations, i.e. polycrystal plasticity and dilatational plasticity, have been efficiently combined, with crystals and voids represented explicitly, to predict porosity evolution. The proposed void growth algorithm is first validated by comparison with corresponding finite-element unit cell results. Next, in order to isolate the influence of microstructure on void growth, the extended formulation is applied to a face-centered cubic polycrystal with uniform texture and intergranular cavities, and to a porous material with homogenous isotropic matrix and identical initial porosity distribution. These simulations allow us to assess the effect of the matrix’s polycrystallinity on porosity evolution. Microstructural effects, such as the influence of the Taylor factor of the crystalline ligaments linking interacting voids, were predicted and qualitatively confirmed by post-shocked microstrostructural characterization of polycrystalline copper.     

Synergistic effect of additives on electrodeposition of copper from cyanide-free electrolytes and its structural and morphological characteristics

Electrodeposition of acid copper plating on mild steel substrate is tedious due to the galvanic displacement reaction of copper on mild steel. This can be avoided by using a proper complexing agent, because the complexing agent tuned the potential of noble direction to less noble direction by complex formation. In this paper, environment friendly electrodeposition of copper from non-cyanide electrolyte using sodium gluconate as complexing agent was investigated in alkaline medium. The effects of additives such as 1, 2, 3-benzotriazole, sodium lauryl sulphate, PEG 8000 and saccharin were studied. These additives are found to reduce the grain size, grain boundaries and improve surface morphology of the copper deposits. Also they improve the throwing power of the depositing electrolytes and hardness of deposits. The electrodeposited copper coatings were characterized by X-ray diffraction technique. XRD results indicate that the electrodeposited copper shows polycrystalline and face centered cubic structure. The crystal size was calculated by XRD and AFM analysis. Among these additives studied, the mixture of benzotriazole and sodium lauryl sulphate acts as the best additive. A uniform pore-free surface observed under SEM and AFM results reveal the grain refining brought about by the additives.     

Emission M?ssbauer spectroscopy of grain boundaries of polycrystalline copper

Grain boundaries of polycrystalline copper have been studied using ⁵⁷Co(⁵⁷Fe) emission M?ssbauer spectroscopy. It is shown that Co atoms are diffused over grain boundaries of polycrystalline copper by a vacancy mechanism. The state of Co atoms in the boundary regions of crystallites has been analyzed. The values of coefficients of the grain-boundary segregation of Co in Cu have been determined.     

High-strain-rate response of ultra-fine-grained copper

The high-strain-rate response of ultra-fine-grained (UFG) copper processed by equal channel angular pressing (ECAP) was characterized by three different dynamic testing methods: reverse Taylor impact, cylindrical compression specimens, and hat-shaped specimens in Hopkinson bar experiments. Upon recovery after impact, the specimens were found to undergo dynamic recrystallization at a calculated temperature of 360 K, indicating that the UFG copper is thermally unstable. Reverse Taylor tests were conducted on as-received oxygen-free high-conductivity copper rod and ECAP specimens with 2 and 8 sequential deformation passes. The dynamic deformation of the samples was modeled using AUTODYN-2D, and a modified Johnson–Cook constitutive equation was found to closely capture the dynamic response. Both the dynamic experiments and analysis from the reverse Taylor tests indicate enhanced strain-rate sensitivity in comparison with conventional polycrystalline copper, in agreement with predictions of reduced activation volume. The shear band thickness, as obtained in forced localization tests, showed a marked decrease, in comparison to conventional polycrystalline copper; this effect is interpreted as due to an accelerated thermal softening and inherent instability exhibited by the UFG structure.