基于弹塑性自洽模型的AZ31镁合金轧制过程的织构模拟

为了更好地研究镁合金轧制过程的织构演变,对商用有限元软件ABAQUS/Explicit的用户材料接口VUMAT做二次开发,实现晶体塑性力学和有限元方法的耦合。对于单个晶粒,通过相关模型计算每个增量步的塑性应变增量,Voce硬化模型计算应变硬化。由于变形机制的不同,分别计算滑移和孪晶引起的晶格旋转。采用弹塑性自洽模型计算单晶体和多晶体之间的联系。应用编制的程序,分别模拟AZ31板材和AZ31铸件的轧制过程。结果表明,该程序能够较好地预测轧制过程的织构演化。     

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。     

面心纯铝单向拉伸的晶体塑性学有限元模拟

基于率相关晶体塑性本构模型,实现了晶体塑性学有限元模拟过程。直接将电子背散射衍射(EBSD)获取的晶粒初始取向输入晶体塑性有限元模型,分别预测了单向拉伸面心1050纯铝过程中的力学响应与织构演化。应力应变响应数值模拟结果与实验结果有较好的一致性,同时也存在一定的偏差。两种多晶模型(Taylor模型和有限单元模型)分别模拟了单向拉伸真应变0.25和0.37时的织构演化。随着真应变的增加,两种丝织构(〈111〉织构和〈100〉织构)变得更加锋锐,模拟结果与EBSD实验测得的织构演化结果有较好的一致性。     

AZ31B镁合金挤压材料的力学性能与本构分析

通过单向拉伸与压缩试验,研究AZ31B镁合金挤压材料的力学性能,并建立相应的本构模型。结果表明:室温下AZ31B镁合金挤压材料的宏观力学性能存在显著的各向异性和拉压非对称屈服的现象,且在塑性流动过程中,屈服面的演化也呈现出各向异性的特点,即畸变硬化特性。基于系统的试验结果,结合考虑了各向异性和拉压非对称性的CPB06屈服面函数,采用解析函数形式的本构参数来描述畸变硬化特性,建立了一种唯象的镁合金材料塑性流动本构模型。通过用户材料子程序VUMAT,将本构模型应用于缺口试件拉伸的有限元模拟中,计算结果与试验结果吻合较好,证实了本构模型的适用性。     

基于晶体塑性有限元模型的异步轧制纯铜箔变形行为晶粒统计效应（英文）

采用一种真实的多晶集合体模型和晶体塑性有限元模型,研究异步轧制纯铜箔塑性变形晶粒统计效应。考虑局部硬化耗散作用,根据取向分布函数在取向空间中的分布规律将晶体取向分配给各个晶粒的单元积分点,建立弹塑性大变形条件下的相关多晶体塑性模型,并将其引入隐式有限元法。对非均匀材料流动、铜箔厚度一定时接触压力和轧制力随晶粒尺寸增加而降低的变形行为进行研究。结果表明,在箔材厚度方向上只有少数几个晶粒时,晶粒尺寸、形貌和取向不再均匀分布于箔材中,材料的变形行为主要受到单个晶粒变形行为的影响,从而导致非均匀变形及模拟和实验结果更加离散。研究变形过程中滑移系的启动过程,晶粒取向对滑移系的启动和滑移带的形成具有重要影响,预测结果与表面层模型一致。随着晶粒尺寸的增加,表层晶粒效应增大,更加有利于降低轧制力和激活表层晶粒内滑移系的开动。通过箔材轧制实验和模拟,可以更加深入地理解异步轧制极薄带微塑性变形的机理。     

逆法识别金属板材颈缩阶段应力应变曲线

金属板料成形过程的仿真需要定义材料在大应变处对应的流动应力曲线,即真应力-应变曲线。文章介绍了一种逆向识别方法,通过结合单向拉伸试验和有限元仿真分析来获得金属板料在大应变区间内的流动应力曲线。仿真分析过程中通过构建合适的有限元模型,正确模拟分散性失稳期间的各向异性塑性变形行为。在得出材料完整的直至颈缩阶段的流动应力曲线后,建立多个不同的硬化模型描述该曲线,并比较拟合结果,发现改进的Hockett-Sherby模型效果最好。     

材料模型对1Cr18Ni9Ti管材拉伸有限元仿真的影响

为研究材料模型对有限元模拟1Cr18Ni9Ti管拉伸的影响,将管材单向拉伸试验获取的真实应力应变曲线分别拟合成线性硬化和指数硬化材料模型,并用于有限元模拟。经对比分析认为,采用真实应力应变模型的分析结果与实验结果吻合良好,并能正确显示出颈缩发生时刻和颈缩形状;采用指数硬化模型的有限元模拟结果接近真实应力应变模型,颈缩区应力应变分布略显分散;采用线性硬化模型的有限元模拟结果未能显示实际管拉伸后期的局部颈缩形状。     

基于真实微观组织的双相钢力学行为多尺度模拟

以两种不同化学成分和马氏体含量的双相钢为研究对象,对其显微组织进行了有限元建模。选取一部分显微组织作为代表性单元RVE来反映宏观材料特性,考虑合金成分和铁素体晶粒尺寸的影响,采用基于位错强化理论的Bailey-Hirsch模型描述铁素体和马氏体单相的流变行为,在ABAQUS中模拟了RVE的单向拉伸过程。结果表明,多尺度模型可以较好的预测宏观力学性能,并能合理的反映双相钢变形过程微观尺度的应力应变行为。     

基于林位错强化的晶体塑性模型的稳定性缺陷及改进

基于林位错强化的晶体塑性有限元模型物理意义清晰简洁,具有广泛的应用。然而,只考虑林位错强化的模型忽略了共面位错之间的相互作用,导致模拟结果常常出现异常的扭曲失稳现象。针对以上问题,通过分析位错之间的相互作用,在林位错强化模型中引入自硬化作用对模型进行改进。对改进前后两个模型模拟单晶体沿单个和多个方向拉伸的变形模式以及冷轧铝板剪切带的形成特征进行了比较。结果显示,对于单晶单滑移,两个模型的稳定性表现一致;但是对于单晶多滑移,改进前的模型出现了扭曲失稳,而引入自硬化的改进模型对于多滑移系的模拟符合真实情况,并改善了模型的稳定性问题。对于冷轧铝板发生剪切变形时剪切带的形成特征,林位错强化模型的不稳定性放大了材料的失稳变形,模型的低稳定性和材料的变形失稳叠加,导致预测的结果失稳程度过高;而引入自硬化作用的改进模型改善了稳定性问题,可以真实地反映材料本身的失稳。     

Simulation of deformation twins and deformation texture in an AZ31 Mg alloy under uniaxial compression

To investigate deformation twins and the evolution of deformation texture during plastic deformation, uniaxial compression tests on a hot-rolled AZ31 Mg alloy were carried out at 200 °C. Cylindrical specimens were then compressed in both the rolling and the normal directions. The findings revealed that texture evolution, work hardening and macroscopic anisotropy are strongly dependent on the loading direction. Electron backscattered diffraction analysis was used to examine the orientation of parent grains and twin bands in the AZ31 Mg alloy under uniaxial compression. A viscoplastic self-consistent model (VPSC) was theoretically employed to calculate the relative activities of slip and twin systems in polycrystalline hexagonal aggregates under uniaxial compression. Each deformed grain exhibited an independent number and type of twin variants under uniaxial compression. Neutron diffraction was used to measure the macroscopic texture of the AZ31 Mg alloy. The VPSC model was used to simulate texture evolution, work hardening and macroscopic anisotropy during the uniaxial compression. A modified predominant twin reorientation (PTR) scheme was suggested to explain the gradual increase in twin volume in deformed grains.     

Microstructure,Tensile Properties and Work Hardening Behavior of an Extruded Mg–Zn–Ca–Mn Magnesium Alloy

A new Mg-2.2 wt% Zn alloy containing 1.8 wt% Ca and 0.5 wt% Mn has been developed and subjected to extrusion under different extrusion parameters.The finest(～0.48 μm) recrystallized grain structures,containing both nano-sized MgZn_2 precipitates and α-Mn nanoparticles,were obtained in the alloy extruded at 270℃/0.01 mm s~(-1).In this alloy,the deformed coarse-grain region possessed a much stronger texture intensity(～32.49 mud) relative to the recrystallized fine-grain region(～13.99 mud).A positive work hardening rate in the third stage of work hardening curve was also evident in the alloy extruded at 270℃,which was related to the sharp basal texture and which provided insufficient active slip systems.The high work hardening rate in the fourth stage contributed to the high ductility extruded at 270℃/1 mm s~(-1).This alloy exhibited a weak texture,and the examination of fracture surface revealed highly dimpled surfaces.The optimum tensile strength was achieved in the alloy extruded at 270℃/0.01 mm s~(-1),and the yield strength,ultimate tensile strength and elongation to failure were～364.1 MPa,～394.5 MPa and～7.2%,respectively.Fine grain strengthening from the recrystallized fine-grain region played the greatest role in the strength increment of this alloy compared with Orowan strengthening and dislocation strengthening in the deformed coarse-grain regions.     

Texture development and hardening characteristics of steel sheets under plane-strain compression

Crystallographic texture development and hardening characteristics of a hot-rolled, low-carbon steel sheet due to cold rolling were investigated by idealizing the cold rolling process as plane-strain compression. The starting anisotropy of the test material was characterized by examination of the grain structure by optical microscopy and the preferred crystal orientation distribution by x-ray diffraction. Various heat treatments were used in an effort to remove the initial deformation texture resulting from hot rolling. The plastic anisotropy of the starting material was investigated with tensile tests on samples with the tensile axis parallel, 45°, and perpendicular to the rolling direction. The grain structure after plane-strain compression was studied by optical microscopy, and the new deformation texture was characterized by x-ray diffraction pole figures. These figures are compared with the theoretical pole figures produced from a Taylor-like polycrystal model based on a pencil-glide slip system. The uniaxial tensile stress-strain curve and the plane-strain, compressive stress-strain curve of the sheet were used to calibrate the material parameters in the model. The experimental pole figures were consistent with the findings in the theoretical study. The experimental and theoretical results suggest that the initial texture due to hot rolling was insignificant as compared with the texture induced by large strains under plane-strain compression.     

Twinning and texture development in two Mg alloys subjected to loading along three different strain paths

The evolution of twinning and texture in two Mg-based (+Al, Mn, Zn) alloys was investigated using uniaxial tension, uniaxial compression and ring hoop tension testing at temperatures from ambient to 250 °C and a strain rate of 0.1 s⁻¹. The results indicate that the initial extrusion texture plays an important role in the formation of different types of twins and that the twinning behavior also depends on the strain path. Contraction and double twinning are the dominant twinning mechanisms in uniaxial tension, while extension twinning prevails in uniaxial compression and ring hoop tension testing. Schmid factor analysis indicates that only components that are favorably oriented (i.e., with the highest SF values) can undergo rapid and complete twinning. The different twinning behaviors are shown to be responsible for the sharply contrasting strain hardening characteristics of the experimental flow curves and dramatic texture changes.     

Post-uniform deformation in uniaxial and equi-biaxial stretching of aluminium alloy sheets

Post-uniform deformation which affects the formability of sheet metals has been studied in the uniaxial and equi-biaxial stretching of aluminium alloy sheets. The materials used were AA5050, AA3105, and AA8014 aluminium alloys having negative, zero and positive strain-rate sensitivities, respectively. Post-uniform deformation has been characterized in terms of ‘neck breadth’ in uniaxial tension, and ‘proportional post-uniform increment in bulge height’ in biaxial tension. The results have been analyzed on the basis of the strain hardening and strain-rate hardening parameters of the materials.     

织构与晶粒尺寸对AZ31镁合金薄板成形性能的影响

通过不同的轧制工艺,制备了4种具有不同晶粒尺寸和织构的镁合金板材;通过单向拉伸试验和室温埃克森试验,探讨了晶粒尺寸与织构对镁合金板材室温成形性能的影响。研究表明,晶粒细化虽然增强了板材的力学性能,但却不利于提高板材的胀形性能;基面织构减弱使板材沿厚向的变形能力提高,具有较好的胀形性能,但却造成板材屈服强度的降低。     

Deformation behavior of thin copper films on deformable substrates

The role of strain hardening for the deformation of thin Cu films was investigated quantitatively by conducting specialized tensile testing allowing the simultaneous characterization of the film stress and the dislocation density as a function of plastic strain. The stress–strain behavior was studied as a function of microstructural parameters of the films, such as film thickness (0.4–3.2 μm), grain size and texture. It was found that the stress–strain behavior can be divided into three regimes, i.e. elastic, plastic with strong strain hardening and plastic with weak hardening. The flow stresses and the hardening rate increase with decreasing film thickness and/or grain size, and are about two times higher in (111)-grains compared to the (100)-grains. These effects will be discussed in the light of existing models for plastic deformation of thin films or fine grained metals.     

On the influence of mechanical environment on the emergence of brass textures in FCC metals

Face-centered cubic metals develop one of two distinct textures, when subjected to a plate rolling process: the copper or brass texture. Copper texture development is relatively well understood; brass texture development is not. One explanation is that slip is more highly planar and deformation is more heterogeneous in metals that develop a brass texture. In this paper, a latent hardening model is introduced to alter the planarity of slip and used in an elastoviscoplastic finite element formulation that permits heterogeneous deformations both within and among crystals to study texture development under plane strain compression. The results support the contention that latent hardening, together with a grain interaction model that allows for heterogeneous deformations, does promote the transition from copper to brass textures at low strains. However, the development of the brass texture does not persist to large strains under the assumed model, suggesting perhaps that revisions are necessary to the underlying form of the hardening model or that deformation mechanisms other than slip intervene at larger strains. These issues are discussed in the context of the influence of latent hardening on the mechanical environment within crystals.