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.     

热变形低碳钢中奥氏体静态再结晶介观尺度模拟

采用晶体塑性有限元（CPFEM）和元胞自动机（CA）耦合的方法模拟了热变形低碳钢的静态再结晶．CPFEM的计算结果定量描述了介观尺度上奥氏体变形储能的不均匀分布，为模拟再结晶的形核和长大提供了依据，从而在再结晶CA模型中考虑了不均匀变形的影响．模拟结果显示：变形储能分布不均匀使得再结晶在不同位置的形核密度不同，形核集中在晶界以及晶内储存能较大的区域；随着临界形核储能的降低，形核数量增加，再结晶晶核的位置分布趋于均匀．对不同形核判据下的再结晶动力学也作了讨论．     

金属材料热锻过程组织演化建模方法的新进展

简要阐述了金属材料热锻过程中组织演变的建模与模拟方法;着重介绍了宏观有限元-介观多层级元胞自动机集成模拟方法,该方法以位错密度为线索,根据"应变/应变速率/温度-位错密度-再结晶-流动应力"之间的宏微观相互影响规律,能够追踪在非均匀/非等温变形条件下,再结晶晶粒的形貌、尺寸和体积分数;同时,介绍了晶体塑性-多层级元胞耦合模拟方法,该方法以实际晶粒的取向为初始组织输入,采用晶体塑性描述晶粒内部变形的应变梯度,并将位错密度和晶粒取向映射到多层级元胞模型中,可以定量描述织构中大晶粒的形成;最后,较为简要地介绍了不锈钢核电大锻件的组织演变数值模拟-物理模拟方法,并归纳了几种方法的优缺点,展望了未来发展趋势。     

GH761变形高温合金的热变形行为

镍基GH761合金热模拟压缩实验表明,当变形温度Td一定时,随应变速率ε的降低,变形峰值应力σp和稳态流动开始应力σs日及与它们对应的应变εp和εs均降低;当应变速率一定时,随Td的升高,σp和σs以及εs均降低,但εp基本不变.细化原始晶粒可提高再结晶形核率,在此基础上降低变形温度和提高变形速率是细化最终晶粒的重要途径.当应变达到完全再结晶时,合金具有最均匀且细小的组织;超过这一应变值,晶粒开始长大.GH761合金的热变形本构方程为:ε=6.5×106σp4.86exp(-461×103/RT).     

Y对Cu-Cr-Zr合金高温热变形行为的影响

利用Gleeble-1500D热模拟试验机对Cu-Cr-Zr和Cu-Cr-Zr-Y合金,进行高温等温压缩试验,研究了在变形温度为650~850℃、应变速率为0.001~10 s-1条件下两种合金的流变应力的变化规律,测定了真应力一应变曲线,从流变应力、应变速率和温度的相关性,得出了该合金热压缩变形时的热变形激活能Q和本构方程,并利用光学显微镜分析了合金在热压缩过程中的组织演变及动态再结晶机制。结果表明:稀土元素Y的加入细化了微观组织,提高了Cu-Cr-Zr合金的动态再结晶体积分数,并且大幅降低了合金的热变形激活能Q,改善了其热加工性能。     

AZ31B镁合金在高应变速率下的热压缩变形行为和微观组织演变

采用Thermecmastor-Z热模拟试验机在变形温度为200~520℃、应变速率为2~60 s-1条件下对AZ31B镁合金厚板进行热压缩变形试验,压缩变形量为60%。结合变形后的微观组织以及热压缩真应力-真应变曲线,分析应变速率和变形温度等工艺参数对其微观组织演变的影响。结果表明:当变形温度高于320℃时,AZ31B镁合金的真应力-真应变曲线呈现典型的动态再结晶特性。当应变速率一定时,流变应力随温度升高而降低;当变形温度一定时,流变应力在高温低应变速率(低于15 s-1)下随应变速率增大而增大。变形后的微观组织显示,压缩变形过程中发生了明显的动态再结晶,动态再结晶体积分数随应变速率的增加而增大。另外,变形组织的均匀性受变形温度的影响十分显著。在热压缩实验的基础上,在温度为300~330℃时对板材进行单道次大压下量的热轧,获得的板材具有均匀细小的晶粒及优异的力学性能。     

TA15钛合金高温变形过程的介观模拟计算

以多晶体位错滑移及塑性流动机制为基础,探究了TA15钛合金在高温变形过程中介观层次上形变不均匀性和力学响应。基于率相关晶体塑性理论,建立了描述体心立方结构金属力学行为的本构模型,同时考虑了主滑移系和次滑移系的运动;确定了合理的材料本构参数,高温压缩实验与模拟得到的真应力-应变曲线基本一致。通过对TA15钛合金高温变形模拟结果进行分析,包括应力和应变分布、滑移系开动情况和晶界面积变化,得出:(1)由于晶粒几何及取向的随机性造成应力和应变分布非均匀性;(2)晶粒间相互作用的复杂性会导致各个滑移系开动的差异性;(3)形变程度越大,晶粒密度越大,晶界面积变化率越大。模拟结果为相变等显微组织演变及多尺度同步耦合提供了参考。     

直径方向上仅有几个晶粒时纯镍丝材的拉伸变形行为（英文）

研究纯镍丝材单向微拉伸塑性变形过程中的流动应力和非均匀变形行为尺寸效应。实验发现,流动应力随晶粒尺寸的增加(或直径方向上晶粒数量的减少)而降低,而非均匀变形程度增加。当直径方向上少于9.3个晶粒时,流动应力随晶粒尺寸增加而快速降低。通过引入晶界尺寸因子构建介观尺度材料本构模型揭示丝材微拉伸流动应力尺寸效应。结果表明,断裂应变和断裂应力随着晶粒尺寸的增加而减小。当试样直径方向上少于14.7个晶粒时,断裂应变和断裂应力快速降低,表明微拉伸过程中的非均匀变形程度随着直径方向上晶粒数量的减小而增加。当试样直径方向上的晶粒数量减少时,断口形貌变得越来越不规则。从材料微观组织分布方面分析了不规则断口形貌的形成机理。     

H156热作模具钢动态再结晶的实验与数值模拟研究

通过Gleeb-1500D热模拟机，对H156热作模具钢进行单道次热压缩实验。实验中，变形温度控制在850～1220℃,应变速率控制在0.001～1 s^-1,真应变为0.7。通过采集数据获得真应力-真应变曲线，得出流变应力与热变形参数之间的数学关系，建立了动态再结晶模型，并借助光学显微镜对不同热变形参数下实验样品的微观组织进行观察分析，建立了晶粒尺寸模型。通过DEFORM-3D有限元软件对H156热作模具钢的动态再结晶行为进行模拟，探究热变形参数对实验材料动态再结晶(DRX)行为的影响。结果表明：随着变形温度的增加，DRX体积分数增加，DRX晶粒分布越均匀；较大的应变速率会导致变形时间缩短，DRX区域减小，细小晶粒缺少充足的时间发生形核长大。因此，提高变形温度、降低应变速率有助于动态再结晶的完成。数值模拟结果与实验结果较吻合，验证了模型的准确性。     

7075铝合金热变形多尺度数值模拟

为揭示7075铝合金热变形介观组织演变与宏观力学性能之间的关系，建立了可以并行计算的7075铝合金热变形多尺度模型。结合唯象型与物理型模型构建了新的并联型介观本构模型。采用ABAQUS有限元软件位移法的umat节点变量进行信息交互，实现不同尺度模型之间的数据耦合。通过实验与模拟结果的校准获得模型所需的材料参数，并在ABAQUS有限元软件上成功实现了7075铝合金试样单轴热拉伸过程的多尺度同平台并行数值模拟，通过对比数值模拟结果与实验结果发现，位错与应变分布趋势接近，均呈现出不均匀的带状分布，表层晶粒的位错明显多于内部晶粒，这主要是由于表层更容易发生滑移位错。     

Experimental and numerical investigations of the plane strain compression of an oligocrystalline pure copper specimen

To evaluate if the deformation in the bulk of an oligocrystalline sample during a plane strain compression test can be accurately simulated numerically with the aid of a crystal plasticity model a detailed comparison between the experimentally found and numerically predicted deformation is made. In the experiment an incremental plane strain compression (PSC) test is conducted on an oligocrystalline specimen. The incremental PSC test is complemented by electron backscatter diffraction (EBSD) measurements. The experimental results enable qualitative observation of the specimen's microstructure and orientation distribution at intermediate stages of the plastic deformation. This incremental PSC test is numerically simulated with the aid of a crystal plasticity finite element model. The numerical model accounts for the specimen's microstructure and the orientations of the crystals. Mapping the microstructure and the grain orientations on the workpiece in the finite element model provides realistic boundary conditions. Both, the experiment and numerical simulation show that the deformation in the oligocrystalline specimen with only a few grains in the deformation zone considerably differs from that of well known continuum like materials. Qualitative comparisons of the incremental PSC test and its numerical simulation showed that most of the experimentally observed behaviour of the grains and their mutual interaction can be seen in the results of the CPFEM simulation as well. From quantitative comparisons it is concluded that the two-dimensional CPFEM simulation can predict the rotations of grains in the bulk of the three-dimensional microstructure of the oligocrystalline specimen only in a qualitative manner. The lack of quantitative agreement is most probably caused by the fact that this two-dimensional, plane strain CPFEM model does not account for the interaction of the grains in the microstructure beyond the analysed cross sectional surface.     

A Multi-phase Field Model for Static Recrystallization of Hot Deformed Austenite in a C–Mn Steel

A multi-phase-field model has been developed to simulate the microstructure evolution and kinetics of the austenite static recrystallization(SRX) in a C–Mn steel. In this model, the bulk free energy that coupling the deformation stored energy with a special interpolation function is incorporated. Both the deformed grain topology and the deformation stored energy have been included in order to investigate the influence of pre-deformation on the subsequent austenite SRX at different hot deformation levels. Diverse scenarios of microstructure evolution show different deformation-dependent recrystallized grain sizes. The transformation kinetics is then discussed by analyzing the overall SRX fraction and the average interface velocity on the recrystallization front.     

奥氏体形变对先共析铁素体等温形成孕育期的影响SCIEI

将Feder相变孕育期表达式引入形变奥氏体的分解中,建立了奥氏体形变条件下先共析铁素体等温形成孕育期的计算方法,计算了奥氏体形变储能和奥氏体晶界能量变化对相变孕育期的影响,计算结果与实验所得规律一致。     

EFFECT OF AUSTENITE DEFORMATION ON ISOTHERMAL INCUBATION TIME OF PROEUTECTOID FERRITE NUCLEATION

A calculation method for the isothermal nucleation incubation time of pro-eutectoid ferriteformed from deformed austenite has been developed by introducing Feder's incubation timeexpression into the decomposition of deformed austenite.The effects of austenite deformationstored energy and austenite grain boundary energy change on the incubation time have beentheoretically calculated and the rule of calculated results is consistent with that ofexperiments,this confirms that austenite deformation stored energy and the change ofaustenite grain boundary energy are the natural effect of austenite deformation on ferritenucleation.     

Modeling the austenite–ferrite diffusive transformation during continuous cooling on a mesoscale using Monte Carlo method

A Q-state Potts model is adopted to simulate the austenite–ferrite diffusive transformation during continuous cooling in a low carbon steel on a mesoscale with Monte Carlo method. The effects of the interfacial energy and the carbon diffusion on the phase transformation are both taken into account. The consideration of the interfacial energy effect makes the simulated ferrite grains polygonal. To simulate the carbon diffusion during the process of phase transformation, a random jumping based mesocopic diffusion model is developed. In this paper, the evolutions of the microstructure and the carbon concentration field are simulated. The simulation results of the effects of the cooling rate on the volume fraction of the ferrite and the ferrite grain size agree well with the experimental results. The simulation results show that increasing the cooling rate is an effective way to refine the microstructure. This Monte Carlo simulation technique provides a novel way for investigating the diffusive transformation on a mesoscale.     

低碳钢低温变形γ→α相变行为的预测模型

从变形使孕育期缩短的角度，讨论了形变诱导相变的发生条件，计算了不同变形条件下几种成分的低碳钢变形过程析出铁素体的开始温度Ar3d，发现同样变形条件下，碳含量越低的钢种，其Ar3d越高；同一成分钢种，随着变形量增加或变形速率减小，Ar3d提高，基于相变动力学理论，在形核速率计算中充分考虑变形和过冷的双重作用，探索了低温变形诱导铁素体相变的动力学模型，用该模型进行的计算机模拟结果和实验结果吻合良好，表明这种理论处理方法可用来模拟这种相变过程。     

Mesoscopic Analysis of Deformation Heterogeneity and Recrystallization Microstructures of a Dual-Phase Steel Using a Coupled Simulation Approach

Microstructure-based numerical modeling of the deformation heterogeneity and ferrite recrystallization in a cold-rolled dual-phase(DP) steel has been performed by using the crystal plasticity finite element method(CPFEM) coupled with a mesoscale cellular automaton(CA) model. The microstructural response of subsequent primary recrystallization with the deformation heterogeneity in two-phase microstructures is studied. The simulations demonstrate that the deformation of multi-phase structures leads to highly strained shear bands formed in the soft ferrite matrix, which produces grain clusters in subsequent primary recrystallization. The early impingement of recrystallization fronts among the clustered grains causes mode conversions in the recrystallization kinetics. Reliable predictions regarding the grain size, microstructure morphology and kinetics can be made by comparison with the experimental results. The infl uence of initial strains on the recrystallization is also obtained by the simulation approach.     

Dependency of Deformation Behavior of Retained Austenite in TRIP Steels on Microstructural and Chemical Homogeneity

The room-temperature stability of the retained austenite against strain-induced martensitic transformation, its deformation behavior, the response to the bainitic isothermal treatment, the appearance of yield point elongation and other peculiarities of plastic flow, and the mechanical properties of transformation-induced plasticity(TRIP) steel were tailored based on the chemical homogeneity and the relative distribution of the retained austenite, bainite, and ferrite in the microstructure. The presence of ferritic-pearlitic banded structure in the initial microstructure resulted in an inhomogeneous TRIP microstructure, in which the retained austenite and bainite were confined to some bands and it was found to be responsible for the resultant inferior mechanical properties. The appearance of discontinuous yielding for the chemically inhomogeneous material was related to the martensitic transformation of unstable retained austenite at the initial stage of tensile deformation. These results are essential for better understanding of the behavior of advanced high-strength steels and their applications.