一个新的用于元胞自动机模拟微观组织的溶质分配模型及其计算验证

在已有的几种溶质再分配计算模型的基础上, 建立了一个新的更为合理的溶质再分配模型.该模型充分考虑了枝晶生长过程中可能出现的固/液界面元胞的各种状态及其邻居元胞的各种状态, 根据这两者的不同状态, 分别建立不同的计算公式展开计算. 利用所建立的模型消除了原有的扩散控制的元胞自动机(CA)生长模型存在的在晶界处元胞不凝固的缺陷. 接着, 为了进一步验证模型的可靠性, 在Kurz-Giovanola-Trivedi方法控制的CA生长模型中引入新的溶质再分配计算模型, 对Al-4.7 wt%Cu 合金铸锭进行了模拟计算, 并与实验结果进行了金相组织和成分分布两方面的对比, 表明新模型具有较好的精确性. 关键词： 元胞自动机方法 微观组织模拟 溶质再分配     

MOLECULAR DYNAMICS SIMULATION ON LOW ANGLE GRAIN BOUNDARIES

Molecular dynamics simulation has been performed to investigate the microstructure and properties of low angle grain boundaries, employing the embedded atom method(EAM) type interatomic potential for Ni-Al alloy. The energies of the low angle grain boundaries with different dislocation densities were calculated, and the results indicate that the low angle grain boundary energy varies as a function of misorientation angle. The simulation was found in good agreement with the calculation on the basis of the dislocation theories in the low angle scale. The low angle grain boundary energy goes up with the increase of misorientation angle and tends to go down after reaching a maximum. An energy cusp exists when the misorientation angle increases further, but in this scale the dislocation theories are invalid for energy calculation due to the strong interaction of the dislocations at the boundaries. The simulation results also indicate that the microstructure of low angle grain boundaries can still be described as dislocations when the misorientation angle is larger than 10°.     

γ-α相变中不同晶界特征下铁素体生长形貌的相场模拟

利用多相场模型模拟了奥氏体(γ)-铁素体(α)相变过程中不同晶界特征下铁素体晶粒的形貌与生长动力学.模型中通过能量梯度系数和耦合项系数的协同变化定量表达晶界能与晶界迁移率的各向异性,同时固定相场界面宽度来保证计算精度.模拟结果显示:随着原奥氏体晶界能与铁素体-奥氏体晶界能比值σ_(γ,γ)/σ_(α,γ)的增加,三叉相界面处的平衡角β减小,铁素体晶粒沿原奥氏体晶界与垂直于奥氏体晶界方向的生长速率差变大.铁素体与奥氏体晶粒间的晶粒取向越接近,铁素体生长越缓慢.模拟结果可描述铁素体晶粒生长形貌的多样性,与实验结果符合.     

Phase-field study of spinodal decomposition under effect of grain boundary

Grain boundary directed spinodal decomposition has a substantial effect on the microstructure evolution and properties of polycrystalline alloys. The morphological selection mechanism of spinodal decomposition at grain boundaries is a major challenge to reveal, and remains elusive so far. In this work, the effect of grain boundaries on spinodal decomposition is investigated by using the phase-field model. The simulation results indicate that the spinodal morphology at the grain boundary is anisotropic bicontinuous microstructures different from the isotropic continuous microstructures of spinodal decomposition in the bulk phase. Moreover, at grain boundaries with higher energy, the decomposed phases are alternating α/β layers that are parallel to the grain boundary. On the contrary, alternating α/β layers are perpendicular to the grain boundary.     

The grain size distribution in nanocomposite films

The grain size distributions and related mechanisms in nanocomposite films with nanostructures comprising a nanocrystalline (nc) phase surrounded by an amorphous (a) matrix under different amorphous phase amounts (V _a) have been analyzed by using a Monte Carlo grain growth model. The results show that with the V _a value increasing to a critical value of ～28%, the grain size distribution approaches lognormality, and it becomes off-lognormal when the V _a value is larger or smaller than ～28%. The simulated results are in a good agreement with the experiment. It is shown that the homogenous or inhomogeneous grain growth mode, determined by the energy exerted on the grain boundary, originates in lognormal or off-lognormal grain size distributions in nanocomposite films. Also, in a system with lognormal grain size distribution, the amorphous phase just covers all grain boundaries (GBs) and the length obtained by summing the boundary circumference of all nanograins is the longest. It is expected that this microstructure can result in exceptional properties of nanocomposite films.     

再结晶和外力场下第二相析出的相场法模拟

在讨论相场法模拟基本方程的基础上,提出了晶界范围宽度的新概念,解释了相场模拟模型中有序化参数梯度范围的物理意义,论证了晶界范围不是晶界原子错排的宽度,而是界面能和界面元素偏析存在范围的观点.建立了一个模拟合金再结晶的相场模型,提出了一系列法则来获得模型中各参数的物理真实值,以AZ31镁合金为例,实现了再结晶过程晶粒长大的真实时间和空间的模拟,通过与试验数据的对比证明了模型的有效性.此外,还列举了相场法模拟Ti-25Al-10Nb合金中O相在外力场作用下析出过程的一系列有趣的新结果,讨论了外力场对第二相析出的重要影响和机理以及模拟结果对合金开发潜在的重要指导意义. 关键词： 相场法 再结晶 析出 外力场     

晶粒生长演变相场法模拟界面表达的物理模型

讨论了当前固态组织演变过程的相场法模拟模型,论证了相场法中界面的概念以及模型中界面的各种处理方法,以AZ31镁合金再结晶系统为例,研究了模型参数取值对界面特征的影响,提出了晶界作用域的概念,阐述了晶界作用域就是相场模型中界面处有序化变量的变化范围,其物理意义是界面能量的分布范围,并对应于成分界面偏析的范围.模拟得出,晶界作用域宽度主要由梯度项系数决定,晶界能则由梯度项系数和耦合项系数共同决定.对于AZ31镁合金,模拟研究了晶界作用域宽度取值的合理性和对显微组织影响的关系,得出取值为1.18 μm时,模拟符 关键词： 相场法 界面 计算机模拟 显微组织     

The Migration of High Angle Grain Boundaries during Recrystallization

When plastically deformed metallic materials are annealed, new strain free grains emerge from the microstructure and grow by means of grain boundary migration until the deformation microstructure is eliminated. This process is called recrystallization. In this paper the various methods by which grain boundary migration rates are measured stereologically in order to characterize the growth process are described and compared using illustrations from recrystallization experiments on commercial AA1050 aluminum. It seems abundantly clear that during recrystallization of cold-deformed materials, isothermal grain boundary migration rates decrease with time and reasons for such a decrease are discussed. A new methodology whereby migration rates of the individual recrystallization texture components may be quantified by combining stereology and orientation imaging by the electron back scattered pattern analysis is outlined. By illustration, recent experiments on aluminum and copper are summarized documenting the slight growth rate advantage the cube texture component (001)[100] possesses during recrystallization of cold rolled material. The role of orientation pinning effects on grain boundary migration is described briefly. It appears that such pinning effects allow recrystallized grains emerging from the weaker deformation texture components to enjoy an average growth rate advantage over those emerging from the stronger deformation texture components.     

相场方法模拟铝合金三维枝晶生长

以相场模型为基础，采用宏微观耦合方法和界面捕获液态方法对铝合金枝晶生长进行模拟计算.为解决试样全场微观计算的困难，采取宏微观耦合的计算方法，试样整体计算温度场，而微观组织计算只在一个确定的宏观单元内进行，宏观微观计算交替耦合进行.在不改变相场模型的条件下，提出界面捕获液态计算方法.赋值计算单元界面标志，只对界面处的单元求解相场变量，当枝晶生长时，捕获液态单元为界面从而推进界面，并对捕获到的单元校正相场变量.通过界面捕获液态方法加速相场模型的计算，实现了铝合金试件局部三维单晶粒和多晶粒的模拟.对模拟结果与实 关键词： 相场方法 液态捕获 微观组织 枝晶     

Grain Boundary Migration in Metals: Recent Developments

Current research on grain boundary migration in metals is reviewed. For individual grain boundaries the dependence of grain boundary migration on misorientation and impurity content are addressed. Impurity drag theory, extended to include the interaction of adsorbed impurities in the boundary, reasonably accounts quantitatively for the observed concentration dependence of grain boundary mobility. For the first time an experimental study of triple junction motion is presented. The kinetics are quantitatively discussed in terms of a triple junction mobility. Their impact on the kinetics of microstructure evolution during grain growth is outlined.     

多晶材料晶粒生长的计算机模拟研究

多晶固体材料显微结构的演化与诸多因素密切相关 ,是一复杂的过程。运用适当的计算机模拟方法进行模拟 ,实现对晶粒生长的预测 ,可为材料研究提供新的方法和重要依据。此文以晶粒生长动力学为基础 ,建立晶粒生长模型 ;分析了晶粒生长过程的计算机模拟理论和方法 ;以MonteCarlo方法为基础 ,提出快速Q statepotts算法 ,简化计算量 ,在PC机上编程实现了正常晶粒生长过程的计算机模拟。得到的显微结构图逼真度较好。通过对结果进行分析处理 ,得到的生长因子为 0 .4 7。     

Effect of finite boundary junction mobility on the growth rate of grains in 3D polycrystals

With decreasing grain size, grain boundary junctions become increasingly important for microstructure evolution. We show that the effect of a limited mobility of triple junctions on the growth rate of polycrystals can be implemented in theories of three-dimensional (3D) grain growth. Respective analytical relations are derived on the basis of the average n-hedra approach introduced by Glicksman to describe the volume rate of change of 3D grains in a polycrystalline aggregate under the impact of a limited triple junction mobility. The theoretical predictions were compared to network-model computer simulations, and good agreement was obtained.     

利用分子动力学方法铁素体-渗碳体相界面效应探究

珠光体是十分重要的组织结构,因此本文构建了含铁素体-渗碳体相界面的模型,并采用分子动力学模拟方法模拟纳米压入的过程。通过对模拟结果的力学性能和组织结构分析,探究了铁素体-渗碳体相界面效应。研究发现,距铁素体-渗碳体晶界不同距离（位置压入）,在压入最初阶段,压头载荷随着压头与晶界距离的增大而增大,当压入深度达到一定深度后,载荷随着距离的增大而减小。杨氏模量和最大剪切模量受压头尖端下方原子结构的直接影响,硬度受到结构完整性和类型的共同影响。铁素体-渗碳体相界面影响了纳米压入过程中位错形核、增殖和扩展,宏观表现为在相同压入深度下,不同压入位置压头载荷的差异。     

Atomistic simulation of plasticity and fracture of nanocrystalline copper under high-rate tension

A molecular dynamics simulation of the plastic deformation and the onset of fracture of nanocrystalline metals is performed using the example of copper. Successive stages of the response of the microstructure of a metal to deformation are considered, namely, grain boundary sliding, the nucleation and gliding of dislocations, and the formation and growth of microdamage nuclei. The influence of the grain size of a nanocrystal on its plasticity and strength is studied.     

Boundary Mobility and Energy Anisotropy Effects on Microstructural Evolution During Grain Growth

We have performed mesoscopic simulations of microstructural evolution during curvature driven grain growth in two-dimensions using anisotropic grain boundary properties obtained from atomistic simulations. Molecular dynamics simulations were employed to determine the energies and mobilities of grain boundaries as a function of boundary misorientation. The mesoscopic simulations were performed both with the Monte Carlo Potts model and the phase field model. The Monte Carlo Potts model and phase field model simulation predictions are in excellent agreement. While the atomistic simulations demonstrate strong anisotropies in both the boundary energy and mobility, both types of microstructural evolution simulations demonstrate that anisotropy in boundary mobility plays little role in the stochastic evolution of the microstructure (other than perhaps setting the overall rate of the evolution. On the other hand, anisotropy in the grain boundary energy strongly modifies both the topology of the polycrystalline microstructure the kinetic law that describes the temporal evolution of the mean grain size. The underlying reasons behind the strongly differing effects of the two types of anisotropy considered here can be understood based largely on geometric and topological arguments.     

金属凝固与晶体生长过程的蒙特卡罗模拟

采用导热的有限差分计算与图形蒙特卡罗方法相结合的手段,模拟了不同的冷却条件,熔体初温以及杂质含量情况下的金属凝固及其晶体生长过程。模拟得了与典型的边界冷却凝固实验相一致的微观组织结构,同时,通过对模拟结果的分析,总结了不同的冷却条件、熔体初温以及含杂质情况对金属凝固组织的一些影响规律。     

Statistical analyses of deformation twinning in magnesium

To extract quantitative and meaningful relationships between material microstructure and deformation twinning in magnesium, we conduct a statistical analysis on large data sets generated by electron backscattering diffraction (EBSD). The analyses show that not all grains of similar orientation and grain size form twins, and twinning does not occur exclusively in grains with high twin Schmid factors or in the relatively large grains of the sample. The number of twins per twinned grain increases with grain area, but twin thickness and the fraction of grains with at least one visible twin are independent of grain area. On the other hand, an analysis of twin pairs joined at a boundary indicates that grain boundary misorientation angle strongly influences twin nucleation and growth. These results question the use of deterministic rules for twin nucleation and Hall–Petch laws for size effects on twinning. Instead, they encourage an examination of the defect structures of grain boundaries and their role in twin nucleation and growth.     

Multi-phase field simulation of competitive grain growth for directional solidification

The multi-phase field model of grain competitive growth during directional solidification of alloy is established. Solving multi-phase field models for thin interface layer thickness conditions, the grain boundary evolution and grain elimination during the competitive growth of SCN-0.24-wt% camphor model alloy bi-crystals are investigated. The effects of different crystal orientations and pulling velocities on grain boundary microstructure evolution are quantitatively analyzed. The obtained results are shown below. In the competitive growth of convergent bi-crystals, when favorably oriented dendrites are in the same direction as the heat flow and the pulling speed is too large, the orientation angle of the bi-crystal from small to large size is the normal elimination phenomenon of the favorably oriented dendrite, blocking the unfavorably oriented dendrite, and the grain boundary is along the growth direction of the favorably oriented dendrite. When the pulling speed becomes small, the grain boundary shows the anomalous elimination phenomenon of the unfavorably oriented dendrite, eliminating the favorably oriented dendrite. In the process of competitive growth of divergent bi-crystal, when the growth direction of favorably oriented dendrites is the same as the heat flow direction and the orientation angle of unfavorably oriented grains is small, the frequency of new spindles of favorably oriented grains is significantly higher than that of unfavorably oriented grains, and as the orientation angle of unfavorably oriented dendrites becomes larger, the unfavorably oriented grains are more likely to have stable secondary dendritic arms, which in turn develop new primary dendritic arms to occupy the liquid phase grain boundary space, but the grain boundary direction is still parallel to favorably oriented dendrites. In addition, the tertiary dendritic arms on the developed secondary dendritic arms may also be blocked by the surrounding lateral branches from further developing into nascent main axes, this blocking of the tertiary dendritic arms has a random nature, which can have aninfluence on the generation of nascent primary main axes in the grain boundaries.     

Grain boundary pinning in two-phase materials

The calculation of the Zener stress involves two parts (1) the determination of the force between a particle and a grain boundary, and the shape of the resulting dimple on the grain boundary and (2) the statistical problem of summing the forces over the boundary. Calculations of the forces from spherical, needle-shaped and plate shaped particles are used to estimate the Zener stress by means of a point obstacle model and statistical dimple model. The results are compared with experimental determinations of stable grain sizes, with a computer simulation growth and with experimental measurements of retarded primary recrystallisation. When the particles are non-spherical or inhomogeneously distributed the Zener stress is anisotropic; this effect has been observed experimentally.