A note on grain size dependent pinning

A rigorous approach to grain size dependent pinning is proposed by accounting for special grain boundary locations such as quadruple points. Using this approach abnormal grain growth is rationalized for a high strength low alloy (HSLA) steel with a stable particle size distribution.     

Mechanism of abnormal grain growth in ultrafine-grained nickel

During annealing of ultrafine nickel powder compacts, abnormal grain growth (AGG) occurred with the formation of cube-shaped grains. The observed AGG is similar to the late-stage abnormal grain growth (LS-AGG) that is commonly observed in electrodeposited (ED) Ni sheets. The cube shape of the abnormal grains with {1 0 0} facets was identical to that of late-stage abnormal grains in ED Ni. As the temperature increased, the time to the appearance of abnormal grains decreased. After impingement of abnormal grains, little further grain growth occurred, indicating stagnant grain growth behavior. The formation of abnormal grains in the present study, however, cannot be explained by the previously suggested AGG mechanisms, i.e. the S phase wetting mechanism and initial texture mechanism, because no amorphous phase containing S was present and no texture developed in our samples. In contrast, the observed grain growth behavior can be well explained in terms of the coupling effect of the maximum driving force for grain growth and the critical driving force for appreciable migration of faceted boundaries. It is concluded that the AGG in ultrafine-grained Ni occurs via mixed migration mechanisms, i.e. diffusion and interface reaction-controlled, of faceted boundaries with respect to the driving force.     

单一晶粒长大过程的元胞自动机模拟

基于Moore型邻居及其相关转换准则的元胞自动机,分别对与多个晶粒相邻的单一正四边形、六边形、八边形小晶粒在晶粒长大过程中的形态变化进行模拟。结果表明：单一小晶粒形态变化规律与总体晶粒形态变化规律一致,即随着晶粒长大过程的延长,晶界边数大于6的晶粒会不断长大且边数趋于6;晶界边数小于6的晶粒则不断变小直至消失。所用模型具有较好的物理机制,可以模拟材料晶粒长大过程。     

Effect of grain boundary energy on surface-energy induced abnormal grain growth in columnar-grained film

In columnar-grained film with anisotropy in the surface energy, abnormal grain growth occurs to minimize the overall energy. We studied the effect of grain boundary energy on the surface-energy driven abnormal grain growth in thin films using the Monte Carlo computer simulation. Our simulation results show that the growth speed of abnormal growth slows down when grain boundary energy increases. It is because the speed of normal growth driven by the grain boundary energy minimization gets faster with increasing grain boundary energy, while that of the abnormal growth induced by the fixed surface energy is consistent. The growth kinetics was monitored based on the growth velocity of the abnormal grains relative to the average growth velocity of all the grains. It was also found that the abnormal growth kinetics was retarded by impingements among abnormally growing grains when there was more than one abnormal grain.     

Kinetics and driving forces of abnormal grain growth in thin Cu films

The abnormal growth of individual (1 0 0) oriented grains is monitored by the in situ electron backscatter diffraction technique for more than 24 h at three different annealing temperatures (90 °C, 104 °C and 118 °C) in 1-5 μm thick Cu films on polyimide substrates. The (1 0 0) grain growth velocity increases with higher film thickness and annealing temperature, as suggested by an earlier model by Thompson and Carel. As a result, the final (1 0 0) texture fraction becomes more dominant for higher annealing temperatures and larger film thicknesses. The Thompson-Carel model, however, predicts that the (1 1 1) grains will preferably grow at temperatures up to 118 °C. Our calculations of the driving forces revealed that in addition to minimization of the strain energy (due to the thermal mismatch between film and substrate) and of the surface energy, the energy stored in the dislocations plays a decisive role in grain growth. Our observations can be understood by the notion that initially available (1 0 0) grain nuclei start to grow very rapidly, due to dislocation annihilation, and thus “overrun” the (1 1 1) grains in size.     

焙烧过程晶粒生长的Monte Carlo模拟

基于Monte Carlo（MC）方法,根据晶粒生长机理建立改进的转换概率模型,在不同焙烧温度、焙烧时间和激活能条件下可实现晶粒生长过程结构演化的计算机模拟,对模拟和实验数据进行分析,结果表明：当晶粒生长指数为2.17时,模拟与实验具有较好的一致性,从而得出焙烧过程晶粒的生长动力学模型;该模型能够较好地解释焙烧过程晶粒的生长过程,对晶粒生长动力学研究等具有一定的指导意义。     

MC方法模拟晶粒长大的改进算法和晶粒数统计的研究

基于材料等温下所有晶粒长大的同步性,认为单元进行再取向尝试时采用随机提取更符合物理模型,且所有单元全部提取并完成一次再取向尝试记为一个MCS,弥补了以往模拟中采用逐一提取单元的不足.晶粒长大模拟结果符合大晶粒不断变大且有相似性,小晶粒的无规则随机变小的规律.采用了对晶粒个数、面积的精确统计算法,得出晶粒长大指数达到0.48～0.51.     

Modification of computer simulation of normal grain growth

A set of principles on transition probability was supplied for the physical process of grain growth. In accord with these principles, a modified transition probability considering the influence of temperature was put forward to simulate the normal grain growth relying on temperature and second phase particles. The modified transition probability correctly reflects the dependence of grain growth on the temperature. The effect of different shapes of second phase particles on the grain growth process was taken into account using the modified transition probability. The relationship between the area fraction of second phase particles and the limit of grain size of the matrix was given. The microstructural evolution patterns employed to 2-D were given. The results agree well with the real grain growth process. All these suggest that the modified transition probability is better than the conventional one.     

用相场法模拟纯铝的枝晶生长

利用相场法微观组织数值模拟技术,对纯铝物质的枝晶生长过程进行了仿真.研究了初始晶核半径、空间步长和各向异性对纯铝枝晶生长的影响.通过计算结果表明,在保证初始晶核半径不被熔化前提下,初始晶核半径不影响枝晶形貌;空间步长取值应在0.2d0～0.6d0范围内;各向异性趋于增大枝晶尖端生长速度,减小枝晶尖端半径.     

最大取向值对晶粒生长元胞自动机模拟结果的影响

为了研究最大取向值Q的大小对晶粒生长的元胞自动机模拟过程及结果的影响,对Q值分别取30、50、70、100、150、200时进行了模拟分析。结果表明,Q值较小时晶粒生长过程中极易发生晶粒碰撞现象导致晶粒异常长大,当Q值大于100时模拟过程平稳且符合晶粒生长理论,当Q值大于150时对模拟过程没有显著影响。不同Q值时晶粒的生长指数相差很小,且与其他研究者给出的生长指数十分接近,说明模拟过程是可信的。     

相场横型原理及其在晶粒长大模拟中的研究进展

晶粒长大是多晶体材料中最基本的显微组织演化过程之一,对其进行研究具有非常明确的理论和实践意义。而相场法作为一种模拟晶粒长大的有效方法,是目前国内外研究的热点。对相场模型的基本原理、数值计算方法以及在晶粒长大模拟中的研究进展进行了综述,提出了相场模型在微观组织模拟中存在的主要问题以及今后的发展方向。     

焊接热影响区晶粒长大过程的微观组织模拟

利用蒙特卡罗方法(MC)结合焊接工艺试验，模拟了单相合金焊接热影响区(HAZ)的晶粒长大过程。文中首先应用试验方法确定了材料的晶粒长大动力学方程，在不同峰值温度和冷却速度等热输入的情况下，模拟结果再现了晶粒长大的动态过程，并体现出热钉扎效应。得出了与焊接实际过程相一致的模拟结果。为以后进行三维焊接热影响区晶粒长大模拟奠定了可靠的基础。     

Phase-field study of competitive dendritic growth of converging grains during directional solidification

The microstructure evolution of grains with different orientations during directional solidification is investigated by the phase-field method. For converging dendrites, in addition to the usually accepted overgrowth pattern wherein the favorably oriented dendrites block the unfavorably oriented ones, the opposite pattern of overgrowth observed in some recent experiments is also found in our simulations. The factors which may induce this unusual overgrowth are analyzed. It is found that in addition to the difference in tip undercooling, the solute interaction of converging dendrites, which has been ignored in the classical theoretical model, also has a significant effect on the nature of the overgrowth at low pulling velocities. Solute interaction can retard the growth of dendrites at the grain boundary (GB) and induce a lag of these dendrites relative to their immediate neighbors, which gives the unfavorably oriented dendrite the possibility to overgrow the favorably oriented one. However, this unusual overgrowth only occurs when the spacing between the favorably oriented GB dendrite and its immediate favorably oriented neighbor decreases to a certain level through lateral motion. These findings can broaden our understanding of the overgrowth mechanism of converging dendrites.     

正常晶粒长大的计算机模拟（I）——晶粒长大动力学跃迁概率的改进

根据晶粒长大真实的物理过程提出了构建跃迁概率所必须满足的条件。在此基础上提出了正确考虑温度影响的跃迁概率，并采用该概率对晶粒长大过程中温度和第二相粒子含量的作用进行了模拟。模拟结果可以更真实地反映晶粒长大的物理过程，又能与已有的计算数据相容，表现出较传统概率更好的优越性。     

织构诱发异常晶粒长大的Monte Carlo模拟

利用欧拉角表示晶粒取向的Monte Carlo方法,模拟了异常晶粒长大过程。通过Voronoi模型产生了晶粒尺寸近似符合Lognormal分布且具有立方织构的初始组织,比较了该组织中不同织构半峰宽和体积分数对晶粒长大过程的影响。模拟结果表明:随着半峰宽变大,异常晶粒长大可能性和平均晶粒尺寸都变大;织构体积分数对异常晶粒长大的影响相对较小,只在半峰宽较小且体积分数很大时,才抑制异常晶粒长大,模拟结果与实验吻合。另外,模拟组织中平均晶粒长大速度不仅受到晶界迁移率的影响,而且受到异常晶粒长大数量的影响。     

“节点修正”FVM-CA方法熔积凝固组织模拟

基于Nastac模型建立了一种优化的元胞捕捉机制,即"节点修正"法元胞自动机(CA)模型.较传统的CA模型能够简便有效地模拟任意主轴方向的柱状晶生长;将其与有限体积法(finite volume method,FVM)的宏观温度场模拟相结合,构建了一种改进的FVM-CA宏微观耦合计算方法.用该改进模型对碳钢电弧熔积组织演变过程进行了模拟,并探讨了熔积速度对熔池枝晶间距及生长方向的影响.试验结果证明了计算模型的合理性,该工作将为熔积增材成形金属零件的组织预测与控制提供科学基础.     

Dendritic grain growth simulation in weld pool of nickel base alloy

Dendritic grain growth at the edge of the weld pool is simulated using a stochastic numerical model of cellular automaton algorithm. The grain growth model is established based upon the balance of solute in the solid/liquid interface of the dendrite tip. Considering the complicated nucleation condition and competitive growth, the dendrite morphologies of different nucleation condition are simulated. The simulated results reproduced the dendrite grain evolution process at the edge of the weld pool. It is indicated that the nucleation condition is an important factor influencing the grain morphologies especially the morphologies of secondary and tertiary arms.