定向倾斜枝晶生长规律及竞争行为的相场法研究

采用多相场模型对定向凝固过程中倾斜枝晶生长进行了研究, 模拟了单一取向枝晶列的演化规律及不同取向枝晶列汇聚生长竞争淘汰行为. 结果表明, 枝晶尖端过冷度随倾斜角度的增大而增大, 即相同条件下倾斜枝晶尖端位置总是低于非倾斜枝晶; 汇聚生长时择优取向枝晶总是阻挡非择优取向枝晶, 但在抽拉速度较低时, 由于溶质扩散场的相互重叠, 晶界处择优取向枝晶的生长受到相邻非择优枝晶的影响而延缓, 这可能导致非择优取向枝晶淘汰择优取向枝晶.     

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.     

基于元胞自动机方法的定向凝固枝晶竞争生长数值模拟

通过耦合温度场模型、溶质扩散方程以及枝晶生长动力学方程等重要因素,建立了一种改进的元胞自动机模型.该模型通过采用偏心算法消除网格各向异性,实现了二维尺度上任意角度枝晶生长的模拟,同时适用于模拟三维尺度上枝晶的生长过程.利用建立的模型开展了定向凝固枝晶竞争生长过程的数值模拟.为了体现本模型的有效性,模拟了透明合金的竞争生长过程,并与实验结果符合良好.镍基高温合金汇聚竞争和发散竞争的模拟结果清楚地展现了不同抽拉速度和枝晶优先生长角度下枝晶的竞争生长过程,并且模拟结果与理论模型相符合.三维枝晶生长的模拟结果表明本模型可以用来模拟三维枝晶一次臂间距的调整过程.     

Seaweed to dendrite transition in directional solidification

We simulate directional solidification using a phase-field model solved with adaptive mesh refinement. For small surface tension anisotropy directed at 45 degrees relative to the pulling direction we observe a crossover from a seaweed to a dendritic morphology as the thermal gradient is lowered, consistent with recent experimental findings. We show that the morphology of crystal structures can be unambiguously characterized through the local interface velocity distribution. We derive semiempirically an estimate for the crossover from seaweed to dendrite as a function of thermal gradient and pulling speed.     

抽拉速度对SCN-DC共晶生长形貌的影响

本文采用类金属透明模型合金丁二腈-23.6 wt%樟脑 (SCN-23.6 wt%DC) 合金, 研究了棒状共晶定向凝固组织的演化行为, 考察了抽拉速度对棒状共晶合金组织形貌演化的影响规律. 结果表明, 在共晶生长初期, 共晶组织首先起源于晶粒晶界或者试样盒型壁处, 随后沿液/固界面和平行于热流方向生长; 在较小的抽拉速度 (0.064–0.44 μm/s)下, 棒状共晶界面前沿呈现平界面形态, 内部两相棒状组织平行生长, 并且随着抽拉速度的增大,棒状共晶逐渐细化, 棒状间距减小; 而在较大的抽拉速度 (0.67–1.56 μm/s)下, 共晶界面前沿呈现胞状生长形貌, 胞内的棒状共晶呈放射状生长, 同样, 随着抽拉速度的增大, 胞内棒状共晶逐渐细化, 棒状间距减小. 关键词： 定向凝固 共晶形貌 抽拉速度 共晶间距     

宏微观耦合模拟熔池不同区域中枝晶竞争生长过程

针对熔化焊过程建立了宏微观耦合模型,模拟了熔池内不同区域凝固过程中随机取向枝晶的竞争生长过程. 通过宏观三维有限元模型计算熔池中瞬态的传热传质过程,利用双线性插值算法将凝固参数传递给微观组织模型. 采用元胞自动机法模拟随机取向的枝晶在熔池凝固条件下的竞争生长过程. 模拟结果表明,所建立的微观模型能够精确模拟任意生长取向的枝晶. 凝固条件中最大温度梯度方向对枝晶竞争过程有明显选择作用,生长方向与最大温度梯度方向相同或接近的枝晶在竞争中具有更大优势. 焊缝中的晶粒组织由枝晶簇发展形成,晶粒组织的形貌演变取决于相邻枝晶簇之间的竞争过程,具有择优取向的枝晶簇会逐渐排挤非择优取向的枝晶簇并最终将其阻挡在凝固组织内部,宏观晶粒的取向与其内部枝晶簇的生长方向并不一定相同. 熔池中心线附近区域在焊接过程中具有更小的温度梯度、更大的凝固速率以及更大的局部冷却速率,凝固后可以获得更加细小的焊缝枝晶组织. 枝晶间距的模拟结果与相应凝固条件下的试验数据符合较好. 关键词： 焊接熔池 枝晶形貌 竞争生长 元胞自动机     

Nucleation and growth of molecular organic crystals in a liquid film under vapor deposition

We report on the nucleation and growth of tetracene in a thin liquid film which is continuously supersaturated by vapor deposition of molecules onto the film. In a first stage, nucleation and fast anisotropic two-dimensional dendritic growth occurs. In a second stage, the dendrites coarsen into pallet-shaped crystals. These are highly oriented with respect to the plane of the liquid film and reach a lateral size of several 100 microm. The two-dimensional growth mode is explained by the confined growth geometry in the liquid in combination with the anisotropy of the crystal structure.     

晶体取向对定向凝固平界面失稳行为的影响

采用丁二腈-丙酮透明模型合金研究了不同晶体取向的晶粒在定向凝固条件下的平界面失稳过程.实验选择了三个界面失稳后具有不同生长形态的典型晶粒作为研究对象, 分别为择优生长枝晶、倾斜枝晶和海藻晶.结果表明可发展为择优生长枝晶的晶粒的平界面失稳孕育时间和初始扰动波长最小,海藻晶次之, 倾斜枝晶最大,这与以往的解析结果和相场模拟结果一致. 同时,实验观察发现可发展为择优生长枝晶和倾斜枝晶的晶粒的界面非稳态演化过程与海藻晶显著不同,这表明平界面失稳的非稳态演化过程与晶体取向相关.     

Dendrite to symmetry-broken dendrite transition in directional solidification of non-axially oriented crystals

In this paper, the morphological transition from dendrite to symmetry-broken dendrite is investigated in the directional ;olidification of non-axially-oriented crystals using a quantitative phase-field model. The effects of pulling velocity and zrystal orientation on the morphological transition are investigated. The results indicate the orientation dependence of the ;ymmetry-broken double dendrites. A dendrite to symmetry-broken dendrite transition is found by varying the pulling telocity at different crystal orientations and the symmetry-broken multiple dendrites emerge as a transition state for the ;ymmetry-broken double dendrites. The state region during the transition can be well characterized through the variations ff the characteristic angle and the average primary dendritic spacing.     

聚乙烯醇水溶液二维定向凝固的微观组织演化

为了深入探究定向多孔聚合物材料的微观组织形成机理,利用定向凝固原位实时观察手段,研究不同浓度及不同分子量聚乙烯醇(PVA)水溶液在不同抽拉速度下的定向凝固形貌演化.PVA水溶液的定向凝固形态在低浓度(1 wt%,2.5 wt%)和小分子量(M_w=24000)情况下,一次枝晶间距随着抽拉速度的增加而减小.随着PVA浓度和分子量的增加,一次枝晶间距随抽拉速度变化不明显,枝晶主轴尺寸则随速度增加呈现减小的趋势.与传统凝固形态形成机理相比,PVA水溶液的凝固形态由PVA分子的扩散引起的凝固界面不稳定性机理和PVA高分子链交联引起的局部相分离机理竞争决定.     

三种变速条件下共晶生长的多相场法模拟

利用多相场模型模拟了共晶合金CBr₄-C₂Cl₆定向凝固变速生长过程，研究了阶跃变速、线性变速以及震荡变速三种变速条件下共晶片层间距的调整以及形态的变化.结果表明：在变速生长过程中，界面平均生长速率与平均过冷度随抽拉速率的变化均产生滞后效应；阶跃增速时，片层间距的调整通过突变分岔形式进行，而阶跃减速时，通过片层的逐步湮没与合并以及自身相的长大方式进行，两个过程表现出强烈的非对称性；线性增速过程，片层间距的调整通过逐步分岔进行，而线性减速过程 关键词： 多相场 共晶 变速生长 片层间距     

深过冷条件下Co_7Mo_6金属间化合物的枝晶生长和维氏硬度研究

采用电磁悬浮和自由落体两种实验技术对二元Co-50%Mo过共晶合金中初生Co_7Mo_6金属间化合物的生长机理和维氏硬度进行了系统研究.电磁悬浮实验中,合金熔体获得的最大过冷度为203 K(0.12T_L),初生Co_7Mo_6枝晶生长速度与过冷度之间呈现幂函数关系.随着过冷度的增大,初生枝晶中Co元素含量单调递增,枝晶尺寸明显减小,并且其维氏硬度逐渐升高.在自由落体状态下,随着液滴直径的减小,合金熔体的过冷度和冷却速率均增大.当液滴直径减小到392μm以下时,初生Co_7Mo_6枝晶从小平面向非小平面形态进行转变.实验发现,深过冷条件下Co_7Mo_6化合物发生了显著的溶质截留效应,其维氏硬度与Co元素分布和形貌特征密切相关.     

六方小面相螺旋在各向异性、表面吸附、界面动力学作用下生长的相场

利用定量相场模型研究了强各向异性、表面吸附率以及界面动力学作用条件下六方Ga N螺旋结构的表面形貌与生长机理.通过引入小面相各向异性的相场修正方程,研究了不同各向异性的稳态螺旋形貌,发现各向异性通过改变台阶尖端的曲率作用影响螺旋生长.弱各向异性下稳态螺距及界面动力学特征相对稳定,各向异性较强时尖端的过饱和度随着各向异性的增强而增大,并使得界面平衡态向着有利于螺旋台阶推进的方向移动.研究了表面吸附率对小面相螺旋生长的作用机理,发现吸附率的增加导致了稳态螺旋间距的降低,通过分析螺旋间距随台阶宽度的变化趋势,发现增强的表面吸附和各向异性强度降低了螺旋间距的收敛性,并且具体分析了收敛性误差;通过探讨界面动力学作用条件下螺旋形貌特征以及螺旋间距变化趋势,发现界面动力学系数通过改变稳态螺旋间距与特征指数因子调控螺旋生长的动力学机理,与各向同性相比小面相螺旋生长表现出较低的界面动力学系数依赖性.     

Computer Simulation of Nonequilibrium Growth of Crystals in a Two-Dimensional Medium with a Phase-Separating Impurity

Two-dimensional nonequilibrium growth of crystals (quasistable faceted and dendritic) in the presence of a phase separating impurity is studied by computer simulation. It is shown that there is a gradual modification in this system from quasistable faceted growth to the formation of dendrites when the impurity concentration increases. If there is dendritic growth in the presence of a phase-separating impurity, the cyclic changes in the morphology, expressed through the periodic occurrence of tertiary branches of a dendrite, are observed when the phase-separating impurity concentration is raised. This behavior of the morphology is considered as a reentrant nonequilibrium phase transition.     

切向流动作用下SCN-3wt% H2O枝晶定向生长过程研究

实时观察了切向流动作用下SCN-3wt% H₂O合金的定向凝固过程,研究了该合金枝晶生长方向、尖端半径、枝晶一次臂间距以及二次臂生长速度的变化规律.实验观察到能够代表枝晶尖端溶质边界层的“亮带”,该亮带在流动作用下的非对称性证实了切向流动能够改变枝晶前沿溶质的对称分布,使溶质边界层厚度沿背流侧向迎流侧逐渐减薄,枝晶生长发生迎流偏转,且偏转角度随抽拉速度的提高而减小.同时,流场与浓度场的耦合促进枝晶间的优胜劣汰,枝晶一次臂间距显著增大.固液界面处的强迫流动还能够引发相邻枝晶间环流,加速迎流处二次臂生长而抑制背流处生长. 关键词： 定向凝固 枝晶生长 切向流动     

基于改进元胞自动机模型的三元合金枝晶生长的数值模拟

在二元合金元胞自动机模型的基础上,通过耦合多元合金热力学相平衡求解器PanEngine, 建立了三元合金改进的元胞自动机模型,可模拟初生相枝晶的生长过程. 模型考虑了曲率过冷和成分过冷对界面平衡溶质成分的影响,通过不同组元的无量纲溶质过饱和度方程和界面溶质守恒方程之间的耦合来求解界面生长速率,并通过PanEngine计算界面处的液相线温度. 采用本模型模拟了Al-7%Si-xMg三元合金自由枝晶的生长形态, 结果表明Mg含量的增加会抑制枝晶一次臂的生长和二次臂的产生.同时模拟了不同抽拉速度下 Al-7%Si-0.5%Mg合金柱状枝晶的竞争生长过程,随着抽拉速度的增大,柱状枝晶一次枝晶臂间距逐渐减小, 与Hunt理论模型符合较好.     

界面能各向异性对定向凝固枝晶生长的影响

采用元胞自动机 (cellular automaton, CA) 模型研究了界面能各向异性对二维定向凝固枝晶生长的影响. 模拟结果显示当晶体的择优生长方向与热流方向一致时, 随着界面能各向异性强度的增大, 凝固组织形态由弱界面能各向异性时的海藻晶转变为强界面能各向异性时的树枝晶. 同时, 界面能各向异性强度会影响稳态枝晶尖端状态的选择, 界面能各向异性越强, 定向凝固稳态枝晶尖端半径越小, 尖端界面前沿的液相浓度和过冷度越小. 稳态枝晶生长的尖端状态选择参数与界面能各向异性强度也存在标度律的指数关系, 而枝晶一次间距则受界面能各向异性强度影响较弱. 当晶体的择优生长方向与热流方向呈-40°夹角时随着界面能各向异性强度的增大, 凝固组织形态由海藻晶逐渐转变为退化枝晶, 后又逐渐转变为倾斜枝晶. 关键词： 元胞自动机 枝晶 界面能各向异性     

CRYSTAL GROWTH OF ISOTACTIC POLYSTYRENE IN ULTRATHIN FILMS: FILM THICKNESS DEPENDENCE

The film thickness dependence of crystal growth is investigated for isotactic polystyrene (it-PS) in thin films for thicknesses from 20 down to 4 nm. The single crystals of it-PS grown at 180°C in the ultrathin films show a morphology typical of diffusion-controlled growth: dense branching morphology and fractal seaweed. The characteristic length of the morphology, i.e., the width of the branch, increases with decreasing film thickness. The thickness dependence of the crystal growth rate shows a crossover around the lamellar thickness of 8 nm. The thickness dependences of the growth rate and morphology are discussed in terms of the diffusion of chain molecules in thin films.     

Influence of initial solid–liquid interface morphology on further microstructure evolution during directional solidification

Preparation of the initial solid–liquid interface on which growth is started is a very critical step in directional solidification experiments. Dedicated experiments concerning preparation of the initial solid–liquid interface morphology and its influence on further directionally solidified microstructure were performed on Cu-20 wt% Sn peritectic alloy in a Bridgman-type furnace. To verify the morphology of the initial solid–liquid interface, steady-state directional dendritic growth was interrupted by thermal stabilization ranging from 0 to 1 h prior to quenching. With thermal stabilization duration increase, the solid–liquid interface morphology degenerated from dendritic to cellular and finally to planar. To verify the influence of the initial state on further solidification microstructure, directional solidification experiments were performed at a low pulling rate of 1 μm/s with different initial solid–liquid interface morphologies. The initial state affects solute redistribution and formation of peritectic coupled growth structure in the subsequent directional solidification process.