热扰动对过冷熔体中二次枝晶生长影响的相场法模拟

用相场法模拟了过冷纯物质二次枝晶的生长过程，并定量地研究了过冷度、各向异性、热扰动振幅等参数对二次枝晶的影响. 通过加入热扰动，相场法能更真实地模拟过冷熔体中二次枝晶的生长，计算得到的二次枝晶间距和幅值与由Wentzel_Kramers_Brillouin方法得到的结果符合较好. 模拟结果表明，过冷度和各向异性对二次枝晶有较大影响. 当过冷度增大时，二次枝晶间距随之减小；当各向异性增大时，二次枝晶间距随之增大，但二次枝晶幅值则随之减小. 热扰动振幅主要影响二次枝晶幅值，而对二次枝晶间距影响较弱. 关键词： 相场法 热扰动 枝晶生长 二次枝晶     

等温凝固多晶粒生长相场法模拟

采用Kim模型，利用耦合溶质场的相场模型对Al-2-mole-Cu合金等温凝固过程中多晶粒相互影响下枝晶的生长过程进行数值模拟，为了提高计算效率，采用差分去实现宏微观场之间的耦合.研究了不同过冷度对多晶粒枝晶形貌和溶质分布的影响，结果表明：成分过冷对枝晶生长速度和溶质分配有着重大的影响，溶质元素Cu在固液界面前沿重新分布，结果导致实际热过冷减小，进而影响枝晶的生长和溶质向外层的扩散，致使相互接触的枝晶产生萎缩而其余没有受到抑制的枝晶生长方向产生择优现象. 关键词： 相场法 模拟 择优生长 等温凝固 二元合金     

受迫流动下的枝晶生长相场法模拟研究

基于耦合流场和热噪声的相场模型及合理高效的三维动态求解域加速算法,定量模拟了在受迫流动下枝晶的非对称生长及流速对迎流、背流两侧的温度分布和层流层分布的影响.计算结果表明,受迫流动使迎流、背流两侧温度的分布与层流层分布呈现不对称状态,导致迎流侧与背流侧的过冷度不同,而熔体施加于枝晶界面前沿迎流侧的力还不足以抑制过冷度的作用,结果造成枝晶迎流方向优先生长,从而产生倾向于散热方向的倾斜,同时,由于迎流侧的实际过冷度大于背流侧,有利于促进迎流一侧枝晶生长速度以及稳定侧向分枝生长,从而导致了侧向分枝的非对称生长.随     

三维枝晶生长的相场法数值模拟研究

基于薄界面限制、耦合界面能各向异性的相场模型,采用动态计算区域的加速算法,对纯物质的三维枝晶生长进行了定量模拟,真实再现了枝晶的生长过程.对枝晶尖端进行了剖切分析,表明主枝截面上各向异性没有主枝方向各向异性明显;对枝晶尖端生长速度、尖端半径、Peclet数及临界稳定性参数σ^*进行模拟计算,并与同条件下报道值进行了对比分析,两者符合良好,并得到了与结晶理论相一致的枝晶生长规律,证实了相场方法模拟三维空间枝晶生长可行有效. 关键词： 相场方法 枝晶生长 微观组织 三维数值模拟     

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.     

深过冷三元Ni-Cu-Co合金的快速枝晶生长

研究了深过冷条件下三元Ni₈₀Cu₁₀Co₁₀合金的快速枝晶生长, 采用电磁悬浮无容器处理方法获得了335 K(0.2T_L)的最大过冷度. X射线衍射分析与差示扫描量热分析均表明,凝固组织为α-Ni单相固溶体. 随过冷度增大, 凝固组织显著细化, 并且当过冷度达110 K时，凝固组织的形态由粗大形枝晶转变为等轴晶. 深过冷条件下溶质截留效应增强, 使得微观偏析程度减小. 对不同过冷度下合金枝晶的生长速度进 关键词： 深过冷 枝晶生长 快速凝固 溶质截留     

相场晶体法模拟过冷熔体中的晶体生长

利用相场晶体(phase-field crystal)模型,采用有限差分法,模拟了过冷熔体中晶体生长过程,研究了不同相区、不同过冷度对晶体生长过程的影响.结果表明,在共存区中,随着演化的进行,晶体生长被抑制,液-晶边界层逐渐变厚;在单相区中,随着过冷度的增大,晶态区面积显著增加,液-晶边界层逐渐变薄.晶体生长速度和过冷度成近似线性的关系. 关键词： 相场晶体 自由能函数 过冷熔体 晶体生长     

相场法模拟多元合金过冷熔体中的枝晶生长

在二元合金相场模型研究的基础上，进行扩展获得了多元合金相场模型.以Al-Si-Mg三元合金为例，采用该相场模型实现了逼真地模拟多元合金凝固过程的等轴枝晶生长，得到了二次或更高次晶臂生长等复杂的枝晶形貌.随着第三组元Mg含量的减少，枝晶的二次枝晶越发达，枝晶中溶质的偏析越严重，枝晶尖端的生长速率和半径越大，与丁二腈-丙酮体系中枝晶尖端生长速率、半径随溶质浓度变化关系的理论计算和实验结果相符合.另外，枝晶初生晶臂中心的溶质浓度最低，在被二次晶臂包围的界面区域的溶质浓度最高；固液界面区域具有较大的浓度梯度，其中枝晶尖端前沿的梯度最大. 关键词： 相场法 多元合金 凝固过程 枝晶生长     

Rapid eutectic growth in undercooled Al-Ge alloy under free fall condition

Eutectic growth in Al-51.6%wt Ge alloy has been investigated during free fall in a drop tube. With decreasing undercooling ΔT, the microstructural evolution has shown a transition from lamellar eutectic to anomalous eutectic. A maximum cooling rate of 4.2×10^4K/s and undercooling of up to 240K (0.35T_E) are obtained in the experiment. The eutectic coupled zone is calculated on the basis of current eutectic and dendritic growth theories, which covers a composition range from 48%－59% Ge and leans towards the Ge-rich side. The two critical undercoolings for the eutectic transition are ΔT_1^*=101K and ΔT_2^*=178K. When ΔT≤ΔT^*_1, the microstructure for Al-51.6% Ge eutectic shows lamellar eutectic. If ΔT≥ΔT^*_2, the microstructure shows anomalous eutectic. In the intermediate range of ΔT^*_1<ΔT<ΔT^*_2, the microstructure is the mixture of the above two types of eutectics.     

外来夹杂物颗粒对枝晶生长形态影响的相场法研究

在KKS相场模型基础上提出了一种耦合取向场的二元合金相场模型.通过该模型分析了外来夹杂物颗粒与晶粒之间的取向错配对于枝晶生长的影响，模拟结果表明外来夹杂物颗粒的大小、位置、数量以及与晶粒的取向差等因素均对枝晶生长形态有较大影响，外来夹杂物颗粒会造成枝晶臂的偏转与分叉，从而最终形成各种形貌的非规则枝晶. 关键词： 相场法 枝晶 外来夹杂物颗粒 晶体取向     

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.     

Effect of elastic strain energy on grain growth and texture in AZ31 magnesium alloy by phase-field simulation

A phase-field model is modified to investigate the grain growth and texture evolution in AZ31 magnesium alloy during stressing at elevated temperatures. The order parameters are defined to represent a physical variable of grain orientation in terms of three angles in spatial coordinates so that the grain volume of different order parameters can be used to indicate the texture of the alloy. The stiffness tensors for different grains are different because of elastic anisotropy of the magnesium lattice. The tensor is defined by transforming the standard stiffness tensor according to the angle between the (0001) plane of a grain and the direction of applied stress. Therefore, different grains contribute to different amounts of work under applied stress. The simulation results are well-explained by using the limited experimental data available, and the texture results are in good agreement with the experimental observations. The simulation results reveal that the applied stress strongly influences AZ31 alloy grain growth and that the grain-growth rate increases with the applied stress increasing, particularly when the stress is less than 400 MPa. A parameter (△d) is introduced to characterize the degree of grain-size variation due to abnormal grain growth; the △d increases with applied stress increasing and becomes considerably large only when the stress is greater than 800 MPa. Moreover, the applied stress also results in an intensive texture of the 〈0001〉 axis parallel to the direction of compressive stress in AZ31 alloy after growing at elevated temperatures, only when the applied stress is greater than 500 MPa.     

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.     

Elasto-plastic phase-field simulation of martensitic transformation in lath martensite steels

Phase-field simulation of lath martensite formation in Fe–0.1C mass% steel was carried out based on the two types of slip deformation (TTSD) model, which is recently developed as a result of analytical solution for the martensitic transformation being composed of Bain deformation and plastic deformation but without lattice rotation matrix. The simulation results reveal that the plastic deformation along the two types of slip system is complementary. The simulation result of the relationship between the two types of slip deformation is consistent with the analytical result calculated by TTSD model, indicating the validity of TTSD model for explaining the formation of lath martensite.     

相场法模拟对流速度对上游枝晶生长的影响

基于Tong和Beckermann等提出的耦合流场的相场模型,采用有限差分法对纯金属凝固过程进行二维模拟计算,研究了不同对流速度对金属枝晶生长的影响.结果表明：在有对流情况下,上游及下游枝晶呈现不对称形貌,上游枝晶生长速度明显比下游方向生长速度快.随着对流速度的增大,上游尖端过冷度也增大,枝晶生长也越快.这是由于流体速度越大,对上游枝晶冲刷越强,上游枝晶尖端实际过冷度越大,枝晶生长越迅速. 关键词： 相场法 对流速度 上游枝晶     

微重力条件下Ni-Cu合金的快速枝晶生长研究

采用落管方法实现了Ni-50%Cu过冷熔体在微重力和无容器条件下的快速枝晶生长.对微重力条件下的晶体形核和快速生长进行了研究，发现随着过冷度的增大，晶体生长形态由粗大枝晶向规划均匀的等轴晶转变.实验中最大冷却速率达到8×103K/s，获得了218K(014TL)的最大过冷度.理论分析表明，过冷熔体中优先发生异质形核，形核率可达1012m-3s-1以上；Ni-50%Cu过冷熔体中的枝晶生长随过冷度的增大发生由溶质扩散控制向热扩散控制的生长动力学机理转变.在68K过冷度条件下，生长界面前沿的偏析程序最大. 关键词： 落管 微重力 深过冷 枝晶 熔体     

BCC枝晶生长原子堆垛过程的晶体相场研究

通过在自由能泛函中引入各向异性参数得到了一个基于高斯内核的改进晶体相场模型, 并采用该模型研究了体心立方结构(BCC)枝晶生长的原子堆垛过程. 结果表明, 在BCC由正十二面体平衡形貌演化为枝晶组织过程中, 形核位置经历了由面心({110}面)到尖端(<100>取向)的转移, 进而发生界面失稳形成枝晶组织; 枝晶生长过程中, 新的固相原子首先在枝晶尖端附近形核, 并快速向尖端及根部生长, 枝晶尖端被新原子完全包覆后将再次诱发液相原子附着形核及生长; 随初始液相密度的增加, 固-液界面移动速率增加, 速率系数的各向异性也增强.     

切向流动作用下的胞晶定向生长机理研究

实时观测了切向流动作用下SCN-0.2%Salol透明合金的定向胞晶生长过程,研究了切向流动作用对胞晶间距调整的作用机理. 实验发现,切向流动使得胞晶阵列发生顺流偏转,并且稳态平均间距随流速的增高而减小. 分析表明,这主要是由于流动作用导致胞端失稳波长减小而引起. 施加切向流动使得胞晶间距的调整过程中的分裂机理多样化. 顶端分裂由均匀分岔转变为多枝分岔和不对称分岔;同时,胞晶迎流侧容易产生分枝,且分枝生长方向在流动作用下转变为与胞晶主干方向相同. 胞晶的淘汰机理由静态条件下的弱势生长胞晶被相邻两侧胞晶淘汰 关键词： 切向流动 定向胞晶生长 间距调整     

Phase-field simulation of dendritic growth in a binary alloy with thermodynamics data

This paper simulates the dendrite growth process during non-isothermal solidification in the Al-Cu binary alloy by using the phase-field model. The heat transfer equation is solved simultaneously. The thermodynamic and kinetic parameters are directly obtained from existing database by using the Calculation of Phase Diagram （CALPHAD） method. The effects of the latent heat and undercooling on the dendrite growth, solute and temperature profile during the solidification of binary alloy are investigated. The results indicate that the dendrite growing morphologies could be simulated realistically by linking the phase-field method to CALPHAD. The secondary arms of solidification dendritic are better developed with the increase of undercooling. Correspondingly, the tip speed and the solute segregation in solid-liquid interface increase, but the tip radius decreases.