二元共晶定向凝固的多相场法数值模拟

借助于多相场模型,利用移动盒算法模拟了小Peclet数下二元共晶模型合金的定向凝固过程.同时研究了共晶生长层片间距调整机制.模拟结果表明:当初始层片间距小于最小过冷层片间距时,层片调整通过两相的竞争生长与层片湮没进行;相反,当初始层片间距大于最小过冷理论时,层尖的形核分叉生长使层片生长趋于稳定;再现了共晶1λ振荡不稳定性生长,通过分析局部动力学得出1λ振荡不稳定性生长原因为溶质再分配与共晶两相的体积调整.     

定向凝固共晶生长的元胞自动机数值模拟

本文在已有的二元初生相元胞自动机（CA）方法的基础上,针对二元共晶凝固过程提出了改进的元胞自动机（MCA）模型.该模型考虑成分过冷和曲率过冷对界面形态的影响,通过界面溶质浓度守恒来获得共晶α相和β相生长速率,模拟了层片的湮灭、分叉与稳态生长.为了验证模型的可靠性,对常见的CBr₄-C₂Cl₆共晶透明合金进行了模拟,研究了抽拉速率对共晶层片间距大小的影响,模拟结果与文献中的实验结果吻合良好;同时模拟了共晶层片间距调整过程的形貌演化以及层片振荡不稳定性现象.本文将MCA模型扩展到三维定向凝固过程中,研究了共晶形态的层棒状转变机制.     

多相场模拟非共晶成分CBr4-C2Cl6合金层片生长的形貌选择

通过KKSM多相场模型,分别研究了定向凝固条件下亚共晶、过共晶成分的CBr4-C2Cl6合金层片生长形貌选择规律.结果表明,随着初始层片间距的不断增加,亚共晶的形貌选择变化依次为:层片湮没→稳态生长→2λ0不稳定性→层片分叉与形核;而过共晶的形貌选择变化依次为:层片湮没→稳态生长→2λ0不稳定性→T-2λ0不稳定性→Tilt 生长→T-1λ0不稳定性.模拟结果不仅与理论计算的形貌选择图相符,而且与实验结果定性地一致.     

试样厚度对共晶组织形态的影响

采用透明模型合金CBr4-11.7?Cl6(摩尔分数),对共晶合金定向凝固界面演化进行了实时观察,研究了初始非稳态凝固过程中试样厚度对于共晶合金界面能各向异性效应及层片间距调整机制的影响规律.结果表明,共晶界面能各向异性效应和层片调节机制与试样厚度显著相关.试样厚度大于48μm时,试样厚度对界面能各向异性的影响可以忽略;当试样厚度为48μm或更小时,界面能各向异性的效应开始显现,由于初始晶粒的取向不同,部分层片偏离温度梯度方向生长;当试样厚度为16μm时,层片间距的调整由初始的多层调节机制转变为准二维调节机制.     

Al-Cu共晶合金定向凝固不同抽拉速度下的组织演化

研究了Al-Cu共晶合金定向凝固下的组织演化规律。结果表明,在一定的温度梯度下,随着凝固速度的增加,共晶层片间距因Al2Cu分叉而发生细化。在定向凝固过程中,层片间距的调整是通过纵向生长上的分叉和横向上的错配边界移动实现的,共晶组织达到稳态的过程是纵向和横向不断调整的过程。     

Al—Si共晶强制性非稳态生长实验研究

本文在定向凝固系统上用变速抽拉方法试验研究了Al-Si共晶强制非稳态生长过程。在加速生长过程中,共晶相间距对生长速度的响应呈滞后效应,响应机制是共晶Si相成簇分枝和局域“终结”延伸。在减速生长过程中,Si相成簇分枝和“终结”伸缩机制仍控制相间距;共晶相间距由初始生长速度决定,对生长速度下降不产生响应。     

共晶形貌选择及界面失稳的多相场模拟

采用KKSM多相场模型,研究了低速下定向凝固CBr4-C2Cl6薄试样的层片共晶界面失稳及形貌选择．多相场模拟结果真实再现了3种共晶层片界面失稳模式及其形貌选择的演化过程,随着初始层片间距的不断增加,其形貌选择变化依次为：层片湮没→稳态生长→1λO不稳定性→2λO不稳定性→层片分叉与形核．模拟结果不仅与理论计算的形貌选择图相符,而且与实验结果定性一致．     

溶质扩散控制层片共晶生长

基于平界面溶质扩散假设,改进界面物质守恒边界条件求得了层片共晶稳态生长的溶质边界层.结果表明,层片共晶组织生长过程中沿生长方向的溶质边界层与沿界面方向的溶质边界层相关.对CBr4-C2Cl6透明合金系的分析表明,共晶合金层片组织生长过程中界面处液、固相平均成分的差值随凝固速率增大而增大.合金成分偏离共晶成分时,界面处液相平均成分与共晶成分非常接近.在给定的凝固速率下,共晶相界面处液、固相平均成分差值与层片宽度的比值随合金成分的不同而不同.     

变速定向生长条件下Pb－Sn共晶组织变化

利用改进后的定向凝固装置，研究了在阶跃增速和线性增、减速的变速生长条件下，Ｐｂ－Ｓｎ共晶合金定向凝固共晶组织的变化。实验结果表明：当共晶试样缓慢线性增速或减速时，定向生长共晶片间距的调节分别通过两相共晶片的逐步分叉或合并缓慢进行；当共晶试样增速和减速的加速度值相同时，实际共晶生长速度的变化过程呈对称的逆曲线。     

变速定向生长条件下Pb—Sn共晶组织变化

利用改进后的定向凝固装置，研究了在阶跃增速和线性增，减速的变速生长条件下，Ｐｂ－Ｓｎ共晶合金定向凝固共晶组织的变化。实验结果表明，当共晶试样缓慢线性增速或减速时，定向生长共晶片间距的调节分别通过两相共晶片的逐步分叉或合并缓慢进行；当共晶试样增速和减速的加速度值相同时，实际共晶生长速度的变化过程呈对称的逆曲线。     

A model of solidification microstructures in nickel-based superalloys: predicting primary dendrite spacing selection

A combined cellular automaton-finite difference (CA-FD) model has been developed to simulate solute diffusion controlled solidification of binary alloys. Constitutional and curvature undercooling were both solved to determine the growth velocity of the solid/liquid interface. A modified decentered square/octahedron (in two or three dimensions) growth technique was implemented in the cellular automaton to account for the effect of crystallographic anisotropy. The resulting model is capable of simulating the growth of equiaxed and columnar dendritic grains in 2D and 3D, with the <100> directions either aligned or inclined with the grid. The algorithm used can also be used on coarser grids, with a concomitant loss in resolution, allowing simulation of sufficiently large numbers of dendrites in 3D to investigate the distribution of spacings, as well as average behavior.Simulations were performed for directional solidification with a range of withdrawal velocities and nucleation conditions, but a constant thermal gradient. The simulations capture the full microstructural development and primary spacing selection by both branching and overgrowth mechanisms. The model illustrates that there is a range of possible stable spacings, and that the final spacing is history dependent. It was also found that a minimum deviation from the steady state dendrite spacing is required before the spacing adjustment mechanisms are activated. The influence of perturbing the withdrawal velocity upon the stability of the spacing was also investigated. It was found that perturbations significantly reduce the range of stable primary dendrite spacing.     

Numerical Simulation of Columnar to Equiaxed Transition for Directionally Solidified Ti-44Al Alloy

Solute diffusion controlled solidification model was applied to simulate the columnar to equiaxed transition （CET） during directional solidification of Ti-44Al alloy. The simulation results show that the solutal interactions from growing equiaxed grains play an important role on CET. The effects of the applied thermal gradient and pulling velocity, the equiaxed seed spacing and nucleation undercooling on the CET are investigated in the present simulation. The simulated results indicated that the columnar branch spacing depends not only on the thermal gradient and the pulling velocity, but also on number of the seeds. A spacing adjustment can occur through initiation of seeds that develop into new columnar grains. The dependence of the CET on the thermal gradient and pulling velocity, qualitatively agrees with the analytical CET model of Hunt,     

NUMERICAL SIMULATION OF COLUMNAR TO EQUIAXED TRANSITION FOR DIRECTIONALLY SOLIDIFIED Ti-44Al ALLOY

Solute diffusion controlled solidification model was applied to simulate the columnar to equiaxed transition (CET) during directional solidification of Ti-44Al alloy. The simulation results show that the solutal interactions from growing equiaxed grains play an important role on CET. The effects of the applied thermal gradient and pulling velocity, the equiaxed seed spacing and nucleation undercooling on the CET are investigated in the present simulation. The simulated results indicated that the columnar branch spacing depends not only on the thermal gradient and the pulling velocity, but also on number of the seeds. A spacing adjustment can occur through initiation of seeds that develop into new columnar grains. The dependence of the CET on the thermal gradient and pulling velocity, qualitatively agrees with the analytical CET model of Hunt.     

Phase field study on crystal orientation effects in eutectoid phase transformation

In this present work, a multi-phase field model was used to simulate the eutectoid transformation process, and on the basis of the nucleation model that was previously proposed by our research team, anisotropic and orientation relationship models were introduced to study the growth mechanism of the pearlite lamellae with anisotropy. It was found that the growth direction of the pearlite lamellae is related to its orientation and spacing. In the process of lamellar growth, deflection growth of pearlite will appear along with the adjustment of lamellar spacing, and the deflection angle is equal to the orientation difference between the austenite and the pearlite. Comparison between experimental and numerical results indicates a good consistency in pearlite morphology.     

Simulation of microstructure evolution in directional solidification of Ti-45at.%Al alloy using cellular automaton method

The microstructural evolution of Ti-45 at.%Al alloy during directional solidification was simulated by applying a solute diffusion controlled solidification model. The obtained results have shown that under high thermal gradients the stable primary spacing can be adjusted via branching or competitive growth. For dendritic structures formed under a high thermal gradient, the secondary dendrite arms are developed not very well in many cases due to the branching mechanism under a constrained dendritic growth condition. Furthermore, it has been observed that, with increasing pulling velocity, there exists a cell/dendrite transition region consisting of cells and dendrites,which varies with the thermal gradient in a contradicting way, i.e. increase of the thermal gradient leading to the decrease of the range of the transition region. The simulations agree reasonably well with experiment results.     

Simulation of microstructural evolution in directional solidification of Ti-45at.%Al alloy using cellular automaton method

The microstructural evolution of Ti-45 at.%Al alloy during directional solidification was simulated by applying a solute diffusion controlled solidification model.The obtained results have shown that under high thermal gradients the stable primary spacing can be adjusted via branching or competitive growth.For dendritic structures formed under a high thermal gradient,the secondary dendrite arms are developed not very well in many cases due to the branching mechanism under a constrained dendritic growth cond...     

Interlamellar spacing and average interface undercooling of irregular eutectic in steady-state growth

The average lamellar spacing and interface undercooling in steady-state irregular eutectic growth were estimated based on the Jackson and Hunt＇s analysis by relaxing the isothermal interface assumption. At low growth rates, the average lamellar spacing and average interface undercooling are dependent only on the characteristic thermo-physical properties of a binary eutectic system. For a general Al-Si eutectic, it is found that the eutectic characteristic length based on the present non-isothermal analysis is consistent with that obtained from isothermal analysis; however, the average interface undercooling is remarkably different between them, and such discrepancy in average interface undercooling increases with increasing of growth rate. The measured interface undercooling obtained from literature is reasonably interpreted by present non-isothermal analysis.     

三维枝晶生长数值模拟及实验验证(英文)

建立了用于模拟立方晶系合金三维枝晶生长的改进元胞自动机模型。该模型将枝晶尖端生长速率、界面曲率和界面能各向异性的二维方程扩展到三维直角坐标系,从而能够描述三维枝晶生长形貌演化。应用本模型模拟在确定温度梯度和抽拉速度条件下三维柱状晶生长过程的一次臂间距调整机制和不同择优取向柱状晶之间的竞争生长。使用NH4Cl?H2O透明合金进行凝固实验,模拟结果和实验结果吻合较好。     

全层状TiAl合金层片生长及其影响因素

利用光学显微镜(OM)和TEM，研究了全层状TiAl合金的层片间距的影响因素及与各因素的关系。实验结果表明，全层状TiAl合金的层片间距与冷却速度和合金中铝含量有关，层片间距与冷却速度呈反比关系，并随着合金中铝含量的增加而增加。同时，以层片生长的台阶机制为基础，推导出了全层状TiAl合金在连续冷却过程中层片间距的数学表达式，推导结果与实验结果相符。