共查询到17条相似文献,搜索用时 93 毫秒
1.
基于Jackson和Hunt二元规则共晶稳态生长理论,在共晶两相的界面溶质守恒条件中引入密度修正项,改进了共晶两相的界面溶质守恒条件.在此基础上,根据二元层片共晶常规凝固过程中层片组织稳态生长时Gibbs自由能的变化,运用极值形态选择原理确定二元层片共晶凝固过程中层片间距特征尺度选择准则.理论分析表明,对于给定二元共晶合金,在常规凝固条件下的层片间距选择通常为一有限区间.此外,理论分析还表明,二元层片共晶稳态生长时其特征尺度的选择可以呈现超稳定性,而且在给定的凝固条件下超稳定性只和给定合金系的物性参数有关.将该形态选择准则分别运用于物性参数精确已知的Al-Al2Cu,Sn-Pb和CBr4-C2Cl6合金系,表明计算结果与实验结果相符合. 相似文献
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对于高密度、导通时间为μs级的柱状等离子体开关,利用磁流体动力学理论(MHD),对其导通阶段的磁场穿透过程进行了模拟,得到了磁场分布随时间的变化;研究了开关导通过程中能量输运导致的温度不均匀分布对磁场穿透过程的影响。模拟结果表明:对于高密度等离子体开关,磁场以远大于磁扩散速率的速度穿透到等离子体中;在磁压对等离子体产生的压缩效应和欧姆加热效应共同作用下,激波区域的等离子体温度显著升高,这进一步加速了磁场穿透;当考虑能量输运方程时,开关导通时间为0.87 μs,比等温模型的结果0.92 μs短,与实验结果0.87 μs相一致。 相似文献
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对于高密度、导通时间为μs级的柱状等离子体开关,利用磁流体动力学理论(MHD),对其导通阶段的磁场穿透过程进行了模拟,得到了磁场分布随时间的变化;研究了开关导通过程中能量输运导致的温度不均匀分布对磁场穿透过程的影响。模拟结果表明:对于高密度等离子体开关,磁场以远大于磁扩散速率的速度穿透到等离子体中;在磁压对等离子体产生的压缩效应和欧姆加热效应共同作用下,激波区域的等离子体温度显著升高,这进一步加速了磁场穿透;当考虑能量输运方程时,开关导通时间为0.87 μs,比等温模型的结果0.92 μs短,与实验结果0.87 μs相一致。 相似文献
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基于Ginzberg-Landau理论,发展了一个与WBM模型和KKS模型一致的新相场模型.并利用该相场模型与溶质场耦合计算,以Al-65%Cu合金为例模拟了不同过冷度条件下,二元合金凝固过程的等轴枝晶生长过程.研究过冷度对二元合金等温凝固过程的等轴枝晶生长以及溶质场分布的影响.结果表明:随着过冷度的增大,枝晶的二次枝晶更加发达,浓度Peclet数和枝晶尖端的生长速率增大,而枝晶尖端的曲率半径减小,枝晶前沿的溶质富集现象也更严重;另外,计算结果与Ivantsov理论符合较好.
关键词:
相场法
枝晶生长
溶质场
Ivantsov理论 相似文献
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采用典型的透明模型合金,即丁二腈-1.5at%水杨酸苯酯(SCN-1.5at%Salol)合金,对界面前沿溶质扩散边界层和界面稳定性进行了实时观测,并结合丁二腈-1.1wt%乙醇(SCN-1.1wt%Eth)、丁二腈0.43wt%氧杂萘邻酮152(SCN-0.43wt%C152)的实验结果,对纯扩散和存在对流情况下的定向凝固界面形态稳定性进行了系统的分析.发现以往所进行的界面稳定性分析能够准确描述实验中所观测到的界面失稳的时间相关特性,并发现界面失稳演化存在两个时间结点,即界面失稳孕育时间和平胞转变孕育时间,所获得的时间相关解能够准确预测实验中观测到的平胞转变孕育时间,而对流效应则明显缩短了界面失稳和平胞转变的孕育时间.实验中获得的单位扰动振幅发展速率比稳态理论预言值小了将近一个数量级,而与时间相关解较为接近.
关键词:
界面稳定性
凝固
扩散
对流 相似文献
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对定向凝固界面前沿非稳态溶质扩散场进行了系统的对比分析,发现无论在纯扩散还是存在对流的情况下,界面前沿的溶质扩散场通常满足指数分布的形式,可以采用一个统一的公式来描述界面前沿的瞬态溶质扩散场.进而在此基础上,对定向凝固界面形态稳定性进行了统一的时间相关的非稳态分析,发现界面临界稳定性条件的数学描述形式与Mullins和Seker ka理论给出的稳态解完全一致, 只是用时间相关的浓度梯度GtC、界面速度Vi、溶质扩散长度l代替了稳态生长中所得到的GC,V,DL/V. 相似文献
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Summary The conditions for the existence of the similarity solution of a two-dimensional boundary layer flow past a plane wall of
an electrically conducting gas, in the presence of a transverse magnetic field at small magnetic Reynolds number, are studied.
It is found that, in general, the similarity requirements involve as many as eleven parameters out of which eight are the
same as these of nonmagnetic gasdynamic boundary layer flow. The solution is obtained for power law velocity distribution,
in Illingworth variables, at the outer edge of the boundary layer and for the corresponding variable magnetic field. The Prandtl
number of the fluid is taken as unity and the magnetic-field effects are confined to the boundary layer only.
相似文献
Riassunto Si studiano le condizioni per l’esistenza della soluzione di somiglianza di un flusso nello stato di confine bidimensionale oltre una parete piana di un gas che conduce electtricità, in presenza di un campo magnetico trasverso con piccolo numero magnetico di Reynolds. Si trova che, in generale, le esigenze di somiglianza coinvolgono undici parametri, otto dei quali sono identici a quelli del flusso della strato di confine non magnetico e gas dinamico. La soluzione è ottenuta per la distribuzione di velocità della legge di potenza, in variabili di Illingworth, nel bordo esterno dello strato limite e per il corrispondente campo magnetico variabile. Il numero di Prandtl del fluido è preso uguale a uno e gli effetti del campo magnetico sono confinati solo allo strato limite.
Резюме Исследуются условия существования подобного решения для двумерного течения в пограничиом слое вдоль плосокой стенки газа, проводящего электричество, в присутствии поперечного магнитного поля при малых магнитных числах Рейнольдса. Получено, что в общем случае требования подобия включают одиннадцать параметров, восемь из которых являются соответствующими параметрами для немагнитного газодинамического течения в пограничном слое. Ппределено решение в случае степенного закона распределения по скоростям на внешнем краю пограничного слоя и для соотвтствуюего переменного магнитного поля. Число Прандтля для жидкости считается равным единице и эффекты магнитного поля ограничиваются только пограничным слоем.
相似文献
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Phase field investigation on the initial planarinstability with surface tension anisotropy during directional solidification of binary alloys 下载免费PDF全文
Phase field investigation reveals that the stability of
the planar interface is related to the anisotropic intensity of
surface tension and the misorientation of preferred crystallographic
orientation with respect to the heat flow direction. The large
anisotropic intensity may compete to determine the stability of the
planar interface. The destabilizing effect or the stabilizing effect
depends on the misorientation. Moreover, the interface morphology of
initial instability is also affected by the surface tension
anisotropy. 相似文献
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《Journal of magnetism and magnetic materials》2001,234(3):437-442
Unidirectional solidification experiments of TbFe2 alloy using a static magnetic field in microgravity were performed in the Japan Microgravity Center. When the magnetic field strength was increased from 0 to 4.5×10−2 T during unidirectional solidification in microgravity, a [1 1 1] crystallographic alignment of the grains dominated, and the maximum magnetostriction constant increased from 1000 to 4000 ppm. For unidirectional solidification in normal gravity, the maximum magnetostriction constant remained at 2000 ppm with increasing of magnetic field. 相似文献
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研究了强磁场复合不同强度和不同频率交变电流对Zn-30wt%Bi合金凝固组织的影响规律.结果表明:在25℃/min的冷却速率下,只施加交变电流无法抑制该合金的分层比重偏析;而单独施加纵向强磁场对合金的偏析有一定的改善作用;如果同时施加纵向强磁场和工频交变电流,在产生的交变洛仑兹力的作用下,合金的分层比重偏析基本得到抑制;增加电流密度、磁场强度和交流电流频率均有利于合金凝固组织的改善.但对上述三个参数而言,均存在一个最佳值,偏离该最佳值时,均难以获得均质的偏晶合金组织.从电磁场动力学角度探讨了复合场影响偏晶
关键词:
强磁场
偏晶合金
重力偏析
低冷却速度 相似文献
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Summary The MHD stability of a liquid jet (radiusR
0 and density ϱ) under the influence of self-gravitating force has traditionally been studied using a normal-mode type of analysis.
The dispersion relation is obtained and studied analytically and numerically. The axial magnetic fields inside and outside
the jet have always stabilizing effects. The transverse magnetic field has a destabilizing effect. It is found that the largest
doman of instability is fastly shrinking (as α> ?) with increasingH
0 /H
G
values,H
G
4(4πGϱ2
R
2
0 μ)1/2, whereG is the gravitational constant. Some reported works are recovered as limiting cases from the present work. 相似文献