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研究各向异性表面张力对定向凝固中共晶生长形态稳定性的影响.应用多重变量展开法导出了共晶界面表达式和扰动振幅的变化率满足的色散关系.结果表明,共晶生长系统有两种整体不稳定性机理:由非震荡导致的"交换稳定性"机理和由震荡导致的"整体波动不稳定性"机理.震荡有四种典型模式,即:反对称-反对称(AA-),对称-反对称(SA-)、反对称-对称(AS-)和对称-对称(SS-)模式.稳定性分析表明:共晶界面形态稳定性取决于Peclet数ε的某一个临界值ε_*,当ε大于临界值ε_*时,共晶界面形态不稳定;当ε小于临界值ε_*时,共晶界面形态稳定.随着各向异性表面张力增大,对应于AA-,SA-和SS-模式的临界值ε_(aa*),ε_(sa*)和ε_(ss*)随之减小,表明各向异性表面张力减小这三种模式的稳定性区域;然而,随着各向异性表面张力增大,对应于AS-模式的临界值ε_(as*)随之增大,表明各向异性表面张力增大AS-模式的稳定性区域. 相似文献
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通过应用匹配渐近展开法和多变量展开法研究了各向异性界面动力学与各向异性表面张力的相互作用对定向凝固过程中深胞晶生长的影响.结果表明:当各向异性界面动力学与各向异性表面张力的偏好方向之间相差角度为θ_0时,θ_0会对深胞晶生长形态产生影响;当0≤θ_0≤π/4时,随着θ_0的增大,深胞晶的指状界面全长减小,深胞晶根部的深度减小,根部附近界面的曲率减小,而曲率半径增大;当π/4≤θ_0≤π/2时,随着θ_0的增大,深胞晶的指状界面全长增大,深胞晶根部的深度增大,根部附近界面的曲率增大,而曲率半径减小. 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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应用匹配渐近展开法和多变量展开法研究各向异性表面张力对定向凝固中深胞晶界面形态稳定性的影响, 通过寻找定向凝固系统的模式解获得了深胞晶界面形态满足的量子化条件. 结果表明, 与各向同性的定向凝固系统中深胞晶界面形态稳定性比较, 考虑各向异性表面张力的定向凝固中深胞晶生长界面形态也有两种整体不稳定性机制: 整体波动不稳定性和低频不稳定性. 随着各向异性表面张力的增加, 中性模式产生强振荡的枝晶结构的整体波动不稳定性的不稳定区域减小, 中性模式产生弱振荡的胞晶结构的低频不稳定性的不稳定区域增加. 相似文献
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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. 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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WANG JianYuan CHEN ChangLe WANG KangNing&JIN KeXin Shaanxi Key Laboratory of Condensed Matter Structures Properties Northwestern Polytechnical Uiversity Xi’an China 《中国科学G辑(英文版)》2009,52(5):742-746
The directional solidification process of SCN-3wt% Salol transparent alloy is investigated in the presence of the shear flow
at the liquid-solid interface. It is found that the shear flow induces a stabilizing effect on planar interface. At higher
pulling rates, oscillation of the growth pattern together with fluctuation of the growth velocity takes place. With the increase
of the pulling rate, the interface growth pattern transits from “planar-cellular” oscillation to “cellular-dendritic” oscillation,
and the periodicity increases. The modification of the growth pattern is due to the effect of the shear flow on solute distribution,
and the time and history dependent character of interface morphology evolution also plays an important role in the formation
of the oscillating growth pattern.
Supported by the National Natural Science Foundation of China (Grant Nos. 50331040 and 50702046) 相似文献
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The effect of interfacial energy anisotropy on planar interface instability in a succinonitrile alloy under a small temperature gradient 下载免费PDF全文
The morphological stability of a planar interface with different crystallographic orientations is studied under a small positive temperature gradient using a transparent model alloy of succinonitrile.Novel experimental apparatus is constructed to provide a temperature gradient of about 0.37 K/mm.Under this small temperature gradient,the planar interface instability depends largely on the crystallographic orientation.It is shown experimentally that the effect of interfacial energy anisotropy on planar interface stability cannot be neglected even in a small temperature gradient system.Higher interfacial energy anisotropy leads the planar interface to become more unstable,which is different from the stabilizing effect of the interfacial energy on the planar interface.The experimental results are in agreement with previous theoretical calculations and phase field simulations. 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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采用类金属透明模型合金丁二腈-1.0 wt%乙醇(SCN-1.0 wt% Eth)合金, 考察了晶体取向对定向凝固过程中晶粒的平界面失稳孕育时间、枝晶形态演化以及枝晶一次间距的影响. 结果表明, 随着枝晶择优生长方向与温度梯度方向夹角的增大, 晶粒的平界面失稳孕育时间增加, 界面的稳定性增强; 对于不同晶体取向的枝晶形态演化, 枝晶择优生长方向与温度梯度方向夹角越大, 枝晶二次臂不对称生长越严重, 同时, 具有生长优势的枝晶二次臂对相邻枝晶的生长的抑制越强烈; 至于不同晶体取向的枝晶一次间距, 随着枝晶择优生长方向与温度梯度方向夹角的增大, 枝晶一次间距增大.
关键词:
定向凝固
平界面失稳
枝晶间距
晶体取向 相似文献
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An experimental apparatus consisting of a crystal growth room and a crystal growth observation system was developed for the
study of the effect of the gravity convection perpendicular to the growth direction on the growth process by use of model
alloy succinonitrile (SCN)-5wt%ethanol. It was found that the convection improves the stability of the interface and causes
the downstream alternation of the cell growth direction because of the dual effect of the Stokes force and the gravity. The
second dendrite arm facing the flow comes into being earlier than that at another side when the interface transforms cell
to dendrite. Then the dendrite at the side facing the flow comes into being earlier. The second dendrite arm facing the flow
grows faster and is more developed than that at another side. In addition, the primary dendrite arm spacing increases and
the dendrite tip radius decreases under the gravity convection.
Supported by the National Natural Science Foundation of China (Grant Nos. 50331040 and 60171043) 相似文献
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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. 相似文献
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Absolute stability of the solidification interface in a laser resolidified Zn--2wt.%Cu hypoperitectic alloy 下载免费PDF全文
This paper reports on laser surface remelting experiments performed on a
Zn--2wt.%}Cu hypoperitectic alloy by employing a 5kW CW CO2 laser at
scanning velocities between 6 and 1207mm/s. The growth velocities of the
microstructures in the laser molten pool were accurately measured. The planar
interface structure caused by the high velocity absolute stability
was achieved at a growth velocity of 210~mm/s. An implicit expression
of the critical solidification velocity for the cellular--planar transition was
carried out by nonlinear stability analyses of the planar interface. The results
showed a better agreement with the measured critical velocity than that predicted by
M--S theory. Cell-free structures were observed throughout the whole molten pool at
a scanning velocity of 652~mm/s and the calculated minimum temperature gradient in
this molten pool was very close to the critical temperature gradient for high
gradient absolute stability (HGAS) of the \eta phase. This indicates that HGAS
was successfully achieved in the present experiments. 相似文献
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