共查询到17条相似文献,搜索用时 484 毫秒
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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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用元胞自动机(CellularAutomaton,CA)模型研究了界面能各向异性对二维定向凝固胞晶的生长形态的影响,建立了判定胞晶生长达到稳态的判据.结果显示,当界面能各向异性强度非常小时,胞晶尖端很容易分岔,胞晶形态不容易稳定.而当界面能各向异性强度足够大时,容易形成稳定的胞晶形态,同时界面能各向异性强度会显著影响稳定胞晶的形态,界面能各向异性越强,稳态胞晶间距越小,胞晶尖端半径越小,尖端半径与胞晶间距的比值越小,固液界面前沿的浓度与过冷度越小. 相似文献
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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 相似文献
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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. 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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本文采用典型的透明模型合金丁二腈-1.1wt%乙醇进行定向凝固实验,对从平界面失去稳定性到建立起新的稳定的界面形态之间时间相关的形态演化过程进行了详细的直接观察研究。发现:1)平界面失稳初始扰动波长在Mullins-Sekerka(缩写为MS)稳定性理论所预言的波长范围内,但与理论预言的振幅发展最快的波长不一致;2)平界面失稳初期,扰动振幅发展速率确实是振幅的线性函数,但在具体数值上实测值比理论值小得多;3)胞晶和枝晶形态演化过程具有不同的特征,导致这种差别的主要原因是稳态一次间距与初始扰动波长之间相对尺
关键词: 相似文献
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The effect of anisotropic surface tension on the morphological stability of planar interface during directional solidification 下载免费PDF全文
This paper considers the effect of the anisotropic surface tension
on the morphological stability of the planar interface during
directional solidification. When the expression exhibiting the
four-fold symmetry is included, the modified absolute stability
criterion is obtained by employing the multi-variable expansion
method. The linear stability analysis reveals that for the given
temperature gradient, as the anisotropic surface tension parameter
increases, the stability zone tends to decrease. 相似文献
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Z. Kozhoukharova C. Rozé 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(1):125-135
The primary stationary and oscillatory Bénard-Marangoni instability is investigated in a fluid layer of infinite horizontal
extent, bounded below by a rigid plane and above by a deformable upper surface, subjected to a vertical temperature gradient.
Since the viscosity is temperature-dependent the consequences of relaxing Oberbeck-Boussinesq approximation and free surface
deformability are theoretically examined by means of small disturbance analysis. The problem has been solved numerically by
the Taylor series expansion method. The results obtained confirm that when the free surface is undeformable, stationary convection
develops in the form of polygonal cells, and oscillatory motion cannot be detected. When the surface deformability is considered,
stationary convection sets in, either as a short-wavelength hexagonal instability or as a long-wavelengh mode or as both,
and oscillatory convection is also possible. The stability threshold for the short-wavelength mode depends mainly on the viscosity
variation while the long-wavelength mode is determined by the surface deformation. Numerically, it is found that the neutral
oscillatory Marangoni numbers are only negative. When a variable-viscosity model is used the theoretical and experimental
results are in better agreement.
Received 15 May 1997 相似文献
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The morphological instability of solidification is analytically studied in the presence of an anisotropic and heterogeneous
phase transition zone with allowance for a liquid flow and convective heat-andmass transfer in this two-phase zone. The mechanism
of breaking the stability of solidification is considered; it consists in a convective heat and impurity transfer during a
liquid flow along channels in the phase transition zone. The morphological instability is subjected to linear analysis with
allowance for a liquid flow in the liquid phase of the system, impurity diffusion in the two-phase zone, and the dependence
of the transfer coefficients on the phase composition. The perturbation evolution parameter is determined for an anisotropic
and heterogeneous two-phase zone, and neutral stability curves of the process are obtained. It is shown that taking into account
impurity diffusion and an increase in the heterogeneity of the phase transition zone broaden the instability region and that
a decrease in the anisotropy narrows this region. A new criterion of convective morphological instability of solidification
with a two-phase zone is found, and it substantially broadens the instability region when the liquid flow velocity increases. 相似文献
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Using laser speckle interferometry and surface profiling methods, the evolution of plastic strain localization under uniaxial
tension of hcp zirconium alloys is studied. It is found that oscillatory instability observed at the parabolic stage of plastic
flow in zirconium alloys arises when the specimen locally and nonuniformly changes shape. It is shown that the cross-sectional
area of the deforming specimen decreases nonlinearly and is determined by the oscillatory variation of contraction and extension
strains at the site of macrolocalization in the hardening-softening mode. 相似文献
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Using the spin-spiral formulation of the tight-binding linear muffin-tin orbital method, the principal components of the exchange stiffness tensor are calculated for typical hard magnets including tetragonal CoPt-type and hexagonal YCo5 alloys. The exchange stiffness is strongly anisotropic in all studied alloys. This anisotropy makes the domain wall surface tension anisotropic. Competition between this anisotropic surface tension and magnetostatic energy controls the formation and dynamics of nanoscale domain structures in hard magnets. Anisotropic domain wall bending is described in detail from the general point of view and with application to cellular Sm–Co magnets. It is shown that the repulsive cell-boundary pinning mechanism in these magnets is feasible only due to the anisotropic exchange stiffness if suitably oriented initial pinning centers are available. In polytwinned CoPt-type magnets the exchange stiffness anisotropy controls the orientation of macrodomain wall segments. These segments may reorient both statically during microstructural coarsening and dynamically during the macrodomain wall splitting in external field. Reorientation of segments may facilitate their pinning at antiphase boundaries. 相似文献
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A high-power, unmodulated frequency standard laser light with low running cost is a powerful tool for industrial measurement involving laser calibration and interferometry in length and shape. To obtain such a laser light, we developed an intermittent frequency offset locked laser (IFOL laser). This laser system, composed of an iodine stabilized He-Ne laser (I2SL) and a high-powered symmetric three-mode He-Ne laser (HP-3ML, optical output=5 mW), provides two stabilization operational modes: (1) independent mode (I-mode) and (2) slave mode (S-mode). Utilizing the frequency pulling effect, HP-3ML is stabilized at the condition of symmetric three-mode oscillation, and a 3 mW single-mode light with frequency instability of 10-12 is obtained (I-mode). When the optical frequency deviates from the allowed limit during I-mode, HP-3ML is temporarily offset-locked to I2SL (S-mode) with frequency resettability of 10-10 and frequency instability of 10-12. After completion of the drift reset in S-mode, HP-3ML is stabilized in I-mode again, and then I2SL is turned off. In a typical case of allowed drift in ±1 MHz, repetitive operation of I-mode for 10 days and S-mode for 5 min was satisfactory. Such an intermittent operation reduces the working time of I2SL dramatically, resulting in low running cost that enables long-term use in industrial measurement. 相似文献