液态三元Fe-Cr-Ni合金中快速枝晶生长与溶质分布规律

采用落管方法实现了液态三元Fe-Cr-Ni合金的深过冷与快速凝固,合金液滴的冷却速率和过冷度均随液滴直径的减小而迅速增大.两种成分合金近平衡凝固组织均为粗大板条状α相.在快速凝固过程中,不同直径Fe_(81.4)Cr_(13.9)Ni_(4.7)合金液滴凝固组织均为板条状α相,其固态相变特征很明显,随着过冷度增大,初生δ相由具有发达主干的粗大枝晶转变为等轴晶.Fe_(81.4)Cr_(4.7)Ni_(13.9)合金液滴凝固组织由α相晶粒组成,随着过冷度增大,初生γ相由具有发达主干的粗大枝晶转变为等轴晶,其枝晶主干长度和二次分枝间距均显著下降,晶粒内溶质的相对偏析度也明显减小,溶质Ni的相对偏析度始终大于溶质Cr.理论计算表明,与γ相相比,δ相枝晶生长速度更大.在实验获得的过冷度范围内,两种Fe-Cr-Ni合金枝晶生长过程均由热扩散控制.     

FeSi2合金在高压下的凝固

研究了压力对FeSi2合金凝固组织的影响.与常压下凝固的典型共晶组织不同，高压条件下的凝固组织为初生ε相树枝晶加离异共晶.高压下凝固组织的变化主要是与压力对相图的影响和对凝固过程中溶质原子扩散的影响有关.通过引入压力参量，推导了高压凝固过程中的成分过冷判据，并应用该判据分析了高压凝固树枝晶组织的形成机理. 关键词： 高压 凝固 FeSi2     

快速凝固Fe62.1Sn27.9Si10合金的分层组织和偏晶胞形成机理

采用自由落体和单辊急冷技术研究了三元Fe_62.1Sn_27.9Si₁₀偏晶合金的相分离和组织形成规律,理论分析了两种快速凝固条件下合金的传热特性.自由落体条件下,由于Marangoni迁移和表面偏析势的作用,液滴凝固组织主要形成富Sn相包裹富Fe相的两层壳核结构.随着液滴直径减小,冷却速率和温度梯度增大,促进偏晶胞快速生长.在单辊急冷条件下,随着辊速的增大,冷却速率从1.1×10⁷增大至6.5×10⁷ K/s,合金熔体内部的液相流动和相分离受到抑制,凝固组织发生"九层结构→两层结构→无分层结构"的转变.同时,凝固过程中FeSn+L₂→FeSn₂包晶反应受到抑制,形成与自由落体条件下不同的相组成.EDS分析显示,αFe相在快速凝固过程中发生显著溶质截留效应. 关键词： Fe-Sn-Si偏晶合金 相分离 快速凝固 溶质截留     

浓度相关的扩散系数对定向凝固枝晶生长的影响

采用定量相场模型模拟了定向凝固过程中浓度相关的扩散系数对枝晶生长的影响.模型中,通过在液相溶质扩散方程中耦合浓度相关的扩散系数实现了浓度依赖的扩散过程.一系列模拟结果证实了浓度相关的扩散过程对枝晶定向生长具有显著的影响.结果表明:溶质扩散系数对溶质浓度耦合强度的增加会增强枝晶间的溶质扩散对尖端排出的溶质原子横向扩散的抑制作用,造成枝晶尖端固-液界面处的溶质富集程度增加,从而增加尖端过冷度;液相中扩散系数的改变对枝晶的尖端半径影响较小,模拟结果与理论模型计算结果较好吻合;液相中溶质扩散系数对溶质浓度依赖性的增加会使侧向分枝的振幅逐渐减小;在对枝晶列的研究中发现,与溶质浓度相关的扩散系数会增加枝晶列的一次间距,降低稳态枝晶的尖端位置.因此,在浓质分配系数严重偏离1的体系中,对现有模型的定量实验检验应考虑浓度相关的扩散对尖端过冷度的影响.     

相场法模拟Fe-C合金定向凝固的液相通道

在定向凝固的研究中,主要是通过改变推进速度或温度梯度以调节凝固组织,提升合金铸件性能.对于不同定向凝固条件下组织的形成及相关性质的研究成为了热点,本文主要研究在特定定向凝固条件下Fe-C合金枝晶尖端分裂后形成的液相通道及推进速度对于液相通道的影响.研究发现:在系统各向异性与材料各向异性的综合作用下,形成了定向凝固液相通道;且随推进速度的增大,液相通道内溶质浓度升高,长度增大,直径基本维持不变.通过液相通道相关尺度以及溶质富集的模拟结果分析其造成的晶内偏析的程度,同时指出可通过适当降低推进速度来减小液相通道溶质偏析的程度.     

微重力条件下Cu-Zr共晶合金的液固相变研究

采用落管方法实现了液态Cu-10 wt.%Zr亚共晶、Cu-12.27 wt.%Zr共晶和Cu-15 wt.%Zr过共晶合金在微重力无容器条件下的快速共晶与枝晶生长.Cu-12.27 wt.%Zr共晶合金的凝固组织随液滴直径减小由层片规则共晶向不规则共晶转变,且层片间距减小;Cu-10 wt.%Zr亚共晶合金的初生(Cu)相随液滴直径减小由粗大树枝晶向棒状晶转变,且所占体积分数增加,部分区域形成花状凝固组织,(Cu)相枝晶辐射向外生长;Cu-15 wt.%Zr过共晶合金初生相则为金属间化合物Cu_9Zr_2相,呈条状生长,随液滴直径减小冷却速率增大,凝固组织由宏观弯曲生长向球状晶胞转变.理论计算表明,三个合金液固相变枝晶与共晶的生长均由溶质扩散控制.测定Cu-10 wt.%Zr亚共晶合金初生(Cu)相显微硬度随液滴直径减小而增大,三个合金的共晶相随合金初始成分增大而增大.     

Fe-Pb合金凝固多相体系内偏析形成过程的三维数值模拟

基于Eulerian-Eulerian方法和流体体积技术,建立了三维多相流体动力学凝固模型,并将其与质量、动量、溶质和热焓守恒方程相耦合,对Fe-Pb合金侧向凝固过程进行了数值模拟.首先,分析了分布面积二次梯度((SPb))和浓度二次梯度((CPb))对偏析模式的影响,结果表明:液、气两相的流动相变使偏析模式表现为上端X形下端V形,X偏析由气相相变驱动力和多取向相变作用下的"散射"形成;ttc时,随(SPb)和(CPb)曲线降低,X偏析的下偏析角增大,上偏析角和V偏析角减小,Pb收得率增大,有利于获得含量稳定弥散的凝固组织.此外,还研究了液、气两相交互流动下通道偏析的形成机理,结果表明:通道偏析仅存在于流动-相变交互作用(ul·cl和ug·cg)为负值的区域,该区域的流动扰动抑制合金的局部凝固,促进偏析通道生长;流动-相变交互作用负值越小,偏析通道持续增长越稳定.模拟结果与实验结果相符合,验证了模型的准确性.     

晶格常数对左手材料的能量透射率及负折射率的影响

研究晶格常数对左手材料的能量透射率、负折射率以及品质因素(FOM)的影响.结果表明,随着晶格常数的减小,左手材料的低阶透射峰出现异常增大,其物理机制为阻抗匹配;左手材料的负折射的中心频率随着晶格常数的减小而蓝移,与理论结果基本一致;晶格常数对左手材料的FOM有重要的影响,当晶格常数减小到一定值时,可以实现完美阻抗匹配,从而得到较高的FOM.     

急冷条件下Cu-Pb偏晶合金的相分离研究

研究了Cu-Pb偏晶合金的急冷快速凝固和组织形成规律，并通过将金属熔体的热传导方程和Navier-Stokes方程相耦合, 理论分析了合金熔体的冷却速率、液固相变时间等物理参量与液相分离之间的相关性. 研究结果表明，在急冷快速凝固条件下，熔体的快速冷却对偏晶合金组织形成的影响要比熔体内部液相流动的影响更为显著. 快速凝固使液相分离受到抑制，Cu-Pb偏晶合金均可获得均匀的微观组织结构. 随着冷速的增大，晶粒尺寸明显减小，凝固组织显著细化，晶体形态由粗大枝晶向均匀细小的等轴晶过渡. 提高冷却速率，缩短液固相变时间是重力场中抑制液相分离、获得均匀偏晶组织结构的重要条件. 关键词： 偏晶合金 快速凝固 液相分离 微观结构     

二元合金等温凝固过程的相场模型

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-65%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     

Ion heating in the field-reversed configuration by rotating magnetic fields near the ion-cyclotron resonance

The trajectories of ions confined in a field-reversed configuration (FRC) equilibrium magnetic geometry and heated with a small-amplitude, odd-parity rotating magnetic field (RMF) have been studied with a Hamiltonian computer code. When the RMF frequency is in the ion-cyclotron range, explosive heating occurs. Higher-energy ions are found to have betatron-type orbits, preferentially localized near the FRC's midplane. These results are relevant to a compact magnetic-fusion-reactor design.     

The effect of toroidal field on the rotating magnetic field current drive in rotamak plasmas

A rotamak is one kind of compact spherically shaped magnetic-confinement device. In a rotamak the plasma current is driven by means of rotating magnetic field （RMF）. The driven current can reverse the original equilibrium field and generate a field-reversed-configuration. In a conventional rotamak, a toroidal field （TF） is not necessary for the RMF to drive plasma current, but it was found that the present of an additional TF can influence the RMF current drive. In this paper the effect of TF on the RMF current drive in a rotamak are investigated in some detail. The experimental results show that addition of TF increases the RMF driven current greatly and enhances the RMF penetration dramatically. Without TF, the RMF can only penetrate into plasma in the edge region. When a TF is added, the RMF can reach almost the whole plasma region. This is an optimal strength of toroidal magnetic field for getting maximum plasma current when Bv and radio frequency generator power are fixed. Besides driving current, the RMF generates high harmonic fields in rotamak plasma. The effect of TF on the harmonic field spectra are also reported.[第一段]     

Effect of rotating magnetic field on microstructure formation of directionally solidified Sn–1.6Cd peritectic alloy

In order to investigate the effect of rotating magnetic field on the microstructure formation of peritectic alloys, directional solidification experiments of Sn–1.6Cd peritectic alloy have been conducted under different rotating magnetic field conditions. The directional solidification microstructure of Sn–1.6Cd peritectic alloy changes from banded structure to axisymmetric isolated banded structure to axisymmetric oscillatory tree-like banded structure and to single primary phase structure when the magnetic Taylor number of forced-melt flow generated by a rotating magnetic field increases from 0 to 91 to 364 and to 1456. The second and third banded structures are observed in a peritectic alloy for the first time. The results indicate that it is possible to control solidification microstructure and prepare a single primary phase structure by using a rotating magnetic field during directional solidification of peritectic alloys. The experiments show that the effect of forced-melt flow on solute distribution transforms from solute buildup to homogenization with an increase in the magnetic Taylor number. The formation mechanisms of those structures are also discussed.     

Enhanced confinement and stability of a field-reversed configuration with rotating magnetic field current drive

A new experiment has been constructed to study the sustainment of a field-reversed configuration (FRC) with a rotating magnetic field (RMF). FRCs were formed with cold, unmagnetized ions and thus without a kinetic ion component that was believed to provide stability to internal tilt modes. No destructive instabilities were observed for the RMF FRC. Only peripheral radial penetration of the RMF was observed. The radially inward flow arising from axial screening currents at the FRC edge reduced convective and conductive losses to the measurement limit of the diagnostics.     

稀释晶场对spin-1和spin-1/2混合自旋纳米管中 Blume-Capel模型磁化强度的研究

利用有效场理论研究了纳米管上稀释晶场中混合自旋Blume-Capel模型格点的磁化强度,得到了系统格点的磁化强度与稀释晶场取值概率、外磁场和晶场的关系.结果表明:取值概率、外磁场、交换相互作用和晶场强度等诸多因素相互竞争,使系统表现出比恒定晶场作用的Blume-Capel模型更为丰富的磁学特性;外磁场能够增大系统格点的磁化强度,导致系统的二级相变消失;负晶场的作用使系统发生一级相变;稀释晶场会抑制系统的磁化强度,导致其基态饱和值小于1.     

Electromagnetic melt flow control during solidification of metallic alloys

In this minireview, we summarize experimental and numerical studies particularly concerned with applications of rotating magnetic fields (RMF) or travelling magnetic fields (TMF) to directional solidification of metal alloys. After introducing some fundamentals of electromagnetic stirring, we review the insights gained into flow-induced modifications of microstructure and the formation of freckles and macrosegregations. We further discuss recent strategies, using time-modulated RMF and TMF, which aim to overcome the deficiencies of conventional stirring, in particular flow-induced macrosegregation, by effectively controlling the flow field. On the microscale, we show that time-varying flows are able to alter the sidebranch characteristics vital to the potential of fragmentation.     

双模随机同向晶场对spin-1和spin-1/2混合自旋纳米管中Blume-Capel模型平均磁化强度的影响

利用有效场理论研究了纳米管上双模随机同向晶场中混合自旋Blume-Capel模型格点的平均磁化强度,得到了系统格点的平均磁化强度与双模随机晶场的取值概率、外磁场、晶场参数和晶场强度比值的关系。结果表明：取值概率、外磁场、交换相互作用、晶场强度比值和晶场强度等诸多因素相互竞争,使系统表现出比恒定晶场作用的Blume-Capel模型更为丰富的磁化现象;双模随机同向晶场会抑制系统的平均磁化强度,使其基态饱和值小于5/6;外磁场导致系统的二级相变消失;一定条件下系统发生一级相变;系统的平均磁化强度呈现部分缺失和负值现象。     

Onset and saturation of ion heating by odd-parity rotating magnetic fields in a field-reversed configuration

Heating of figure-8 orbit ions by odd-parity rotating magnetic fields (RMF(O)) applied to an elongated field-reversed configuration (FRC) is investigated. The largest energy gain occurs at resonances (s congruent to omega(R)/omega) of the RMF(O) frequency, omega(R), with the figure-8 orbital frequency, omega, and is proportional to s2 for s-even resonances and to s for s-odd resonances. The threshold for the transition from regular to stochastic orbits explains both the onset and saturation of heating. The FRC magnetic geometry lowers the threshold for heating below that in the tokamak by an order of magnitude.     

Linear and non-linear stability of melt flows in magnetic fields

This review considers the stability of melt motion in two simplified models of semiconductor crystal growth by either vertical gradient freeze (VGF) or Czochralski (Cz) processes under the influence of various magnetic fields. In VGF the crystal is grown at the bottom of the crucible, resulting in a stable thermal stratification of the melt. The presence of a stabilizing temperature gradient surprisingly decreases the stability of the flow driven by a rotating magnetic field (RMF). The instability of the travelling magnetic field (TMF)-driven flow, in contrast, is significantly delayed by thermal stratification in VGF. The TMF may, thus, be used in VGF to control the shape of the solidification interface or the radial dopant distribution without causing undesirable flow oscillations. The crystal is pulled out from the melt in the Cz process, producing an unstable temperature gradient below the crystal. The RMF is able to force the resulting unstable buoyant flow into a state of small-scale, high-frequency turbulence that may be regarded as stable for practical purposes. This effect is experimentally observed over a wide range of Grashof numbers, up to 10⁹, characteristic for a large Cz system.     

Model experiments for the Czochralski crystal growth technique

A lot of the physical and the numerical modeling of Czochralski crystal growth is done on the generic Rayleigh-Bénard system. To better approximate the conditions in a Czochralski puller, the influences of a rounded crucible bottom, deviations of the thermal boundary conditions from the generic case, crucible and/or crystal rotation, and the influence of magnetic fields are often studied separately. The present contribution reviews some of these topics while concentrating on studies of the flow and related temperature fluctuations in systems where a rotating magnetic field (RMF) was applied. The three-dimensional convective patterns and the resulting temperature fluctuations will be discussed both for the mere buoyant case and for the application of an RMF. It is shown that a system between a Rayleigh-Bénard and a more realistic configuration, which is still cylindrical but whose surface is partially covered by a crystal model, behaves much the same as a Rayleigh-Bénard system. An RMF can be used to damp the temperature fluctuations. Secondly, a more Czochralski-like system is examined. It turns out that the RMF does not provide the desired damping of the temperature fluctutions in the parameter range considered.