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1.
基于Al-Bi过偏晶合金凝固通过难混溶区阶段产生的液-液相分解及分离的运动行为,采用两相体积平均法,对质量、动量、能量、组分及液滴密度守恒方程进行数值模拟,计算中考虑了形核、扩散长大、Stokes运动及Marangoni运动等多种复杂物理现象的耦合作用,分析了两相运动速度、第二相尺寸分布、第二相体积分数分布以及液滴密度分布对过偏晶合金凝固的显微组织演变及宏观偏析的影响。结果表明,过偏晶合金凝固过程中显微组织演化在不同阶段的主要影响因素不同:凝固初始阶段主要以形核和扩散长大为主;凝固中期和后期第二相迁移运动行为将逐渐占主导作用。凝固过程中,铸件顶角位置首先获得过冷度驱动形核,并以较快的形核速率达到最大形核密度。随着凝固过程不断地进行,第二相小液滴受到的Marangoni力约为Stokes粘滞阻力的两倍,开始由铸件顶角和边缘低温区向中心高温区聚集。凝固时间为1 s时,铸件顶角和边缘第二相小液滴的生长直径和第二相体积分数大于铸件中心位置,而凝固时间为5和7 s时,第二相小液滴直径随铸件中心距离变化的曲线斜率随凝固过程的进行而逐渐变缓,长大速率逐渐变慢。 相似文献
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《钢铁研究学报》2020,(9)
采用所开发的微观元胞自动机模型,耦合热力学模型计算获得的凝固路径,将浓度分布、温降与晶体生长相结合,模拟了亚共晶球墨铸铁(Fe-4wt.%C合金)离异共晶组织生长过程。结果表明,奥氏体与石墨生长相互促进,奥氏体晶粒快速包裹石墨,生长形态由枝晶状转为团簇状,石墨晶粒最终孤立分布于奥氏体基体中。等温下仅依靠溶质扩散的奥氏体与石墨生长速度略低于温降和溶质扩散共同作用下数值。二者生长速度随熔体初始过冷度增大均增快;随冷却速率增加均呈线性增长,增幅大致相同。本计算参数范围内,冷却速率对石墨生长速度的影响强度大于熔体初始过冷度。所预测的石墨平均半径与文献实验值相符。研究结果可为控制球墨铸铁凝固组织形貌及性能提供依据。 相似文献
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摘要:采用所开发的微观元胞自动机模型,耦合热力学模型计算获得的凝固路径,将浓度分布、温降与晶体生长相结合,模拟了亚共晶球墨铸铁(Fe-4wt%C合金)离异共晶组织生长过程。结果表明,奥氏体与石墨生长相互促进,奥氏体晶粒快速包裹石墨,生长形态由枝晶状转为团簇状,石墨晶粒最终孤立分布于奥氏体基体中。等温下仅依靠溶质扩散的奥氏体与石墨生长速度略低于温降和溶质扩散共同作用下数值。二者生长速度随熔体初始过冷度增大均增快;随冷却速率增加均呈线性增长,增幅大致相同。本计算参数范围内,冷却速率对石墨生长速度的影响强度大于熔体初始过冷度。所预测的石墨平均半径与文献实验值相符。研究结果可为控制球墨铸铁凝固组织形貌及性能提供依据。 相似文献
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《稀有金属》2017,(7)
从固/液界面的动力学方程出发,通过设计不同的计算模型,系统地研究了平面型界面和圆柱型界面的动力学行为,并计算了固/液界面的动力学因子(μ)。模拟平面型界面的运动和涨落行为,并分别提取了μ,两种模拟给出的结果非常接近,都有:μ100μ110μ111。模拟平面型界面回复现象,当界面偏离平衡位置时,会在界面张力的作用下发生回复,界面形貌在回复过程满足Gauss衰减规律。模拟圆柱型短晶核的界面运动行为,发现晶核在凝固或熔化过程中,晶核半径的变化速度逐渐加快,但存在一个函数,变化速度为匀速。研究了圆柱型长晶核的熔断行为,解释了界面涨落导致晶核熔断的内在机制,并发现熔断断口形貌近似服从Gauss分布。 相似文献
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994001A2工具钢在雾化沉积中孔隙的评估/Jean-PierreDelplanque等//TheInternationalJournalofPowderMetallurgy.l999,35(2)提供了一个数学模型,用于确定工具钢喷射成形的最佳工艺参数。用探索式模型预测孔隙度,综合考虑了液流时间和凝固时间对孔隙形成过程的影响。根据这些假设,将孔隙度表达为颗粒尺寸分布。喷射的平均固化百分数和凝固收缩率的函数。由液滴的尺寸分布和给定的尺寸凝固百分数来评估平均固化百分数。后者由液滴动力学和加热过程计算所决定。研究了工艺参数对孔隙度大小的影响(过热。沉积距离和熔流速度)。… 相似文献
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The solidification of undercooled spherical droplets with a discrete melting temperature is analyzed using both a Newtonian
and a non-Newtonian (Enthalpy) model. Relationships are established between atomization parameters, the growth kinetics, the
interface velocity and undercooling, and other important solidification variables. A new mathematical formulation and solution
methodology is developed for simulating the solidification process in an undercooled droplet from a single nucleation event
occurring at its surface. The computational mesh used in the enthalpy model is defined on a superimposed bispherical coordinate
system. Numerical solutions for the solidification of pure aluminum droplets based on the enthalpy model are developed, and
their results are compared to the trends predicted from the Newtonian model. The implications of single vs multiple nucleation
events are also discussed. In general, the results indicate that when substantial undercoolings are achieved in a droplet
prior to nucleation, the thermal history consists of two distinct solidification regimes. In the first, the interface velocities
are high, the droplet absorbs most of the latent heat released, and the external cooling usually plays a minor role. The second
regime is one of slower growth, and strongly depends on the heat extraction at the droplet surface. The extent of “rapid solidification”,
as determined from the fraction of material solidified at temperatures below a certain critical undercooling, is a function
of the nucleation temperature, the particle size, a kinetic parameter, and the heat translow as 10~4.
Formerly a Research Associate at the University of Illinois, 相似文献
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Morphologies of silicon crystals solidified on a chill plate 总被引:1,自引:0,他引:1
R. P. Liu D. M. Herlach M. Vandyoussefi A. L. Greer 《Metallurgical and Materials Transactions A》2004,35(3):1067-1073
Electromagnetically levitated liquid droplets of pure Si or a Si-Ge alloy were cooled to different temperatures and then dropped
onto a chill plate of Cu. Droplet oscillations mark the solid/liquid interface during solidification and permit the different
crystal morphologies of silicon to be observed on the quenched surface by scanning electron microscopy (SEM). A spherical
morphology found on the quenched surface represents the initial stage of crystal growth. Further growth leads to octahedral
crystals bounded by {111} faces near equilibrium and to other polyhedra or even faceted dendrites further from equilibrium.
The spherical growth can be observed only when the initial melt undercooling is moderately high. The critical size at which
spherical crystals start to develop dendritic growth is much bigger than that calculated from the Mullins and Sekerka model,
and is bigger than the Coriell and Parker model when kinetic undercooling is taken into account. 相似文献
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R. P. Liu D. M. Herlach M. Vandyoussefi A. L. Greer 《Metallurgical and Materials Transactions A》2004,35(13):1067-1073
Electromagnetically levitated liquid droplets of pure Si or a Si-Ge alloy were cooled to different temperatures and then dropped
onto a chill plate of Cu. Droplet oscillations mark the solid/liquid interface during solidification and permit the different
crystal morphologies of silicon to be observed on the quenched surface by scanning electron microscopy (SEM). A spherical
morphology found on the quenched surface represents the initial stage of crystal growth. Further growth leads to octahedral
crystals bounded by {111} faces near equilibrium and to other polyhedra or even faceted dendrites further from equilibrium.
The spherical growth can be observed only when the initial melt undercooling is moderately high. The critical size at which
spherical crystals start to develop dendritic growth is much higger than that calculated from the Mullins and Sekerka model,
and is bigger than the Coriell and Parker model when kinetic undercooling is taken into account. 相似文献
14.
The crystalline solidification during rapid substrate quenching in planar-flow casting was simulated by using a numerical model based on a rapid solidification algorithm and the infinite viscosity approximation. The calculation shows that the existence of a real melt puddle shape suppresses undercooling and recalescence on the melt surface as well as on the solidifying ribbon. The melt puddle length is mainly determined by the heat-transfer coefficient. With increasing heat transfer across the melt – substrate interface the melt puddle length decreases. If the formation rate of critical nuclei on the substrate surface is low compared to the present cooling rate a large undercooling may occur. The performed calculations reveal that an undercooling of up to 600 K does neither affect the temperature distribution on the surface of the melt nor the melt puddle length, perceptibly. Therefore, investigations on microstructural features of rapidly quenched metals might give detailed information on the amount of undercooling present at the beginning of solidification. 相似文献
15.
Masaki Sumida Yuh Shiohara Takateru Umeda 《Metallurgical and Materials Transactions B》2000,31(1):141-149
The interface stability of the Sm 123 superconductor was analyzed in accordance with the constitutional undercooling criterion.
As the single-crystal growth of the 123 phase is largely dependent on the growth-interface stability, a quantitative analysis
was very much required. From this analysis, it was clarified that the constitutional undercooling must exist in the liquid
when the 123 growth interface comes close to a 211 particle during the peritectic solidification. It was also predicted that
the larger 211 particle radius, smaller volume fraction of the 211 particles, larger growth rate, or smaller imposed temperature
gradient would cause easy occurrence of the constitutional undercooling ahead of the 123 growth interface. Taking into account
the nucleation at the L/211 interface just ahead of the 123 growth front due to the constitutional undercooling, the transition of 123 growth from
a planar-interface morphology to an equiaxed blocky morphology was investigated quantitatively and qualitatively. 相似文献
16.
Solidification of highly undercooled Fe-P alloys 总被引:1,自引:0,他引:1
M. Suzuki T. J. Piccone M. C. Flemings H. D. Brody 《Metallurgical and Materials Transactions A》1991,22(11):2761-2768
Rapid solidification behavior of highly undercooled iron-phosphorus alloys was investigated by using a high-speed optical
temperature measurement system. The experimental results on solidification rate as a function of bulk undercooling agree well
with a model which includes a treatment of nonequilibrium effects during the solidification process. The model is based on
an earlier analysis by Boettinger, Coriell, and Trivedi[81] (BCT) and employs temperature-dependent values of equilibrium liquidus slope, equilibrium solute distribution coefficient,
and solute interdiffusion coefficient. Values of the kinetic parameters,a
0
andV
0
, in the analysis which best fit the experimental results are 5 x 10-10 m and 600 m/s, respectively. Comparison of experimental results and theory suggests that a transition from local equilibrium
to nonequilibrium solidification takes place with increasing undercooling and that interface kinetic effects become predominant
at higher undercooling (or growth velocity). 相似文献
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Spangle formation in galvanized sheet steel coatings 总被引:2,自引:0,他引:2
Very large grains, termed “spangles,” are produced on galvanized sheet steel coatings when lead is added to the zinc bath.
The spangles have been attributed to melt undercooling prior to solidification. The present results indicate this is not the
case, undercooling being less than 1 °C. The spangle diameter is shown to be dependent on the alloy addition to the bath,
large spangles being obtained with Bi and Sb as well as Pb. The spangle size is related to the surface tension of the alloying
addition, the size decreasing as the melt vapor surface tension of the alloying element increases. It is proposed that spangles
form dendritically from a nucleus in the melt. Alloy additions with low interfacial energies and very limited solid solubility
are highly concentrated ahead of the dendrite tip. This decreases the tip radius and increases the dendrite velocity, producing
large grains. The basal plane orientation of the samples varies between 17 and 80 deg with respect to the steel sheet surface,
which is inconsistent with basal plane dendritic growth in Zn along (1010) directions. It is proposed that solute additions
to the melt and growth in a thin liquid layer can modify the dendrite growth direction, accounting for the spangle orientation.
On leave from Obafemi Awolowo University, lie Ife, Oyo State, Nigeria 相似文献
18.
Zengyun Jian Kosuke Nagashio Kazuhiko Kuribayashi 《Metallurgical and Materials Transactions A》2002,33(9):2947-2953
Using an electromagnetic levitation facility with a laser heating unit, silicon droplets were highly undercooled in the containerless
state. The crystal morphologies on the surface of the undercooled droplets during the solidification process and after solidification
were recorded live by using a high-speed camera and were observed by scanning electron microscopy. The growth behavior of
silicon was found to vary not only with the nucleation undercooling, but also with the time after nucleation. In the earlier
stage of solidification, the silicon grew in lateral, intermediary, and continuous modes at low, medium, and high undercoolings,
respectively. In the later stage of solidification, the growth of highly undercooled silicon can transform to the lateral
mode from the nonlateral one. The transition time of the sample with 320 K of undercooling was about 535 ms after recalescence,
which was much later than the time where recalescence was completed. 相似文献
19.
宽淬透性带和大尺寸TiN析出物严重危害20CrMnTi齿轮钢的产品质量,其控制的关键基础是掌握连铸过程中凝固组织演变行为机理。传统高温激光共聚焦扫描显微镜(HT-CSLM)原位观察研究通常将糊状区冷却速率设定为固定值,这不能有效反映连铸凝固过程中冷却速率的变化。为此,以国内某钢厂20CrMnTi 160 mm×160 mm小方坯为研究对象,首先通过二维切片凝固传热计算,确定内弧表面下方20、40、60 mm位置处糊状区的热历程、瞬态与平均冷却速率,进而设计HT-CSLM试验升温与降温方案。然后,开展这些位置处糊状区瞬态与平均冷却速率条件下HT-CSLM试验,研究揭示不同冷却条件下20CrMnTi的凝固过程、δ晶粒生长动力学和包晶相变机制。最后,通过电子探针分析(EPMA),考察冷却条件对凝固组织尺寸的影响规律。结果表明,由于凝固潜热的补偿,内弧皮下20、40、60 mm位置处初始凝固阶段冷却速率较小,凝固中后期逐渐增大,且越深入方坯内部越显著。这些位置处的平均冷却速率分别为102.81、44.63和34.93 ℃/min。δ晶粒率先从钢液中析出,其平均生长速率随着冷却速率的提升而增大。在瞬态冷却速率条件下,随着凝固的进行,瞬时生长速率呈增大的趋势,但是在平均冷却速率条件下瞬时生长速率则略微降低。这是因为在瞬态冷却速率条件下,糊状区冷却速率由慢至快,不断补偿了凝固潜热,同时初始形核数量少,生长空间大,溶质过冷度的影响相对较弱。当熔体温度降低至包晶相变临界温度时,δ晶粒快速转变为γ晶粒,即发生块状转变,导致固相率迅速增加,且伴随有部分γ晶粒快速聚合。整体上讲,包晶相变临界温度随着冷却速率的增大而降低,但是也受溶质初始含量的影响。此后,剩余液相向γ相转变,直至完全凝固。晶粒半径随着冷却速率的提升而减小,且在平均冷却速率条件下比瞬态冷却速率条件更小,这取决于初始凝固阶段的形核数量。 相似文献
20.
H. Li A. McLean J. W. Rutter I. D. Sommerville 《Metallurgical and Materials Transactions B》1988,19(3):383-395
Two series of experiments have been conducted to determine the influence of rare earth additions on the nucleation and crystallization
behavior of pure iron and 1045 steel. In the first series, additions of rare earth suicide or cerium dioxide powder to two-Kg
1045 steel ingots indicated that rare earth suicide can refine the as-cast structure of such ingots. However, if the holding
time after rare earth silicide addition is over two minutes, the grain refinement decreases. With cerium dioxide additions,
a relatively large columnar zone was obtained. In the second series, the effects of cerium metal or cerium dioxide powder
additions on the degree of undercooling obtainable in pure iron and 1045 steel were examined by the lévitation melting method.
Surface tension measurements of the levitated droplets were carried out at the same time to investigate the possible effects
of surface tension variations on the nucleation and crystallization behavior of the metals. The experimental data show that
rare earth inclusions can greatly reduce the degree of undercooling of iron and steel, and that a small amount of dissolved
cerium can further reduce the degree of undercooling of levitated droplets. The structure and reaction products obtained with
Fe-Ce levitated droplets were examined with both optical and scanning electron microscopy as well as X-ray diffraction analysis.
The experimental results clearly indicated that cerium solute redistribution during solidification is the dominant factor
in refining the as-cast structure. A nucleation and solidification model for the Fe-Ce levitated droplets has been developed,
which can successfully explain the experimental results.
on research leave from Beijing University of Iron and Steel Technology 相似文献