基于均匀形核的金属液滴凝固过程

研究了均匀形核的金属液滴凝固过程,应用渐近分析法求得金属液滴内晶核生长数学模型的渐近解,分析了表面张力、界面动力学参数、初始晶核尺寸和过冷度对晶核界面生长速度、晶核半径以及液滴凝固时间的影响.在一定的过冷条件下,表面张力和界面动力学参数显著减缓了晶核界面生长速度.在凝固开始的很短时间内晶核界面生长速度迅速上升,当速度上升到最大值后,随着晶核半径的增大,界面生长速度逐渐减慢,表面张力和界面动力学参数对晶核生长速度的作用也逐渐减小.过冷度越大,液滴凝固时间越短.经过在开始的瞬变凝固阶段之后,温度场从设定的初始分布迅速地调整为由过冷度、表面张力、界面动力学参数等所确定的特定温度分布.      

各向异性表面张力对柱晶界面形态的影响

本文研究了拟稳态时各向异性表面张力作用下柱晶界面的生长行为.在运用渐近分析方法求解柱晶界面形态数学表达式的基础上,分析在不同各向异性表面张力强度影响下柱晶界面形态的变化.各向异性表面张力对柱晶界面形态有显著影响.在柱晶生长开始阶段,柱晶部分界面首先向内收缩,当收缩达到某个临界值后,界面开始向外生长.柱晶界面沿着优先生长方向更快地生长.柱晶界面形态上形成突起变形,各向异性表面张力强度越大,柱晶界面形态的突起变形越明显.      

WC_p/2024铝合金复合材料界面反应的分析

采用真空热压法在不同温度下制备了体积分数为12%的WCp/2024Al复合材料,试验中所用WC原始粉末的平均粒径分别为2μm和8μm.利用XRD、SEM、EDS等方法对增强颗粒与基体金属之间的界面反应进行了研究.结果表明,界面反应的主要产物为WAl12,但是当制备温度较高时,界面反应产物中出现少量Al5W,并且WCp(2μm)/2024Al复合材料界面反应的起始温度低于WCp(8μm)/2024Al复合材料.硬度测试结果表明,界面反应发生后,复合材料的硬度提高,最高比例达50%.     

保护气氛下电渣重熔过程中界面脱硫传质动力学

电渣重熔过程中渣金反应温度高,接触面积大,具有良好的脱硫传质条件.然而,渣金界面处存在着传质边界层,边界层的厚度会影响界面脱硫传质速率,这对研究界面脱硫传质具有重要的意义.本文基于菲克第二定律、渗透理论和薄膜理论,建立保护气氛条件下电渣重熔过程渣-金界面脱硫传质方程,研究温度对渣-金间脱硫传质速率的影响规律.研究结果表明:随着温度的升高,渣-金界面处硫的浓度减小,界面脱硫传质速率增大;渣-金界面硫扩散的传质边界层厚度不随温度发生变化.     

电磁出钢装置中填装料对固-液界面位置的影响

 在电磁出钢系统中 ,为了提高感应加热效果以实现快速出钢 ,需将固-液界面控制在感应线圈的有效加热区内。设计了模拟钢包上水口处固-液界面测量装置,考察了上水口内在高中低碳钢的出钢温度分别为1550、1600、1620℃时,铁碳合金颗粒的成分、形状、大小对固-液界面的影响规律。结果表明：使用铸铁填装料比使用铸钢填装料时固-液界面的下移量要大,并且成分的影响随着钢液温度的升高有增大的趋势;规则球形颗粒比不规则的砂形颗粒对固-液界面位置影响要大,而且固-液界面的下移量随着填装料粒径的增大而增大;选用粒径为2.0mm的铸铁颗粒作为填充料,当出钢温度为1600和1620℃时,固-液界面位置能够进入有效加热区;当出钢温度为1550℃时,使用粒径为4.0mm的铸铁颗粒做填充料时,固-液界面位置也能够进入有效加热区。     

热压温度对TC4/Ta层状复合材料界面元素扩散行为及微观结构的影响

在不同温度下通过高温热压复合制备了TC4/Ta/TC4层状金属复合材料(LMCs),并讨论了界面元素扩散行为、微观结构随热压温度的变化关系.结果 表明,在热压和保温过程中两组元元素在界面处发生了明显的扩散行为,两组元实现了良好的冶金结合.高温热压促进了Al、V、Ti、Ta各元素在界面处的扩散,其扩散程度显著影响了界面附近的显微组织.每种元素的扩散深度与原子半径紧密相关,随着原子半径减小,扩散行为发生的更为强烈.元素扩散行为导致界面附近钛基体的相变温度降低,在低于TC4相变温度的950℃出现了网篮组织,随着与界面距离的变化呈现不同的微观组织形貌.     

老化对Sn—Pb／Cu界面的疲劳裂纹生长的影响

本文研究了Ｓｎ－Ｐｂ焊料／铜界面的疲劳裂纹生长行为的等温疲劳的影响。重点是界面微晶结构的作用。弯曲剥离界面裂纹样品是通过结合易熔Ｓｎ－Ｐｂ焊料和铜而制得，然后在４３Ｋ的条件下老化７天至３０天。用弯曲剥离样品测得了焊料／铜界面的疲劳裂纹生长动力学，该动力学是一个应变能释放率函数。     

铁液中Al2O3纳米颗粒溶解和熔化的热力学研究

与块体材料相比,纳米材料尺寸小、界面能大,导致其溶解和熔化过程的热力学不同于块体材料.从理论上推导了Al2O3纳米颗粒在铁液中的元素平衡溶度积以及熔化温度与颗粒尺寸之间的关系,结果表明,随颗粒尺寸减小,平衡溶度积逐渐增加,熔化温度逐渐下降,而且粒径越小,平衡溶度积和熔化温度的变化率越大.经计算,1873K铁液中Al2O...     

退火温度对铜/铝异步冷轧复合带组织与拉伸性能的影响

采用异步冷轧复合工艺制备铜/铝复合带,研究轧后退火温度和拉伸应变速率对复合界面组织和力学性能的影响.利用扫描电镜观察界面显微组织和拉伸断口形貌,并通过线扫描分析界面元素分布,进行不同应变速率的拉伸实验研究复合带的力学性能.结果表明,较高的退火温度可促进界面扩散层的生长,在350℃时形成三层结构;退火温度的提高使复合带抗拉强度减小而延伸率增大,应变速率的提高使抗拉强度和延伸率均增大,但在退火温度超过350℃后增幅减小;退火温度和应变速率越大,复合带拉伸断口的开裂程度越大,复合界面的过渡作用越弱.     

极端非平衡条件下过冷熔体枝晶生长模型

考虑界面非平衡、弯曲界面对驱动力的影响,并且摒弃液固相线线性假设,得到了界面移动热力学驱动力;从液固相线非线性条件下的动力学边界条件出发,对稳定性判据进行了修正;在溶质截留模型中,综合考虑弛豫效应和非线性液固相线的影响,对Aziz模型进行了修正;基于相图分析,得到过冷度分配.综合以上分析,在非线性液固相线基础上,得到了新的二元过冷熔体自由枝晶生长模型.将该模型应用到Ni-0.7%B合金中,结果表明无论从定性上还是定量上都与实验结果吻合得很好.     

The coarsening kinetics of two misfitting particles in an anisotropic crystal

We examine the influence of elastic stress on the Ostwald ripening kinetics of two elastically and diffusionally interacting, misfitting spherical particles in an anisotropic crystal. The coupled equations of elasticity and diffusion are solved analytically in series form in a bispherical coordinate system when the matrix supersaturation is small, local equilibrium obtains at the interface, particle and matrix possess the same cubic elastic constants, and the stress engenered by compositional inhomogeneity is negligible. Expressions are obtained for the matrix composition field, the local normal interfacial velocities of the particles, the isotropic particle growth rates, and the velocity of the particles' centers of mass. Inverse coarsening, or the growth of a smaller particle at the expense of a larger particle, is predicted for particle alignments along the elasticity soft 〈100〉 directions in nickel and for the 〈110〉 and 〈111〉 directions in molybdenum. Coarsening rates are often significantly enhanced for other particle orientations with respect to those of the stress-free case. The elastic stresses also change the functional dependence of the particle growth rate on particle size suggesting that the temporal exponents observed during classical ripening may not obtain in stressed systems. These predictions indicate that elastically-induced preferential coarsening strongly influences microstructural development in two-phase coherent alloys.     

Theory for growth of needle-shaped particles in multicomponent systems

A solution is presented for the growth of needle-shaped particles (paraboloids of revolution) in multicomponent systems that obey Henry’s law. Interface kinetics and capillarity effects are incorporated, and it is demonstrated that the maximum velocity hypothesis cannot be sustained if it is assumed that there is local equilibrium at the interface. The particle is unable to grow with equilibrium for small supersaturations when capillarity effects are prominent and for large supersaturations when the interface kinetics effect is large. A method to obtain the lengthening rate and tip radius is provided.     

Dynamics of solid/liquid interface shape evolution near an insoluble particle—An X-ray transmission microscopy investigation

In this article, for the first time, in situ and real-time experimental observations of changes in solid/liquid (s/l) interface shape during interactions with a particle or void are reported for metallic systems. Real-time interface shape evolution for both stationary and growing interfaces was observed by use of a state-of-the-art X-ray transmission microscope. Localized interfacial perturbations were studied as a function of the particle or void diameter, the distance between the s/l interface and the particle or void, and the thermal conductivity ratio between the matrix and the particle or void. In particular, the sensitivity of interfacial perturbation to the thermal conductivity ratio is critically analyzed. Analytical predictions of interface shape are compared to the real-time, in situ experimental data. A good agreement between the experimentally observed and predicted interface shapes was found for stationary interfaces. Based on the differences in experimental observations, between a moving and a stationary interface, an alternate hypothesis is suggested to explain the observed kinetics of particle engulfment by a growing interface.     

The effects of interface kinetics anisotropy on the growth direction of cellular microstructures

Directional solidification studies have been carried out in the pivalic acid-ethanol system in which significant anisotropies in interface kinetics and interfacial free energy are present. These anisotropic properties influence the microstructure formation and often lead to the formation of cells and dendrites which are tilted with respect to the heat flow direction. It is shown that dendrites always form in the preferred crystallographic direction, whereas the angle of tilt for cells is governed by the relative effects of heat flow and the anisotropic property of the crystal. This tilt angle for a given crystal orientation is found to increase as the velocity is increased. The angle of tilt reaches its largest value when the cell growth direction coincides with the preferred crystallographic direction,i.e., 〈001〈 direction for the cubic pivalic acid crystals. At this point, a transition from cellular to dendritic morphology occurs. The variation in the angle of tilt as a function of velocity is examined for the steady-state cellular structures. These results are then compared with the linear and the weakly nonlinear analyses of the planar interface stability to obtain the magnitude of the kinetic anisotropy effects. It is also shown that the cellular spacings as well as the amplitude of cells alter significantly with the angle of tilt under identical conditions of growth rate, temperature gradient, and composition. Formerly Research Associate with Ames Laboratory, Formerly Graduate Research Assistant with Ames Laboratory,.     

Growth kinetics of dispersed thoria in Ni and Ni-Cr alloys

The growth rates of dispersed thoria particles in nickel have been measured as a function of time at 1350°C in gases of two separate oxygen partialpressures. Samples were prepared for electron microscopic study after these treatments for particle size distribution measurements. The diffusion coefficient and solubility of thorium in nickel placed in contact with thoria were also determined under the same conditions. The results of the measurements then provide the necessary information for the elucidation of the kinetics of Ostwald ripening in the ThO₂-Ni system. A linear relationship was found between the cube of the mean radius of the dispersed particles and the time at temperature. Substitution of the experimental results into the Lifshitz-Wagner equation for particle growth under diffusion control yielded values of the ThO₂-Ni solid-solid interfacial energy, which are in good agreement with what is expected by comparison with the corresponding solid oxide-liquid metal interface. Further studies were made of the coarsening of thoria in 20 pct Cr-Ni alloys. It was found that the enhanced rate of particle growth could be accounted for by the increased value of the diffusion coefficient of thorium in the alloy over the value in pure nickel.     

The growth of gamma prime precipitates in aged Ni−Ti alloys

The kinetics of growth of the γ′ precipitate in a Ni-8.74 wt pct Ti alloy were studied by magnetic analysis and transmission electron microscopy. The variation of the titanium content of the nickel-rich matrix as a function of aging time was studied by measuring the ferromagnetic Curie temperature of alloys aged at 692°, 593°, and 525°C. The kinetics of this process accurately obeyed the predictions of the Lifshitz-Wagner theory of diffusion controlled coarsening after relatively short aging times at all aging temperatures. Dark-field transmission electron microscopy was used to measure the particle-size distributions and the average particle sizes of samples aged for various times at 692°C. The kinetics of particle growth also obeyed the time law predicted by the Lifshitz-Wagner theory within the limits of experimental error. Additional analysis of the data provided a value of approximately 21 erg per sq cm for the interfacial free energy of the γ′-matrix interface, and a value for the diffusion coefficient of titanium in nickel which is in very good agreement with an independently determined value. The distribution of γ′ particle sizes was found to be significantly broader than the theoretical distribution of the Lifshitz-Wagner theory. It is suggested that this is due to the relatively large lattice parameter mismatch between γ′ and the Ni−Ti matrix. The results and conclusions of this study are critically compared with those of other investigations. A. J. ARDELL, formerly Assistant Professor, California Institute of Technology     

Growth kinetics of solid-liquid Ga interfaces: Part I. Experimental

A technique based on the Seebeck effect was used to determine directly the solid-liquid (S/L) interface supercooling and toin situ monitor the interfacial conditions during growth of high-purity Ga single crystals from a supercooled melt. Using this nonintrusive technique, the growth kinetics of faceted (111) and (001) interfaces were studied as a function of the interface supercooling in the range of 0.2 to 4.6 K, corresponding to bulk supercoolings of about 0.2 to 53 K. In addition, the growth kinetics have been determined as a function of crystal perfection related to the emergence of dislocations at the S/L interface. The results show that at low super-coolings, the faceted interfaces grow with either of the lateral growth mechanisms: two-dimensional nucleation-assisted (2DNG) or screw dislocation-assisted (SDG), depending on the perfection of the interface. At increased interfacial supercoolings, however, both growth rates (2DNG and SDG) become a linear function of the supercooling. Application of the existing growth theories to the experimental results gives only qualitative agreement and fails to predict the observed deviation in the kinetics at high supercoolings. A theoretical treatment of the growth of faceted interfaces will be given in Part II of this series.¹ Formerly Research Assistant with the Department of Materials Science and Engineering, University of Florida. This paper is based on a presentation made in the symposium “The Role of Ledges in Phase Transformations” presented as part of the 1989 Fall Meeting of TMS-MSD, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Phase Transformations Committee of the Materials Science Division, ASM INTERNATIONAL.     

Effects of shear flow and anisotropic kinetics on the morphological stability of a binary alloy

The effect of a parallel shear flow and anisotropic interface kinetics on the onset of instability during the directional solidification of a binary alloy at constant velocity is calculated. The model for anisotropy is based on the motion of steps. A shear flow (linear Couette flow or asymptotic suction profile), parallel to the crystal-melt interface in the same direction as the step motion, decreases interface stability in that the critical solute concentration decreases. A shear flow counter to the step motion enhances stability for small shear rates; for larger shear rates, the neutral curve develops a bimodal structure, and the critical solute concentration slowly decreases with shear rate.     

The influence of buoyant forces and volume fraction of particles on the particle pushing/entrapment transition during directional solidification of Al/SiC and Al/graphite composites

Directional solidification experiments in a Bridgman-type furnace were used to study particle behavior at the liquid/solid interface in aluminum metal matrix composites. Graphite or siliconcarbide particles were first dispersed in aluminum-base alloysvia a mechanically stirred vortex. Then, 100-mm-diameter and 120-mm-long samples were cast in steel dies and used for directional solidification. The processing variables controlled were the direction and velocity of solidification and the temperature gradient at the interface. The material variables monitored were the interface energy, the liquid/particle density difference, the particle/liquid thermal conductivity ratio, and the volume fraction of particles. These properties were changed by selecting combinations of particles (graphite or silicon carbide) and alloys (Al-Cu, Al-Mg, Al-Ni). A model which considers process thermodynamics, process kinetics (including the role of buoyant forces), and thermophysical properties was developed. Based on solidification direction and velocity, and on materials properties, four types of behavior were predicted. Sessile drop experiments were also used to determine some of the interface energies required in calculation with the proposed model. Experimental results compared favorably with model predictions. BRU K. DHINDAW Visiting Scholar with the Solidification Laboratory at the time this work was performed.