Rapid growth of ternary eutectic under high undercooling conditions

~~Rapid growth of ternary eutectic un der high undercooling conditions1.Offerman, S.E., Dijk, N.H., Sietsma, J.et al., Grain nucleation and growth during phase transformations, Science, 2002, 298: 1003-1005. 2.Warren, J.A., Langer, J.S., Prediction of dendritic spacings in a directional-solidification experiment, Phys.Rev.E, 1993, 47: 2702-2712. 3.Cao, C.D., Wang, N., Wei, B., Containerless rapid solidification of undercooled Cu-Co peritectic alloys, Science in China, Ser.A, …     

Dendritic and eutectic growth in Sb<Subscript>60</Subscript>Ag<Subscript>20</Subscript>Cu<Subscript>20</Subscript> ternary alloy

The rapid solidification of Sb60Ag20Cu20 ternary alloy was realized by high undercooling method, and the maximum undercooling is up to 142 K (0.18TL). Within the wide undercooling range of 40-142 K, the solidified microstructures are composed of (Sb), θand ε phases. High undercooling enlarges the solute solubility of (Sb) phase, which causes its crystal lattice to expand and its crystal lattice constants to increase. Primary (Sb) phase grows in two modes at small undercoolings non-faceted dendrite growth is the main growth form; whereas at large undercoolings faceted dendrite growth takes the dominant place. The remarkable difference of crystal structures between (Sb) and θphases leads to (θ Sb) pseudobinary eutectic hard to form, whereas strips of θform when the alloy melt reaches the (θ Sb) pseudobinary eutectic line. The cooperative growth of θand ε phases contributes to the formation of (ε θ) pseudobinary eutectic easily. In addition, the crystallization route has been determined via microstructural characteristic analysis and DSC experiment.     

Formation mechanism of the primary faceted phase and complex eutectic structure within an undercooled Ag–Cu–Ge alloy

The solidified microstructure of bulk undercooled Ag₄₀Cu₃₀Ge₃₀ alloy consists of three parts: primary (Ge) phase, the complex structure of (Ag + Ge) and (Ag + ε ₂) pseudobinary eutectics, and (Ag + Ge + ε ₂) ternary eutectic. In comparison, the pseudobinary eutectic no longer appears in an alloy droplet solidified in a drop tube. Once the undercooling exceeds 225 K and the cooling rate is greater than 2×10³ K s⁻¹, the microstructure of the solidified droplet is totally composed of anomalous ternary eutectic. In both cases, the primary (Ge) phase exhibits various faceted growth morphologies at different undercoolings, such as columnar block, long dendrite, equiaxed dendrite and rod-like crystal. Some refined side branches grow from the equiaxed (Ge) dendritic branches composed of {111} twins, which is ascribed to the rapid epitaxial growth of (Ag + Ge) pseudobinary eutectic from the (Ge) dendritic branches. Moreover, both the primary (Ge) phase and the (Ge) phase in the (Ag + Ge) pseudobinary eutectic are effective heterogeneous nuclei for the (Ag+ε ₂) pseudobinary eutectic. As undercooling increases, the (Ge) phase in the (Ag + Ge+ε ₂) ternary eutectic transforms from faceted to non-faceted phase, while the independent nucleation and growth of the (Ag) and ε ₂ phases in the ternary eutectic displaces their previous cooperative growth. These growth kinetics transitions result in the formation of anomalous ternary eutectic.     

Diffusionless crystal growth in a eutectic system during rapid solidification

Experiments on nonequilibrium rapid eutectic growth are surveyed. The applicability limits of the modern theoretical models describing rapid solidification of binary systems are assessed. A problem of rapid eutectic growth when the local equilibrium is violated in the solute diffusion field (in the bulk liquid and at the solid-liquid interface) is formulated. An analytical solution to the problem of rapid lamellar eutectic growth under local nonequilibrium conditions in the solute diffusion field is found. It is shown that the diffusion-limited growth of a eutectic pattern ceases as soon as a chemically homogeneous crystalline phase begins to grow when the critical point V=V _D is achieved (V is the solid-liquid interface velocity and V _D is the solute diffusion speed in the bulk liquid). At V ≥ V _D, eutectic decomposition is suppressed and the nascent homogeneous crystalline phase has the initial (nominal) chemical composition of the binary system.     

急冷条件下NiAl-Mo三元共晶合金的组织形成机制

采用单辊急冷技术实现了NiAl-Mo三元两相共晶合金的快速凝固, 同时与常规条件下的凝固组织进行了对比研究. 实验发现, 单辊急冷的合金条带与常规条件的凝固样品均由B2结构的NiAl金属间化合物和bcc结构的Mo固溶体两相组成, 两相均具有(110)晶面优先生长的趋势, 并呈现出(110)NiAl//(110)Mo取向关系. 常规条件下得到的微观结构主要由规则的两相共晶组织组成, 形成了类似菊花状的共晶胞. 而单辊急冷条件下形成的组织结构主要是由近辊面的柱状晶区和近自由面的等轴晶区组成的凝固组织. 理论计算发现, 合金熔体的单辊辊速由10 m/s增大至50 m/s后, 其冷却速率从1.01×10⁷ K/s逐渐增大到2.46×10⁷ K/s, 冷却速率明显高于常规铸造过程, 因而形成了差别很大的凝固组织. 随着辊速(冷却速率)的增加, 合金条带的厚度从54.4 μm减小至22 μm, 近辊面柱状晶区的厚度所占比例也逐渐增大, 晶粒发生了明显细化. 关键词： 快速凝固 三元共晶 共晶转变 冷却速率     

Rapid solidification of Al-Cu-Ag ternary alloy under the free fall condition

The rapid solidification of Al-30%Cu-18%Ag ternary alloy is investigated by using the free fall method. Its solidified microstructure is composed of θ(Al₂Cu), α(Al) and ξ(Ag₂Al) phases. The liquidus temperature and solidus temperature are determined as 778 and 827 K, respectively. The alloy melt undercooled amounts up to ΔT _Max=171 K (0.20T _L). Its microstructural evolution is investigated based on the theoretical analysis of undercooling behavior and nucleation mechanics. It is found that the undercooling increases with the decrease of the diameter of the alloy droplet. When ΔT<78 K, the primary θ(Al₂Cu) phase of the alloy grows into coarse dendrite. When 78 K⩽ΔT⩽171 K, its refined θ(Al₂Cu) phase grows alternatively with α(Al) phase. Once ΔT⩾171 K, its microstructure is characterized by the anomalous (θ+α+ξ) ternary eutectic. Supported by the National Natural Science Foundation of China (Grant Nos. 50121101 and 50395105)     

深过冷Ag-Cu-Ge三元共晶合金的相组成与凝固特征

在Ag_38.5Cu_33.4Ge_28.1三元共晶合金的深过冷实验中，获得的最大过冷度为175 K(0.22T_E). XRD分析表明，不同过冷条件下其共晶组织均由(Ag),(Ge)和η(Cu₃Ge)三相组成. 在小过冷条件下，三个共晶相协同生长，凝固组织粗大.随着过冷度的增大，共晶组织明显细化，(Ge)相与其他两相分离，以初生相方式生长，而(Ag)相与η相始终呈二相层片共晶方式共生生长. 当过冷度超过80 K时，初生相(Ge)由小过冷时的块状转变为具有小面相特征的枝晶方式生长. 部分小面相(Ge)枝晶出现规则的花状，花瓣数介于5—8之间，并且过冷度越大(Ge)相越容易分瓣. 花状(Ge)枝晶的晶体表面为{111}晶面簇，择优生长方向为〈100〉晶向族. 关键词： 三元共晶 晶体形核 深过冷 快速凝固     

Formation mechanism of primary phases and eutectic structures within undercooled Pb-Sb-Sn ternary alloys

The solidification characteristics of three types of Pb-Sb-Sn ternary alloys with different primary phases were studied under substantial undercooling conditions. The experimental results show that primary (Pb) and SbSn phases grow in the dendritic mode, whereas primary (Sb) phase exhibits faceted growth in the form of polygonal blocks and long strips. (Pb) solid solution phase displays strong affinity with SbSn intermetallic compound so that they produce various morphologies of pseudobinary eutectics, but it can only grow in the divorced eutectic mode together with (Sb) phase. Although (Sb) solid solution phase and SbSn intermetallic compound may grow cooperatively within ternary eutectic microstructures, they seldom form pseudobinary eutectics independently. The (Pb)+(Sb)+SbSn ternary eutectic structure usually shows lamellar morphology, but appears as anomalous eutectic when its volume fraction becomes small. EDS analyses reveal that all of the three primary (Pb), (Sb) and SbSn phases exhibit conspicuous solute trapping effect during rapid solidification, which results in the remarkable extension of solute solubility. Supported by the National Natural Science Foundation of China (Grant Nos. 50121101 and 50395105)     

Asymmetrical contribution of eutectic growth kinetics on the coupled growth behaviour in rapid eutectic solidification

The interface undercooling in classical eutectic growth theories consists of solute and curvature undercooling with different expression formats. However, a significant difference in interface kinetic coefficients arises for a disordered non-facetted solid solution and an ordered facetted intermetallic compound in that different growth kinetics govern the attachment kinetics at the solid–liquid interface, which correspond to a typical eutectic reaction with a solid solution and an intermetallic compound as its terminal eutectic phases. Following the pioneering work of Jackson and Hunt (Trans. Metall. Soc. AIME 236 1129 (1966) ), the kinetic undercooling is supplemented to interface undercooling and two eutectic phases are considered separately so as to diagnose the effect of the asymmetrical contribution of kinetic undercooling on the coupled eutectic growth behaviour. Further analysis indicates that it is the asymmetrical contribution of kinetic undercoolings of the facetted and non-facetted phases that enables the coupled eutectic composition shift to the facetted phase side so as to weaken the solute undercooling of the facetted phase and balance the kinetic contribution in the rapid solidification of coupled eutectics.     

Microstructure and physical properties of bismuth–lead–tin ternary eutectic alloy

Using different experimental techniques, microstructure, electrical resistivity, attenuation coefficient, and mechanical and thermal properties of the quenched Bi–Pb–Sn ternary eutectic alloy have been investigated. From the X-ray analysis, Bi₃Pb₇ and Bi–Sn meta-stable phases are detected, in addition to rhombohedral bismuth and Sn body-centered tetragonal phases. This study also compared the physical properties of the Bi–Sn–Pb ternary eutectic alloys with the base binary Bi–Sn and Bi–Pb eutectic alloys.     

采用元胞自动机法模拟二元规则共晶生长

基于元胞自动机方法,建立了二元规则共晶生长的数值模拟模型.该模型耦合宏观温度场,考虑了溶质扩散、成分过冷以及曲率过冷等重要因素,实现了二元规则共晶的稳态层片生长.以共晶模型合金为对象,研究了定向凝固条件下不同过冷度、初始层片间距、温度梯度及凝固速率对共晶两相生长形貌及层片间距的影响,再现了两相在溶质扩散及界面能的相互作用下通过形核、分岔、湮没及合并等机理实现共晶层片间距的调整.模拟得到的结果与Jackson-Hunt模型及前人实验结果规律一致.将模型扩展到三维系统,验证了二元规则共晶生长三维模拟的可行性. 关键词： 规则共晶 生长形貌 层片间距 CA方法     

三维多相场数值模拟共晶CBr4-C2Cl6合金在不同抽拉速度下的形态选择

利用KKSO多相场模型对定向凝固共晶CBr₄-C₂Cl₆合金的三维恒速及变速生长过程进行了研究,再现了不同抽拉速度下共晶形态演化及选择过程,建立了形态选择图,研究了变速过程的界面平均生长速度及界面平均过冷度的变化.结果表明,变速前后的形态选择与恒速下的形态选择一致;变速过程的形态演变、界面平均生长速度和界面平均过冷度的变化均产生滞后效应;界面平均生长速度和界面平均过冷度之间的关系与理论结果符合较好. 关键词： 多相场模型 共晶生长 抽拉速度     

弱熔体对流对定向凝固中棒状共晶生长的影响

利用渐近方法求出在弱对流熔体中定向凝固棒状共晶生长的浓度场的渐近解,研究了弱熔体对流对定向凝固中棒状共晶生长的影响.结果表明,弱熔体对流对定向凝固中棒状共晶生长有显著的作用;平均界面过冷度不仅与棒状共晶的棒间距、生长速度有关,还与流动强度有关;当生长速度一定时,随着流动强度增大,棒状共晶的平均界面过冷度减小.利用最小过冷原则,获得棒间距与生长速度和流动强度的关系.结果表明,当生长速度比较小时,随着流动强度增大,棒状共晶的棒间距增大;当生长速度比较大时,随着流动强度增大,棒状共晶的棒间距变化减弱;棒状共晶的生长速度越小,流动对棒状共晶生长的影响越大.利用本文的解析结果计算在对流条件下Al-Cu共晶的棒间距,结果显示随着转速增大或径向距离增大,共晶的间距增大,这与Junze等的实验结果相符合.     

Rapid solidification and dendrite growth of ternary Fe-Sn-Ge and Cu-Pb-Ge monotectic alloys

The phase separation and dendrite growth characteristics of ternary Fe-43.9%Sn- 10%Ge and Cu-35.5%Pb-5%Ge monotectic alloys were studied systematically by the glass fluxing method under substantial undercooling conditions. The maximum undercoolings obtained in this work are 245 and 257 K, respectively, for these two alloys. All of the solidified samples exhibit serious macrosegregation, indicating that the homogenous alloy melt is separated into two liquid phases prior to rapid solidification. The solidification structures consist of four phases including α-Fe, (Sn), FeSn and FeSn2 in Fe-43.9%Sn-10%Ge ternary alloy, whereas only (Cu) and (Pb) solid solution phases in Cu-35.5%Pb-5%Ge alloy under different undercoolings. In the process of rapid monotectic solidification, α-Fe and (Cu) phases grow in a dendritic mode, and the transition "dendrite→monotectic cell" happens when alloy undercoolings become sufficiently large. The dendrite growth velocities of α-Fe and (Cu) phases are found to increase with undercooling according to an exponential relation.     

Directional growth of Al−Mg<Subscript>2</Subscript>Si eutectic alloy in the SZ2 unmanned spaceship

This paper reports the investigation results on directional growth of Al−Mg₂Si eutectic alloy in the SZ2 unmanned spaceship. The analysis shows that microgravity has influence on the directional growth of Al−Mg₂Si eutectic alloy. The eutectic colony in space-grown sample is larger than that in ground-grown sample, and the directional effect is not so good as that of terrestrial sample. But there is no obvious difference in the lamellar thickness between these samples, and lamellar interspacing in space sample is smaller than that grown on the ground. A simple discussion is done on the gravity effect on the directional growth of Al−Mg₂Si eutectic alloy.     

Behavior of sound velocities in the compounds La1−x SrxMnO3 near magnetic and structural phase transitions

The temperature dependences of the transverse V _t and longitudinal V _l sound velocities in single crystals of the perovskites La_1−x Sr_xMnO₃ (x=0.1, 0.175, 0.2, 0.25) in the temperature interval T=70–350 K are investigated by the resonance method. Anomalies — small minima and kinks in the temperature dependences V _l (T) and V _t (T) — are observed at the Curie points. A strong jumplike increase (by up to 30%) in both the longitudinal and transverse sound velocities, which attests to sizable hardening of the acoustic branches of the phonon spectrum, is observed near the temperatures of the structural transitions between the rhombohedral and orthorhombic phases (for x=0.175, 0.2, 0.25) and at the phase transition to the polaron-ordered state (for x=0.1). Pis’ma Zh. éksp. Teor. Fiz. 68, No. 2, 141–146 (25 July 1998)     

Segregation during crystal growth from melt and absorption cross section determination by optical absorption method

Segregation during crystal growth from melt under two conditions is studied by using crystal mass, which can be measured easily, as an independent variable, and a method to determine the effective segregation coefficient and absorption cross section of optical dopant is given. When the segregated solute disperses into the whole or just a part of melt homogenously, the concentration C _s in solid interface will change by different formulas. If the crystal growth interface is conical and segregated solute disperses into melt in total or part, the solute concentration at r = 2/3R, where r is the distance from the growth cross section center and R the crystal radius, is independent on the shape of the crystal growth interface, and its variation at r = 2/3R can be regarded as the result from crystal growth in flat interface. With C _s variation formula in solid and absorption cross section σ for optical dopant, the absorption coefficients along the crystal growth direction can be calculated, and the corresponding experimental value can be obtained through the crystal optical absorption spectra. By minimizing the half sum, whose independent variables are k, ΔW or σ, of the difference square between the calculated and experimental absorption coefficients from one or more absorption peaks along the crystal growth direction, k and σ, or k and ΔW, can be determined at the same time through the Levenberg-Marquardt iteration method. Finally, the effective segregation coefficient k, ΔW and absorption cross sections of Nd:GGG were determined, the results fitted by two formula gave more closed effective segregation coefficient, and the value ΔW also indicates that the segregated dopant had nearly dispersed into the whole melt. Experimental results show that the method to determine effective segregation coefficient k, ΔW and absorption cross sections σ is convenient and reliable, and the two segregation formulas can describe the segregation during the crystal growth from melt relatively commendably. Supported by the National Natural Science Foundation of China (Grant No. 50772112)     

Adsorption effect in non-reaction wetting: In–Ti on CaF2

The experiments show that the alloying liquid In with only (0.1–0.5) at% Ti dramatically reduces the equilibrium contact angle Θ _∞ formed by In on the surface of CaF₂. The aim of this paper is to clarify whether this practically important and conceptually challenging effect can be explained solely by Ti adsorption at the F-terminated solid–liquid interface without resorting to any other Ti-induced effect. The combination of ab initio calculations and regular solution approximation was proposed for finding the binding energy, ΔE _Ti of Ti adatom with the interface “CaF₂/liquid solutions In–Ti.” With thus obtained ΔE _Ti=1.16 eV, we calculated from the Shishkovsky isotherm the reduction in the solid–liquid interface energy, Δγ _SL induced by Ti adsorption from liquid In with various Ti concentration, C. It was found that Δγ _SL(C) dependence demonstrated close inverse correspondence with Θ _∞(C) and that the theory fitted very well all available experimental data on the concentration and temperature dependence of Δγ _SL. It was concluded that the Ti adsorption effect is large enough to account for the observed wetting improvement. The proposed multiscale modeling approach to the role of adsorption in wetting can be applied also to other nonreactive systems “liquid metal–ceramics” where the substrate determines the surface density of the adsorption sites for the active element.     

三种变速条件下共晶生长的多相场法模拟

利用多相场模型模拟了共晶合金CBr₄-C₂Cl₆定向凝固变速生长过程，研究了阶跃变速、线性变速以及震荡变速三种变速条件下共晶片层间距的调整以及形态的变化.结果表明：在变速生长过程中，界面平均生长速率与平均过冷度随抽拉速率的变化均产生滞后效应；阶跃增速时，片层间距的调整通过突变分岔形式进行，而阶跃减速时，通过片层的逐步湮没与合并以及自身相的长大方式进行，两个过程表现出强烈的非对称性；线性增速过程，片层间距的调整通过逐步分岔进行，而线性减速过程 关键词： 多相场 共晶 变速生长 片层间距     

Diffusionless crystal growth in rapidly solidifying eutectic systems

Using a local nonequilibrium model of solidification, experiments on rapid eutectic growth are analyzed. An analytical solution of a problem of rapid lamellar eutectic growth under local nonequilibrium conditions in the solute diffusion field is found. It is shown that solute diffusion-limited growth of a eutectic pattern is completely finished, and diffusionless growth of the chemically homogeneous crystalline phase begins to proceed at a critical point V = V(D), where V is the solid-liquid interface velocity and V(D) is the solute diffusion speed in bulk liquid. A suppression of eutectic decomposition occurs in the range V > or = V(D) that results in a growth of homogeneous crystal phase with the initial (nominal) chemical composition of the binary system.