深过冷三元Ni-Cu-Co合金的快速枝晶生长

研究了深过冷条件下三元Ni₈₀Cu₁₀Co₁₀合金的快速枝晶生长, 采用电磁悬浮无容器处理方法获得了335 K(0.2T_L)的最大过冷度. X射线衍射分析与差示扫描量热分析均表明,凝固组织为α-Ni单相固溶体. 随过冷度增大, 凝固组织显著细化, 并且当过冷度达110 K时，凝固组织的形态由粗大形枝晶转变为等轴晶. 深过冷条件下溶质截留效应增强, 使得微观偏析程度减小. 对不同过冷度下合金枝晶的生长速度进 关键词： 深过冷 枝晶生长 快速凝固 溶质截留     

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.     

液态三元Fe-Sn-Si/Ge偏晶合金相分离过程的实验和模拟研究

本文采用自由落体实验技术和格子玻尔兹曼计算方法研究了低重力条件下液态Fe-Sn-Si/Ge合金的相分离过程. 实验发现, 二种合金液滴在自由下落过程中均发生显著的液相分离, 形成了壳核和弥散组织. 当Fe-Sn-Si合金中的Si元素被等量的Ge元素替换后, 壳核组织中富Fe区和富Sn区的分布次序会发生反转. 计算表明, 在液相分离过程中冷却速率、Marangoni对流和表面偏析对壳核构型的选择和弥散组织的形成起决定性作用.     

电磁悬浮条件下液态Fe_(50)Cu_(50)合金的对流和凝固规律研究

基于轴对称电磁悬浮模型,理论计算了二元Fe_(50)Cu_(50)合金熔体内部的磁感应强度和感应电流,分析了其时均洛伦兹力分布特征,进一步耦合Navier-Stokes方程组计算求解了合金熔体内部流场分布规律.计算结果表明,电磁悬浮状态下合金内部流场呈现环形管状分布,并且电流强度、电流频率或合金过冷度的增加,均会导致熔体内部流动速率峰值减小,平均流动速率增大,并使流动速率大于100 mm·s~(-1)区域显著增大.通过与静态凝固实验对比发现,电磁悬浮条件下熔体中强制对流使得合金内部富Fe和富Cu区的相界面呈波浪状起伏形貌,并且富Cu相颗粒在熔体上部分出现的概率增加.     

Simulated evolution process of core-shell microstructures

The evolution process of core-shell microstructures formed in monotectic alloys under the space environment condition was investigated by the numerical simulation method. In order to account for the effect of surface segregation on phase separation, Model H was modified by introducing a surface free energy term into the total free energy of alloy droplet. Three Fe-Cu alloys were taken as simulated examples, which usually exhibit metastable phase separation in undercooled and microgravity states. It was revealed by the dynamic simulation process that the formation of core-shell microstructures depends mainly on surface segregation and Marangoni convection. The phase separation of Fe65Cu35 alloy starts from a dispersed structure and gradually evolves into a triple-layer core-shell microstructure. Similarly, Fe50Cu50 alloy experiences a structural evolution process of "bicontinuous phase → quadruple-layer core-shell → triple-layer core-shell", while the microstructures of Fe35Cu65 alloy transfer from the dispersed structure into the final double-layer core-shell morphology. The Cu-rich phase always forms the outer layer because of surface segregation, whereas the internal microstructural evolution is controlled mainly by the Marangoni convection resulting from the temperature gradient.     

深过冷Cu60Sn30Pb10偏晶合金的快速凝固

实现了大体积Cu₆₀Sn₃₀Pb₁₀偏晶合金的深过冷与快速凝固. 实验获得的最大过冷度为173 K(0.17T_L). 凝固组织发生了明显的宏观偏析,XRD分析表明,试样上部是由固溶体(Sn),(Pb)相和金属间化合物ε(Cu₃Sn)相组成的三相区,下部为富(Pb)相区. 在小过冷条件下,三相区中ε(Cu₃Sn)相的凝固组织为粗大的枝晶,随着过冷度的增大,ε(Cu₃Sn)相细化成层片状组织,且层片间距随过冷度的增大而减小,而(Sn),(Pb)两相始终以离异共晶的方式存在. 富(Pb)相区中分布有少量的ε(Cu₃Sn)枝晶,枝晶长度随过冷度的增大而增大,且在大过冷条件下发生碎断. (Sn)相在ε(Cu₃Sn)相表面形核、长大,其形态类似于包晶凝固组织. 关键词： 深过冷 快速凝固 偏晶合金 层片组织     

Phase separation and rapid solidification of liquid Cu<Subscript>60</Subscript>Fe<Subscript>30</Subscript>Co<Subscript>10</Subscript> ternary peritectic alloy

The metastable liquid phase separation and rapid solidification of Cu₆₀Fe₃₀Co₁₀ ternary peritectic alloy were investigated by using the drop tube technique and the differential scanning calorimetry method. It was found that the critical temperature of metastable liquid phase separation in this alloy is 1623.5 K, and the two separated liquid phases solidify as Cu(Fe,Co) and Fe(Cu,Co) solid solutions, respectively. The undercooling and cooling rate of droplets processed in the drop tube increase with the decrease of their diameters. During the drop tube processing, the structural morphologies of undercooled droplets are strongly dependent on the cooling rate. With the increase of the cooling rate, Fe(Cu,Co) spheres are refined greatly and become uniformly dispersed in the Cu-rich matrix. The calculations of Marangoni migration velocity (V _M) and Stokes motion velocity (V _S) of Fe(Cu,Co) droplets indicated that Marangoni migration contributes more to the coarsening and congregation of the minor phase during free fall. At the same undercooling, the V _M/V _S ratio increases drastically as Fe(Cu,Co) droplet size decreases. On the other hand, a larger undercooling tends to increase the V _M/V _S value for Fe(Cu,Co) droplets with the same size. Supported by the National Natural Science Foundation of China (Grant Nos. 50121101 and 50395105) and the Scientific and Technological Creative Foundation of Youth in Northwestern Polytechnical University of China (Grant No. W016223)     

落管中Al4Mn合金的形核研究

用1.2米长的落管对Al₄Mn合金进行了形核与过冷的研究.在落管底部收集到的样品中得到了正交Al₆Mn,β-Mn,两个十次准晶相关相、一个二十面体五次准晶相关相和十次准晶畴.发现这些相的含量与样品的尺寸大小有关,根据传统的形核理论对相含量与样品尺寸大小之间的关系作了定性的分析和讨论. 关键词：     

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     

Solidification of tin on quasicrystalline surfaces

A two-phase alloy of β-Sn and Al₆₃Cu₂₅Fe₁₂ quasicrystal produced by melt-spinning was annealed and aged to form various microstructures of tin in a quasicrystalline (QC) or microcrystalline (MC) matrix. The morphology and structure of the interfaces was studied by scanning and transmission electron microscopy and was related to melting and solidification behavior of tin studied by differential scanning calorimetry. In a MC matrix the tin phase occurred as nanoparticles and solidified with an undercooling of about 35°C. In a QC matrix, tin formed intergranular layers on faceted matrix grains. Tin showed multiple solidification peaks in undercooling ranging from 8°C to 43°C, indicating several distinct nucleation sites which compete with each other and are selected kinetically. The interfacial energy (depending on the structural state of the matrix) had a more dominating effect on the solidification of tin than the size, shape and the distribution of the tin particles. It was also concluded that solidification of tin is easier on quasicrystalline surfaces than on aluminum.     

Dendrite growth characteristics within liquid Fe-Sb alloy

Bulk samples and small droplets of liquid Fe-10%Sb alloys are undercooled up to 429 K (0.24TL) and 568 K (0.32TL), respectively, with glass fluxing and free fall techniques. The high undercooling does not change the phase constitution, and only the αFe solid solution is found in the rapidly solidified alloy. The experimental results show that when the undercooling is below 296 K, the growth velocity of αFe dendrite rises exponentially with the increase of undercooling and reaches a maximum value 1.38 m/s. S...     

Dendritic growth and solute trapping in rapidly solidified Cu-based alloys

Rapid solidification of binary Cu-22%Sn peritectic alloys and Cu-5%Sn-5%Ni-5%Ag quaternary alloys was accomplished by glass fluxing, drop tube and melt spinning methods. The undercooled, by glass fluxing method, Cu-22%Sn peritectic alloy was composed of α(Cu) and δ(Cu41Sn11) phases. If rapidly solidified in a drop tube, the alloy phase constitution changed from α(Cu) and δ(Cu41Sn11) phases into a single supersaturated (Cu) phase with the reducing of droplet diameter, and the maximum solubility of Sn in (Cu)...     

液态Ti-Al合金的深过冷与快速枝晶生长

采用电磁悬浮和自由落体两种试验技术研究了液态Ti-25 wt.%Al合金的亚稳过冷能力、晶体形核机制和枝晶生长过程. 试验发现, 即使电磁悬浮无容器状态下仍难以消除润湿角θ ≥60°的异质晶核, 合金熔体过冷度可达210 K (0.11T_L). β-Ti相形核的热力学驱动力随过冷度近似以线性方式增大, 其枝晶生长速度高达11.2 m/s, 从而在慢速冷却条件下实现了快速凝固. 理论计算表明, 随着过冷度的逐步增大, β相枝晶生长从溶质扩散控制转变为热扩散控制. 当过冷度超过100 K时, 非平衡溶质截留效应可使合金熔体发生无偏析凝固. 然而, 单靠深过冷状态不足以抑制β相的后续固态相变. 对于落管中快速凝固的直径77-1048 μm合金液滴, 其冷却速率最高达1.05×10⁵ K/s, 深过冷与快速冷却的耦合作用能更有效地调控凝固组织形成过程.     

Undercooling and metastable phase formation in a Bi95Sb5 melt

High undercoolings have been obtained in bulk Bi₉₅Sb₅ alloy melts by the cyclic superheating and cooling technology. The highest undercooling that was achieved in this paper is 121 K. The influence of various processing factors on the undercooling behavior is examined. Undercooling of 121 K leads to the formation of a metastable solid phase with the tetragonal crystal structure. The phase selection and the metastable phase formation have been discussed based on the classical nucleation theory. A criterion that contains the relative melting temperature, the relative molar volume of the solid, the relative structure-dependent factor, and the undercooling has been developed to interpret the formation of the metastable tetragonal phase. Received: 12 January 2000 / Accepted: 28 March 2000 / Published online: 13 July 2000     

Phase formation in undercooled NdFeB alloy droplets

Small droplets of Nd_xFe_100−1.5xB_0.5x alloys (x=11.8–15) were undercooled and solidified using the drop tube technique for the purpose of studying metastable phase formation in this technically important alloy system. It was found that primary γ-Fe phase has been suppressed in most of the droplets due to significant undercooling levels achieved prior to solidification. Consequently, the primary crystallization either of metastable Nd₂Fe₁₇B_y (y≈1) phase or of Nd₂Fe₁₄B phase has been favored. The former occurs predominantly to low Nd alloys (x=11.8–13), while the latter prevails in Nd-rich alloys (x=14–15). By means of thermomagnetic analysis and powder X-ray diffraction analysis, it was determined that the metastable Nd₂Fe₁₇B_y phase has a Curie temperature of about 100°C and a hexagonal structure with lattice parameters as a=0.496 and c=0.416 nm.     

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, …     

A study of the microstructure,thermal properties and wetting kinetics of Sn–3Ag–<Emphasis Type="Italic">x</Emphasis>Zn lead-free solders

Microstructure, thermal properties and wetting kinetics of Sn–3Ag–xZn solders (x = 0.4, 0.6, 0.8, 1, 2 and 4 wt%) were systematically investigated. The results indicate that a small amount of Zn (Zn wt% ≤ 1 wt%) has a rather moderate effect on the microstructure morphology of the Sn–3Ag–xZn solders. The microstructures are composed of a β-Sn phase and the mixture of Ag₃Sn and ζ-AgZn particles. However, the β-Sn phase reduces its volume fraction in the entire microstructure and the intermetallic compounds population increases with the increasing of Zn content. The microstructure is dramatically changed with a further increase in the Zn content. The γ-AgZn phase is formed in a Sn–3Ag–2Zn solder. The ε-AgZn phase is formed in a Sn–3Ag–4Zn solder. The melting temperature and the undercooling of the Sn–3Ag–xZn solder alloys decrease with the increase in Zn content, reach to a minimum value when the content of Zn is 1 wt%, and then increase with further increase in Zn content. The Sn–3Ag–1Zn demonstrates the minimum value of 228.13 °C in the melting temperature and 13.87 °C in undercooling. The wetting kinetics of the main spreading stage features the power law of R ⁿ  ~ t (n = 1), which is controlled by chemical reactions at the triple line.     

Solute redistribution profiles during rapid solidification of undercooled ternary Co-Cu-Pb alloy

The solute redistribution and phase separation of liquid ternary Co-35%Cu-32.5%Pb immiscible alloy have been investigated using glass fluxing method. A bulk undercooling of 125 K was achieved and the macrosegregation pattern was characterized by a top Co-rich zone and a bottom Cu-rich zone. The average solute contents of the two separated zones decreased with the increase of undercooling, except for the solute Pb in Cu-rich zone. With the enhancement of undercooling, a morphological transition from dendrites into equaxied grains occurred to the primary α(Co) phase in Co-rich zone. The solute redistribution of Cu in primary α(Co) phase was found to depend upon both the undercooling and composition of Co-rich zone. Stokes migration is shown to be the main dynamic mechanism of droplet movement during liquid phase separation.     

Microstructural evolution during containerless rapid solidification of Co-Si alloys

The Co-12%Si hypoeutectic, Co-12.52%Si eutectic and Co-13%Si hypereutectic alloys are rapidly solidified in a containerless environment in a drop tube. Undercoolings up to 207K (0.14T_E) are obtained, which play a dominant role in dendritic and eutectic growth. The coupled zone around Co-12.52%Si eutectic alloy has been calculated, which covers a composition range from 11.6 to 12.7%Si. A microstructural transition from lamellar eutectic to divorced eutectic occurs to Co-12.52%Si eutectic droplets with increasing undercooling. The lamellar eutectic structure of the Co-12.52%Si alloy consists of εCo and Co_3Si phases at small undercooling. The Co_3Si phase cannot decompose completely into εCo and αCo_2Si phases. As undercooling becomes larger, the Co_3Si phase grows very rapidly from the highly undercooled alloy melt to form a divorced eutectic. The structural morphology of the Co-12%Si alloy droplets transforms from εCo primary phase plus lamellar eutectic to anomalous eutectic, whereas the microstructure of Co-13%Si alloy droplets experiences a `dendritic to equiaxed' structural transition. No matter how large the undercooling is, the εCo solid solution is the primary nucleation phase. In the highly undercooled alloy melts, the growth of εCo and Co_3Si phases is controlled by solutal diffusion.     

Metastable phase separation and rapid solidification of undercooled Co_(40)Fe_(40)Cu_(20) alloy

The metastable liquid phase separation and rapid solidification behaviors of Co_(40) Fe_(40) Cu_(20) alloy were investigated by using differential thermal analysis(DTA) in combination with glass fluxing and electromagnetic levitation(EML) techniques. The critical liquid phase separation undercooling for this alloy was determined by DTA to be 174 K. Macrosegregation morphologies are formed in the bulk samples processed by both DTA and EML. It is revealed that undercooling level, cooling rate, convection, and surface tension difference between the two separated phases play a dominant role in the coalescence and segregation of the separated phases. The growth velocity of the(Fe,Co) dendrite has been measured as a function of undercooling up to 275 K. The temperature rise resulting from recalescence increases linearly with the increase of undercooling because of the enhancement of recalescence. The slope change of the recalescence temperature rise versus undercooling at the critical undercooling also implies the occurrence of liquid demixing.