DENUCLEATION AND HIGH UNDERCOOLING OF Ni-32.5%Sn EUTECTIC ALLOY

Undercoolings up to 397 K(0.283 T_E)have been obtained for Ni-32.5% Sn eutectic alloy melted by superheating-cooling cycles and denucleating with inorganic glasses.The predomi- nant dissipation of heat for highly undercooled alloy melt is through radiation.An approxi- mate method is consequently derived to calculate its mean specific heat from measured cooling curves.With the aid of high speed cinematography,it is revealed that the surface or interface heterogeneous nucleation takes place in preference to homogeneous nucleation even though the undercooling has exceeded 0.2 T_E.

Rapid growth of ternary eutectic un der high undercooling conditions

Rapid solidification of bulk Ag42.4Cu21.6Sb36 ternary eutectic alloy is accomplished by glass fluxing method,during which the maximum undercooling attains 114 K (0.16 TE). Under high undercooling conditions,the ternary eutectic consists ofε (Ag3Sb),(Sb)and θ(Cu2Sb)phases,instead of (Ag),(Sb)and θphases as predicted by the phase diagram.In the sample of small undercooling,the alloy microstructure is characterized by the mixture of primary θ(Cu2Sb),(ε+θ) and (ε+Sb) pseudobinary eutectics,and regular (ε+θ+Sb) ternary eutectic.With the increase of undercooling, θ (Cu2Sb) primary phase and pseudobinary eutectics disappear gradually,and ternary eutectic transfers from regular to anomalous structure.When undercooling exceeds 102 K,anomalous (ε+θ+Sb) ternary eutectic is the unique microstructure.Competitive nucleation and growth of these three eutectic phases is the main cause for the formation of complex growth morphologies.Based on the current experiments and theoretical calculations,it can be concluded that the intermetallic compound phaseθ(Cu2Sb) is the leading nucleating phase.

MICROSTRUCTURAL EVOLUTION OF HIGHLY UNDERCOOLED EUTECTIC Ni78.6 Si21.4 ALLOY

A large undercooling level up to 550K （0.386Te） was achieved in eutectic Ni78.6Si21.4melt by the combination of molten-glass and cyclic superheating. A microcrystaUine structure is obtained at large undercooling. Surprisingly, the morphology of ct（Ni） phase transits from the non-faceted phaseto faceted phase at large undercooling of 390K. Based on the classical nucleation theory and transient nucleation theory, the process of microstructure evolution and competitive nucleation was analyzed, and the refinement of crystal structure is determined by the high nucleation rate under large undercooling.

Rapid solidification of acoustically levitated Al-Cu-Si eutectic alloy under laser irradiation

Al-27%Cu-5.3%Si ternary eutectic alloy was melted using a YAG laser and then solidified while being acoustically levitated. A maximum undercooling to 195 K (0.24 TL) was achieved with a cooling rate of 76 K/s. The solidification microstructure was composed of (Al+θ+Si) ternary eutectics and (Al+θ) pseudobinary eutectics. During acoustic levitation, the (Al+θ+Si) ternary eutectics are refined and the (Al+θ) pseudobinary eutectics have morphological diversity. On the surface of the alloys, surface oscillations and acoustic streaming promote the nucleation of the three eutectic phases and expedite the cooling process. This results in the refinement of the ternary eutectic microstructure. During experiments, the reflector decreases with increasing alloy temperature, and the levitation distance always exceeds the resonant distance. Because of the acoustic radiation pressure, the melted alloy was flattened, and deformation increases with increasing sound pressure. The maximum aspect ratio achieved was 6.64, corresponding to a sound pressure of 1.8×104 Pa.

Rapid solidification of undercooled Al-Cu-Si eutectic alloys

Under the conventional solidification condition,a liquid aluminium alloy can be hardly undercooled because of oxidation. In this work,rapid solidification of an undercooled liquid Al80.4Cu13.6Si6 ternary eutectic alloy was realized by the glass fluxing method combined with recycled superheating. The re-lationship between superheating and undercooling was investigated at a certain cooling rate of the alloy melt. The maximum undercooling is 147 K (0.18TE). The undercooled ternary eutectic is composed of α(Al) solid solution,(Si) semiconductor and θ(CuAl2) intermetallic compound. In the (Al+Si+θ) ternary eutectic,(Si) faceted phase grows independently,while (Al) and θ non-faceted phases grow coopera-tively in the lamellar mode. When undercooling is small,only (Al) solid solution forms as the leading phase. Once undercooling exceeds 73 K,(Si) phase nucleates firstly and grows as the primary phase. The alloy microstructure consists of primary (Al) dendrite,(Al+θ) pseudobinary eutectic and (Al+Si+θ) ternary eutectic at small undercooling,while at large undercooling primary (Si) block,(Al+θ) pseudo-binary eutectic and (Al+Si+θ) ternary eutectic coexist. As undercooling increases,the volume fraction of primary (Al) dendrite decreases and that of primary (Si) block increases.

Rapid solidification mecha-nism of Ag_(60)Sb_(34)Cu_(6) ternaryalloy in drop tube

Droplets of Ag60Sb34Cu6 ternary alloy within the diameter range of 60—800 μm were rapidly solidified by means of drop tube containerless processing, and the solidi- fication mechanism is analyzed. With a decrease in droplet size, the cooling rate increases from 57 to 5.8×104 K/s. The maximum undercooling is determined to be 180 K (0.23TL) and the microstructure of primary ε(Ag3Sb) dendrite refines drastically until homogenous equiaxed dendrite forms. Ex- perimental results indicate that (ε+Ag) pseudobinary eutectic cannot form under high undercooling conditions and the solubility of Ag in primary ε phase increases as undercooling increases. Based on thermal analysis and crystal growth morphology, it is found that this alloy is solidified in two ways corresponding to different undercooling levels.

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 ΔTMax=171 K (0.20TL). 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.

落管中Fe-Mo-Si三元包共晶合金的快速凝固机制

采用落管无容器处理技术实现了Fe-27．5％Mo-20．5％Si三元包共晶合金的快速凝固，获得直径介于50～900μm的合金小球，对其凝固机制进行了探讨。理论计算出不同直径液滴在落管中自由下落时的冷却速率和过冷度，实验中获得的最大过冷度达339K（0．21TL）。EDS和SEM测试结果表明，初生相为FeSi，包共晶相为τ1（Fe5MoSi4）和R（Fe2MoSi2）相。当300pm〈D≤900μm时，凝固组织由残余初生相、包共晶相和三相共晶组成；当直径≤300μm时，三相共晶受到抑制。根据晶体生长形态与相图，提出了合金液滴在不同粒径下的两种凝固方式。

Fe-18.5%Mo-28.5%Si三元共晶合金的快速凝固研究

采用落管无容器处理和单辊急冷技术研究了三元Fe-18.5%Mo-28.5%Si共晶合金的深过冷快速凝固。结果表明,快速凝固合金的微观组织均由MoSi2、τ1(Fe5MoSi4)和FeSi 3相组成。在自由落体条件下,随着液滴直径的减小,合金的凝固组织由两种二相共晶(MoSi2+τ1)和(MoSi2+FeSi)向细小不规则的3相(MoSi2+τ1+FeSi)共晶转变;而在单辊急冷快速凝固条件下,三元共晶的凝固组织则以柱状晶为特征。随着冷却速率的增大,三元共晶组织都发生了显著的细化。

Ni-32.5%Sn共晶合金的净化与深过冷

本文采用循环高温过热和无机盐玻璃净化剂去除液态Ni-32.5%Sn共晶合金中的异质晶核,获得高达397K(0.283T_E)的过冷度,发现深过冷熔体主要以辐射方式散热,从而导出一种根据冷却曲线近似计算其平均比热的方法,采用高速摄影技术研究了其再辉过程,发现即使过冷度超过0.2T_E,液态合金仍然优先发生界面异质形核。

Ag-Cu共晶合金在落管中的快速凝固

采用3m小型落管实现了Ag—28．1（质量分数，％下同）Cu共晶合金的微重力快速凝固，在高纯He和Ar的混合气氛中获得的合金小球直径在80—1000mm范围内．随着小球直径的减小，晶粒及层片共晶得到细化，凝固组织由层片共晶向不规则共品转变．TMK快速共晶生长理论的计算结果显示，Ag—28．1Cu层片共晶的最大生长速度、最小层片间距和最大过冷度分别为0．291m／s，0．017μm和193K．

深过冷熔体中的晶体形核与快速长大

以特制的无机盐玻璃作为净化剂去除液态Ni-0.39％B-6.58％Si合金中的异质晶核,获得363K(0.229T_L)过冷度,采用高速摄影及快速红外测温技术研究了深过冷熔体的快速凝固行为。

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.

三元Pb-Sb-Sn合金的快速凝固及组织特征

实现了三元Pb-12%Sb-4%Sn和Pb-15%Sb-10%Sn合金在深过冷条件下的快速凝固,实验中获得的最大过冷度分别为65 K(0.13TE)和70 K(0.13TL)。XRD分析表明,这两种合金在实验过冷度范围内均由(Pb)固溶体、(Sb)固溶体和SbSn金属间化合物三相组成。在小过冷条件下,Pb-12%Sb-4%Sn共晶合金的三相以层片状交替分布,协同生长。随着过冷度增大,(Sb)作为初生相以小平面方式生长,且三元层片共晶组织显著细化。对于Pb-15%Sb-10%Sn合金,其凝固组织由初生相、二相共晶和三元共晶组成,初生SbSn相以具有小平面特征的枝晶方式生长。在实验基础上,对三元Pb-Sb-Sn合金的形核特征和生长规律进行了理论分析。

Dendritic and eutectic growth in Sb_(60)Ag_(20)Cu_(20) ternary alloy

The rapid solidification of Sb60Ag20Cu20 ternary alloy was realized by high under- cooling 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), θ and 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 undercool- ings faceted dendrite growth takes the dominant place. The remarkable difference of crystal structures between (Sb) and θ phases leads to (θ + Sb) pseudobinary eutectic hard to form, whereas strips of θ form when the alloy melt reaches the (θ + Sb) pseudobinary eutectic line. The cooperative growth of θ and ε phases contrib- utes to the formation of (ε + θ ) pseudobinary eutectic easily. In addition, the crys- tallization route has been determined via microstructural characteristic analysis and DSC experiment.

自由落体条件下三元Cu_(60)Sn_(20)Pb_(20)偏晶合金的快速凝固

采用3m落管超高真空无容器装置研究三元Cu60Sn20Pb20偏晶合金在自由落体条件下的深过冷和快速凝固。结果表明,凝固组织由金属间化合物ε(Cu3Sn)相和固溶体(Pb)相组成,合金的液相线温度为1032K。液滴的冷却速率和过冷度都随液滴直径的增大而减小,液滴的最大过冷度为282K,且过冷度与液滴直径呈指数变化关系。直径较大的液滴的凝固组织出现壳核结构,外层为表面张力较小的富(Pb)相,内层为ε(Cu3Sn)相。直径较小的液滴内部的Marangon i迁移不能够充分进行,ε(Cu3Sn)相凝固成许多小球状组织。

Solidification microstructure and phase constitution of Fe-7.5%Mo-16.5%Si ternary quasiperitectic alloy

Solidification of Fe-7.5%Mo-16.5%Si ternary quasiperitectic alloy is investigated by using differential scanning calorimetry (DSC) and drop tube containerless processing techniques.The primary phase is identified as R (Fe5Mo3Si2) and the quasiperitectic phases are τ1 (Fe5MoSi4) and Fe3Si.With the decrease of droplet diameter, the cooling rate and undercooling of the droplets in-crease rapidly.The experiment result indicates that the solidification microstructure is composed of remnant primary phase, qua-siperitectic phases and ternary eutectic when the droplet diameter exceeds 400 μm, whereas the ternary eutectic is suppressed when the droplet is smaller than 400 μm in diameter.

深过冷Ag-Cu-Ge三元共晶合金的相组成与凝固特征

在Ag38.5Cu33.4Ge28.1三元共晶合金的深过冷实验中,获得的最大过冷度为175K(0.22TE).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80K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.

快速凝固过程的高速摄影研究

以特制的无机盐玻璃70%Na_2SiO_3+17.7%Na_2B_4O_7+12.3%B_2O_3作为净化剂,去除液态Ni-32.5%Sn 共晶合金中的异质晶核,使过冷度达302K(0.215T_E)。采用高速摄影及快速红外测温技术研究了深过冷熔体的快速凝固行为。发现尽管过冷度超过了以往认为的均质形核临界过冷度0.2T_m,但是 Ni-32.5%Sn 共晶合金仍然优先发生界面异质形核,再辉过程中瞬时凝固速度最大可达784mm/s。

深过冷液态Al-Ni合金中枝晶与共晶生长机理

在自由落体条件下实现了液态Al-4 wt.%Ni亚共晶、Al-5.69 wt.%Ni共晶和Al-8 wt.%Ni过共晶合金的深过冷与快速凝固.计算表明,(Al+Al3Ni)规则纤维状共晶的共生区是4.8—15 wt.%Ni成分范围内不闭合区域,且强烈偏向Al3Ni相一侧.实验发现,随液滴直径的减小,合金熔体冷却速率和过冷度增大,(Al)和Al3Ni相枝晶与其共晶的竞争生长引发了Al-Ni共晶型合金微观组织演化.在快速凝固过程中,Al-4 wt.%Ni亚共晶合金发生完全溶质截留效应,从而形成亚稳单相固溶体.当过冷度超过58K时,Al-5.69 wt.%Ni共晶合金呈现从纤维状共晶向初生(Al)枝晶为主的亚共晶组织演变.若过冷度连续增大,Al-8 wt.%Ni过共晶合金可以形成全部纤维状共晶组织,并且最终演变为粒状共晶.