Nb42Ti21Co37包共晶合金定向凝固组织演化规律研究

包共晶转变兼具共晶转变和包晶转变双重特征,存在于众多的工业合金中。然而,迄今为止尚未建立起相对完整的包共晶凝固理论模型,关于其凝固机理的相关研究较少。基于此,本文针对Nb42Ti21Co37包共晶合金开展了不同抽拉速度(V=1, 3, 5, 15, 30, 70 μm/s)下的定向凝固实验,旨在研究不同抽拉速度下合金的微观组织演化规律,并构建相应的凝固机制。研究结果表明：Nb42Ti21Co37包共晶合金常规铸态和定向凝固组织中均含有α-Nb、Co6Nb7和TiCo＋Co6Nb7包共晶相,随着抽拉速率的逐渐增加,初生相α-Nb依次经历了圆球状→花瓣状→团簇状→枝晶状的转变;伴随着上述过程,淬火界面经历了胞状界面到胞枝晶状界面的转变,并且,在抽拉速率V=70 μm/s时固/液界面消失;其次,定向凝固稳态生长区内包共晶的组织逐渐细化,其层间距与生长速率呈指数线性关系,即λ=-1＋5×e2.5;当抽拉速率低于5μm/s时,合金的定向凝固过程与平衡凝固相类似;另外,各相在稳定生长区的生长机制为共生生长,随着抽拉速率的增加,包共晶组织的定向排列性逐渐变差。     

非平衡凝固反常共晶研究进展

对共晶生长的凝固理论及非平衡凝固条件下产生的反常共晶的生长机制进行了评述。通过比较分析发现,现有的共晶生长理论模型不能够很好地解释反常共晶的形成,此外,发现不同的研究者对反常共晶的生长机制还有争议,关于两种反常共晶的划分标准未统一,对同一合金体系在深过冷快速凝固下产生反常共晶的临界过冷度有差异。综合现有研究结果,进一步研究反常共晶的生长机制对共晶凝固理论科学的发展将具有重要意义。     

Ni－NbC合金中强化相的形核与分支特点

本文利用光学及扫描电子显微技术、Ｘ射线择优取向分析等手段，对Ｍｏ杂质作用下的Ｎｉ－ＮｂＣ合金的形貌、取向等特点进行了研究．结果表明：在自由凝固下，领先形核与生长的ＮｂＣ初生晶依杂质含量不同，呈六面体和八面体两种形态．它们分别以＜１１１＞和＜１００＞晶向为择优生长方向．共晶团生长过程中，ＮｂＣ相的分支机制主要表现为择优生长方向的分支，而缺陷的作用是次要的．     

Ni—NbC合金中强化相的形核与分支特点

本文光学及扫描电子显微技术，Ｘ射线择优取向分析等手段，对Ｍｏ杂质作用下的Ｎｉ－Ｎｂｃ合金的形貌、取向等特点进行了研究，结果表明，在自由凝固下，领行形核与生长的Ｎｂｃ初生晶依杂质含量不同，呈六面体和八面体两种形态，它们分别以＜１１１＞和＜１００＞晶向为择优生长方向，共晶团生长过程中，ＮｂＣ相的分支机制主要表现为择优生长方向的分支，而缺陷的作用是次要的。     

Fe含量对金属型铸造Al-Fe合金组织形态的影响

研究了不同Fe含量的Al-Fe合金在金属型铸造条件下的组织形态.在金属型铸造条件下,亚共晶Al-Fe组织由发达α-Al枝晶与枝晶间隙链接呈网状的细小共晶Al3Fe相所组成;共晶Al-Fe合金组织为弯曲针棒状共晶Al3Fe相和共晶α-Al相共生生长组织;过共晶Al-Fe合金随着Fe含量增加初生Al3Fe相的形态由针棒状向针片状再向长针状转变,初生相周围共晶Al3Fe相依附在初生相表面生长而形成离异共晶组织.并对不同含Fe量时合金的凝固过程进行了分析探讨.     

共晶硅变质机理研究进展

随着对共晶Si变质机理的深入研究,相继发现了一些新的证据,提出了一些新的假说。共晶Si的变质机理主要存在两种：形核理论和生长理论。“小面-非小面转变说”的核心是Si相的形核受到抑制,而“毒化”机制、Zigzag孪晶生长模型及杂质诱发孪晶机制揭示了变质时共晶Si晶体的生长行为。文中对共晶Si的变质机理研究进展进行了综述。     

激光快速熔凝Ni-Sn共晶合金的组织演变

采用Ni 28%Sn,Ni 30%Sn,Ni 33%Sn和Ni-35%Sn(质量分数)共4种成分的亚共晶和过共晶合金,考察了Ni Sn共晶合金在激光快速熔凝过程中的组织演化过程.当激光扫描速率较低时,Ni 28%Sn亚共晶合金和Ni-35%Sn过共晶合金的熔凝组织主要由细化的初生枝晶(亚共晶合金为α-Ni相,过共晶合金为Ni3Sn相)和枝晶间共晶(α-Ni+Ni3Sn)组成;近共晶合金Ni-30%Sn和Ni 33%Sn的熔凝组织基本相似,熔池从底部到顶部存在柱状共晶团向等轴状共晶团的转变.Ni-Sn亚共晶和过共晶合金熔池底部皆存在少量粗大的残留初生相.激光快速熔凝后,相比基材中所存在的层片和棒状共晶的混合组织,熔池内的共晶组织皆为层片状共晶,层片间距相比基材明显减小,并呈现垂直于熔池底部外延生长的特征.通过对比4种成分合金在不同激光扫描速率下的熔凝组织,获得了激光快速熔凝条件下Ni-Sn合金共生生长区的成分范围以及临界激光扫描速率.应用描述快速枝晶生长的KGT模型和快速共晶生长的TMK模型对熔凝组织进行了分析,模型计算结果与实验结果符合良好.     

铝硅二元合金的共生生长机制

观察分析了铝硅二元共晶合金的显微结构。发现在不同凝固条件下凝固的共晶硅形貌虽然有极显著的差别，但它们的基本结构是相同的－－铝硅二元合金共生生长成共晶团；共晶团的核心部分基本上相同，共晶团的尺寸与冷却速度和变质处理有关。稀土元素主要富集在共晶团的周界处，促使晶体缺陷增多，导致分枝急剧增加，从而使共晶硅由各向异性生长特性转变为各向同自发性生长特性。     

氧化铝纤维对锌合金复合材料凝固组织的影响

利用挤压铸造制备了氧化铝纤维增强锌合金复合材料，研究了氧化铝纤维对锌合金复合材料凝固组织的影响。结果表明，在复合材料中，纤维与基体间存在致密界面层，合金元素通过适当的化学反应可改善纤维与基体的结合；在凝固过程中，氧化铝纤维可作为锌合金共晶体非自发形核的衬底，纤维／基体界面上的硅在共晶体的共生生长过程中起了领先相作用，导致复合材料的共晶转变由铝硅共晶转变和锌铝共晶转变两者组成     

B对近共晶Al-13.0%Si合金共晶凝固过程的影响

研究了约2℃/s冷却条件下,Al-3B中间合金不同加入量对近共晶Al-13.0%Si合金凝固过程的影响。分析认为B的加入为细小共晶体的形核提供了大量形核基底,致使共晶形核温度(TN)和再辉前最低温度(TM)升高,对共晶生长温度(TG)影响很小。B的加入对Al-13.0%Si合金的凝固过程产生了显著影响,使合金由未加B时从型壁区至中心区凝固,转变为加B后在整个试样范围内同时大量形核和生长,对共晶Si片有一定的细化作用。     

Microstructural evolution during containerless rapid solidification of Ni–Mo eutectic alloys

Ni–45%Mo hypoeutectic, Ni–47.7%Mo eutectic and Ni–50%Mo hypereutectic alloys are rapidly solidified during containerless processing in drop tube. The microstructures of Ni–47.7%Mo eutectic alloy are composed of lamellar eutectic plus anomalous eutectic of Ni and NiMo phases. When the droplet size decreases, the volume fraction of anomalous eutectic becomes larger. The structural morphology transforms into Ni dendrite plus lamellar eutectic in very small droplets which are highly undercooled. The microstructures of Ni–45%Mo hypoeutectic alloy are characterized by primary Ni dendrite plus lamellar eutectic, whereas those of Ni–50%Mo hypereutectic alloy consist of NiMo dendrite plus lamellar eutectic. For both off-eutectic alloys, the experimental results show that the microstructure evolution depends mainly on droplet size. In the case of Ni–45%Mo hypoeutectic alloy, with the decrease of droplet size, the primary Ni phase transforms from dendrites to equiaxed grains. As for Ni–50%Mo hypereutectic alloy, when droplets become smaller and smaller the microstructural transition proceeds from primary NiMo dendrite plus lamellar eutectic to anomalous eutectic. The calculated highest undercoolings of the three alloys are 226, 182 and 135 K, respectively. By classical nucleation theory, Ni phase is the primary phase to nucleate for Ni–47.7%Mo eutectic alloy. The TMK eutectic growth and LKT/BCT dendritic growth theories are applied to analyze the rapid solidification process and investigate the microstructural transition mechanisms. The coupled zone of Ni–Mo eutectic alloy has also been calculated on the basis of TMK and LKT/BCT models, which covers a composition range from 45.7% to 57.1% Mo.     

Reexamination of the solidification behavior of undercooled Ni–Sn eutectic melts

Employing a high-speed video, in situ recalescence behaviors of Ni–Sn eutectic melts under different undercooling conditions have been investigated. Based on the surface morphologies and cross-sectional microstructures that consist of independent eutectic colonies, copious nucleation is proposed to take place in Ni–Sn eutectic melt regardless of melt undercooling in the unconstrained solidification. The boundary with sharp contrast between the crystallized bright solid and undercooled dark liquid during recalescence is not the solid/liquid interface but the simultaneous thermal release of concentrated crystallizing eutectic colonies. It may not be feasible to measure growth velocities in free solidification of the system due to copious nucleation. The fluctuations in undercooled melts are proposed to activate a chain-like successive nucleation reaction once an effective nucleus is formed. The current observation urges that we should deliberate further the conventional concept that once an effective nucleus is formed in a deeply undercooled melt, the growth will be initiated promptly and the solid/liquid interface will sweep across the entire sample rapidly. The origin of anomalous eutectic formation and the growth have been discussed when the current nucleation phenomenon is taken into account. Based on the copious nucleation theory and the improved driving force for growth, the present recalescence frames and some other experimental phenomena concerning the free growth of the undercooled eutectic system can be well clarified.     

Electrolytic deposition of Zn coatings from ionic liquids based on choline chloride

Abstract

Reported here are results of a systematic study into the electrolytic deposition of Zn metal from ionic liquids based on eutectic mixtures of choline chloride (ChCl) with either ethylene glycol (Eg) or urea. Cyclic voltammetry and gravimetric acoustic impedance spectroscopy (EQCM) techniques have been used to probe mass-transfer and charge-transfer mechanisms. The results show that nucleation in the urea based liquid is fast but bulk growth is slow, whereas nucleation in the glycol based liquid is slow but bulk growth is relatively fast. Critical surface coverage necessary for the transition from nucleation to bulk growth is similar for both liquids. Additionally, ex situ atomic force microscopy (AFM) has been used to probe the morphology of the metallic deposit at two different potentials during a linear sweep voltammogram.     

Eutectic nucleation in Al-25wt.%Si alloy through DSC

Eutectic nucleation in hypereutectic Al-25wt.%Si alloy with the addition of P was studied by calculating the activation energy and nucleation work. The results indicate that P refinement, which can decrease the activation energy as well as nucleation work of eutectic solidification, can increase the eutectic nucleation frequency of Al-25wt.%Si alloy, so the addition of P can refine the eutectic structures of hypereutectic Al-Si alloys. According to the calculations, there is a certain relationship between the precipitation of primary silicon and eutectic solidifica-tion, and the eutectic nucleation mode is independent of morphology transition.     

凝固过程晶粒生长动力学及拓扑转变机制的相场研究

基于连续相场动力学模型,研究了凝固过程形核、长大及粗化阶段的组织形貌演化,动力学转变,以及粗化过程拓扑转变,分析了形核与长大过程的关系。研究结果表明,形核长大过程中,晶粒体积分数逐渐增大至平衡值,总表面积先增大后减小,体积自由能是形核的驱动力,表面能是形核的阻力。形核伴随着长大,两者是相互重叠相互竞争的两个过程。晶粒生长过程中,边数大于六的晶粒持续长大,而边数少于六的晶粒不断缩小。小晶粒消失机制有:临近切换机制;三边﹑四边及五边晶粒直接消失机制;四边交叉点分离并最终导致小晶粒消失机制;晶界直接消失机制。模拟结果与实验结果符合较好。     

Computer Simulation of the Kinetics of Eutectic Crystallization1

The state-of-the-art in the phenomenological theory of eutectic crystallization is considered. The theory is used for developing a method for computer simulation of the kinetics of crystallization of castings from eutectic alloys. The kinetic aspect of the process is considered in terms of the geometry of nucleation, growth, and competition in the process of growth of a set of crystals.     

Eutectic nucleation in Al-7 wt pct Si-Mg casting alloys

The nucleation of eutectic crystals of hypoeutectic Al-7 wt pct Si-Mg casting alloys modified by Ce and Sr was studied by using differential scanning calorimeter (DSC) and scanning electron microscope (SEM). DSC results were applied to calculate the values of activity energy and nucleation work of eutectic nucleation for the alloys. These values were decreased and the eutectic nucleation frequency was increased with the addition of Ce and Sr to the alloys. Moreover, the morphology of eutectic silicon in the modified alloys was partially or fully modified as fine, fibrous or coral, and DSC plots show that the eutectic undercooling of Al-7 wt pct Si-Mg alloys were increased with additions of modified agents. The mechanism of eutectic modification is combined actions of the accelerated nucleation and the restricted growth.     

Gasar工艺及其衍生技术的研究进展

固-气共晶定向凝固（Gasar工艺）是一种可制备出藕状或放射状规则多孔结构（Gasarite）的新颖工艺方法。在该工艺基础上,目前已衍生出热分解法、连续区熔技术和连续铸造技术。本文介绍了这些技术的主要研究进展,总结了固-气共晶凝固过程中气泡形核和固-气共生生长的理论研究结果以及Gasarite孔隙结构控制与优化的工艺措施,同时简要介绍了Gasarite的性能特点和应用前景。     

共晶反应定向凝固工艺制备多孔材料气孔形成和长大机理

本文采用热分析法研究几种金属一氢气共晶反应定向凝固制备藕状多孔材料气孔的形成和长大机理。结果表明,这种工艺气孔是借助熔融金属中的活性杂质非匀质形核的,而长大则是沿固液界面的法线方向向具有粗糙界面的气固相转移,最终形成气孔规则定向排列于金属基体中的多孔材料。