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     

过冷水滴碰撞结冰的实验与模拟研究

以宏观结冰/霜过程中过冷水滴的碰撞结冰现象为背景,实验对比了亲水和超疏水表面上常温水滴碰撞、常温水滴碰撞结冰和过冷水滴碰撞结冰的过程,建立了过冷水滴碰撞结冰过程的数值模型,研究了We数和接触角对碰撞结冰的影响。结果表明:相比于常温水滴的碰撞及其碰撞结冰过程,过冷水滴碰撞结冰过程的稳态铺展系数更大;随着过冷度和We数的增大以及接触角的减小,过冷水滴的碰撞结冰与常温水滴的碰撞在水滴形态和铺展系数上的差异逐渐增大。     

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.     

深过冷液态Mg-Ca合金的微观动力学行为

采用广义非定域模型赝势确定的原子间相互作用的有效偶势及等温-等压系综下的分子动力学模拟技术,研究了深过冷液态Mg-Ca合金的微观动力学行为。考察了原子体系密度自相关函数随时间、波矢及温度的变化特点。结果表明,随着温度的降低,自关联函数的关联范围增长,在深过冷区体系表现出一定程度的结构慢化现象。此外还发现,模式耦合的理论结果,在本文所涉及的过冷区范围内适用于液态Mg-Ca合金。 关键词：     

微重力下Fe-Al-Nb合金液滴的快速凝固机理及其对显微硬度的影响

采用落管无容器处理技术实现了Fe_(67.5)Al_(22.8)Nb_(9.7)三元合金在微重力条件下的快速凝固,获得了直径为40—1000μm的合金液滴.实验中合金液滴的过冷度范围为50—216 K,冷却速率随着液滴直径的减小由1.23×10~3K·s~(-1)增大到5.53×10~5K·s~(-1).研究发现,Fe_(67.5)Al_(22.8)Nb_(9.7)合金液滴的凝固组织均由Nb(Fe,Al)_2相和(αFe)相组成,且随着液滴直径的减小,初生Nb(Fe,Al)_2相由树枝晶转变为等轴晶,共晶组织发生了约3倍的细化且生长特征由层片共晶向碎断共晶转变;硬质初生Nb(Fe,Al)_2相的析出有效提高了合金的显微硬度.与电磁悬浮条件下同过冷合金的凝固组织对比发现,落管条件下的合金液滴凝固组织更细化,使得合金显微硬度提高了2%—6%.     

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(0.17TL).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),(Ph)相和金属间化合物ε(Cu3Sn)相组成的三相区,下部为富(Ph)相区.在小过冷条件下,三相区中ε(CuSn)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),(Ph)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.     

Sn-Pb合金颗粒异质成核及其冷却凝固行为预测

采用均匀颗粒成型法 (UniformDropletSpray ,UDS)在氮气氛下 (氧含量为 1.3 6μmol/L)制备了 15 0和 185μmSn 5 %Pb合金颗粒 .采用光学显微镜观测颗粒的外观形貌 ,结果表明 ,UDS方法制备的微粒是粒度均匀的球状颗粒 ;利用非绝热容量法确定了颗粒的形核点及过冷度 ;计算了以时间和温度为函数的颗粒表面被氧化的比例 ,提出了以颗粒表面氧化为催化媒质的异质成核理论模型 ,合理反映了颗粒的异质成核过程 .在此基础上计算了微粒表面异质形核条件下的连续冷却转变 (ContinuousCoolingTransformation ,CCT)曲线 ,同时以线性冷却条件为例预测了颗粒的冷却凝固行为 .     

过冷沸腾气泡行为的实验研究

本文采用拍摄速度为10000帧/秒的高速摄影仪对不锈钢箔表面的过冷沸腾现象进行了可视化实验研究。实验结果与用微液层模型理论预测的结果一致。高过冷度区域的沸腾换热机理主要是由气泡生长、消失过程中温度边界层的强制排除(所谓强制对流)引起的。气泡周期主要由等待时间构成,这在过冷度高的情况下尤为显著。对等热流密度换热面,微液层模型预测的气泡周期与实验值比较吻合。     

具有宽过冷液相区的Fe62 Co8-x(Cr,Mo)xNb4 Zr6 B2非晶态合金的热稳定性与磁性

根据制备块状非晶态合金的三条经验准则,选择了成分为Fe62Co8-xCrxNb4Zr6B20和Fe62Co8-xMoxNb4Zr6B20(x=0,2,4)的合金系.利用单辊急冷法制备出厚为30μm宽为5mm左右的条带,并用差热分析、X射线衍射以及Faraday磁天平、静态磁性测量仪等研究了合金的热稳定性、非晶结构和磁性能.发现含2at%Cr的Fe62Co6Co6Nb4Zr20B20非晶态合金的过冷液相区ΔTx最宽,达到85K,但是合金系的饱和比磁化强度σs随Cr或Mo含量的增加而急剧下降.合金系经973K真空退火900s后,由于晶化相α-Fe等晶相析出,使得各合金的σx和Hc都迅速升高.     

混合工质过冷核化沸腾气泡行为特性

本文采用控制热流密度方式对乙醇和水二元混合物在直径0．1 mm铂丝上过冷沸腾核化进行可视化实验观察, 分析不同混合比、热流密度下过冷沸腾核化实验现象。实验证明,小气泡射流是混合工质沸腾换热中主要的换热形式,并且其形态具有多样化,如连续射流、间断射流、离散射流、非常规射流、分叉射流等。     

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)     

Formation mechanism of anomalous eutectics in highly undercooled Ag--39.9 at%Cu alloy

<正>This paper investigates the solidification behaviour of the Ag—Cu eutectic alloy melt undercooled up to 100 K.It is revealed that lamellar eutectics grow in a dendritic form in the Ag-Cu eutectic melt with undercooling equal to or greater than 76 K.As undercooling increases,the remelted fraction of the primary eutectics during recalescence rises. The severe remelting and the subsequent ripening of the primary eutectic dendrites lead to the formation of anomalous eutectics.     

Rapid eutectic growth in undercooled Al-Ge alloy under free fall condition

Eutectic growth in Al-51.6%wt Ge alloy has been investigated during free fall in a drop tube. With decreasing undercooling ΔT, the microstructural evolution has shown a transition from lamellar eutectic to anomalous eutectic. A maximum cooling rate of 4.2×10^4K/s and undercooling of up to 240K (0.35T_E) are obtained in the experiment. The eutectic coupled zone is calculated on the basis of current eutectic and dendritic growth theories, which covers a composition range from 48%－59% Ge and leans towards the Ge-rich side. The two critical undercoolings for the eutectic transition are ΔT_1^*=101K and ΔT_2^*=178K. When ΔT≤ΔT^*_1, the microstructure for Al-51.6% Ge eutectic shows lamellar eutectic. If ΔT≥ΔT^*_2, the microstructure shows anomalous eutectic. In the intermediate range of ΔT^*_1<ΔT<ΔT^*_2, the microstructure is the mixture of the above two types of eutectics.     

Microstructure evolution of undercooled Fe-Co-Cu alloys

Rapid solidification of undercooled Fe-Co-Cu alloys was investigated by means of fluxing purification and cyclic superheating technique. A transition in microstructure from dendrites to phase-separation occurred above a phase-separation undercooling ΔT_sep. When ΔT<ΔT_sep, dendrite was observed, the trunks were rich in Fe and Co, while Cu was rich at inter-dendrites. However, the phase-separated microstructure was obtained once ΔT>ΔT_sep, with a large sphere of L1 phase located almost at the center of the sample and enwrapped by L2 phase. ΔT_sep was 222, 88 and 45 K for Fe₅₀Co₃₀Cu₂₀, Fe₂₅Co₂₅Cu₅₀ and Fe₁₅Co₁₅Cu₇₀ alloys in this work, respectively. It was investigated that L1 phase solidified before L2 phase after liquid separation and followed different ways.     

Atomic structure insight into crystallization of undercooled liquid metal Zr during isothermal relaxation processes

ABSTRACT

The crystallization of undercooled liquid zirconium (Zr) was investigated via molecular dynamics simulations. The atomic structures were characterised and analysed by means of pair distribution function, angular distribution function, and the largest standard cluster analysis. At low temperature (T?=?1000?K), it is found that the crystallization begins at the initial stage of relaxation and takes the pathway of undercooled liquid (UCL)→BCC→HCP instead of UCL→HCP. The HCP-type medium-range orders (MROs) are formed from the inside of the precursors formed by the BCC-type MROs, and growing at the cost of the precursors, in agreement with the Ostwald’s step rule. The number of MROs is found to play a key role in forming the crystal phases in this process. However, at high temperature (T?=?1200?K) the time-scale to reach crystallization is two orders of magnitude bigger than that of T?=?1000?K. No intermediate stage of crystallization is observed. The undercooled Zr melts are directly transformed into BCC crystal phases. The max size of BCC-MROs is intimately correlated with the crystallization.     

Thermophysical properties of undercooled liquid Co-Mo alloys

Using electromagnetic levitation in combination with the oscillating drop technique and drop calorimeter method, the surface tensions and specific heats of undercooled liquid Co-10 wt% Mo, Co-26.3 wt% Mo, and Co-37.6 wt% Mo alloys were measured. The containerless state during levitation produces substantial undercoolings up to 223 K (0.13 T _L), 213 K (0.13 T _L) and 110 K (0.07 T _L) respectively for these three alloys. In their respective undercooling ranges, the surface tensions were determined to be 1895 m0.31(T m1744), 1932 m0.33(T m1682), and 1989 m0.34(T m1607) mN m^?1. According to the Butler equation, the surface tensions of these three Co-Mo alloys were also calculated, and the results agree well with the experimental data. The specific heats of these three alloys are determined to be 41.85, 43.75 and 44.92 J mol^?1 K^?1. Based on the determined surface tensions and specific heats, the changes in thermodynamics functions such as enthalpy, entropy and Gibbs free energy are predicted. Furthermore, the crystal nucleation, dendrite growth and Marangoni convection of undercooled Co-Mo alloys are investigated in the light of these measured thermophysical properties.     

In situ observation of containerless solidification from undercooled Lu2O3 and Y2O3 melts

Containerless solidification processes in undercooled Lu₂O₃ and Y₂O₃ melts were investigated using an aerodynamic levitation furnace and a high-speed video. Double recalescence, indicative of two successive phase transitions, was observed for both oxides. The melting point following the first recalescence (T _M1) was higher than that of the second (T _M2) for Y₂O₃, which suggests that Y₂O₃ has a phase transition from a melt to a high temperature phase at T _M1, and then to a room temperature phase at T _M2. Two solid phases of Lu₂O₃ were also identified at T _M1 and T _M2. However, in contrast to Y₂O₃, T _M1 was lower than T _M2 for Lu₂O₃. This implies that the first crystallized phase from a melt is a metastable phase. The life duration of this metastable Lu₂O₃ was in the order of tens of milliseconds.