Ti3Al合金凝固过程晶核形成及演变过程的模拟研究

采用分子动力学方法对Ti₃Al合金的形核机理进行了模拟研究,采用团簇类型指数法(CTIM),对凝固过程不同尺度的原子团簇结构进行了识别和表征,深入研究了临界晶核的形成和长大过程.结果表明,凝固过程体系包含了数万种不同类型的原子团簇结构,但其中22种团簇结构类型对结晶形核过程起关键性作用.在晶核的形成和长大过程,类二十面体(ICO)原子团簇、类BCC原子团簇和缺陷FCC及缺陷HCP原子团簇在3个特征温度点T₁ (1110 K), T₂ (1085 K)和T₃ (1010 K)时达到数量上的饱和,并根据数量和空间分布随温度的变化,得到了它们在形核和长大过程相互竞争的关系.跟踪平行孪生晶粒形成和长大的过程发现,临界晶核是由FCC原子构成的单相结构,并未观察到亚稳BCC相优先形核的过程;平行孪生结构是由FCC单相晶核在沿密排面逐层生长过程中形成的.结果还表明, CTIM相比于其他微观结构表示方法,能更为准确地揭示凝固过程微观结构的转变特征.     

定向结晶条件下聚乙二醇6000的强动力学效应

在单向温度场条件下, 采用不同抽拉速度实现了聚乙二醇6000的定向生长、界面形貌的实时观测及界面温度的测量, 进而揭示了其生长机制. 实验结果表明, 随着抽拉速度的增大, 界面的温度逐渐减小, 过冷度逐渐增大. 运用高聚物结晶的次级形核理论模型, 对实验数据进行了计算, 得到在界面过冷度为13.5 K左右时, 生长机制发生了由区域Ⅱ向区域Ⅲ的转变. 实验数据与等温结晶数据的比较发现等温结晶方法中获得过冷度相对较大, 是因为其包含了热过冷. 聚乙二醇6000定向结晶过程中需要的最大动力学过冷度为20 K, 说明由于高聚物的二维形核, 其生长主要由界面动力学控制, 具有较强的动力学效应.     

压强对Zr_(67)Ni_(33)非晶合金结构和动力学性能的影响

采用分子动力学的方法模拟Zr_(67)Ni_(33)合金液体在不同压强下快速凝固过程,通过结构分析方法如对分布函数、配位数、Honeycutt-Andersen(HA)键型指数法,以及动力学参数如均方位移、自散射关联函数、非高斯参数研究压强对Zr_(67)Ni_(33)非晶合金局域原子结构和其过冷液体动力学性能的影响.研究结果表明:压强越大,Zr_(67)Ni_(33)非晶合金中Zr-Ni原子间的相互作用越强,体系结构有序性越强,过冷液体中动力学减慢和动力学不均匀现象越显著.     

Al-Ti-B细化工业纯铝凝固组织演变过程数值模拟

采用耦合群体动力学方法与元胞自动机方法建立了细化处理条件下铝合金凝固微观组织演变的数值模型.该模型考虑了a-Al的非均匀形核过程、晶粒的初始球形长大以及之后的枝晶生长过程.利用建立的模型模拟了Al-5Ti-1B中间合金细化工业纯铝凝固组织演变过程.结果表明:形核初始阶段,熔体中存在充足数量的有效形核粒子, a-Al形核率随着熔体过冷度的增大逐渐增高;形核开始不久后, a-Al的异质形核过程由熔体中有效形核粒子数量控制,直到再辉发生,形核停止.模拟分析了中间合金添加量以及熔体冷却速度对工业纯铝凝固组织演变过程的影响,模拟结果与实验结果相符,验证了模型的准确性.     

BCC枝晶生长原子堆垛过程的晶体相场研究

通过在自由能泛函中引入各向异性参数得到了一个基于高斯内核的改进晶体相场模型, 并采用该模型研究了体心立方结构(BCC)枝晶生长的原子堆垛过程. 结果表明, 在BCC由正十二面体平衡形貌演化为枝晶组织过程中, 形核位置经历了由面心({110}面)到尖端(<100>取向)的转移, 进而发生界面失稳形成枝晶组织; 枝晶生长过程中, 新的固相原子首先在枝晶尖端附近形核, 并快速向尖端及根部生长, 枝晶尖端被新原子完全包覆后将再次诱发液相原子附着形核及生长; 随初始液相密度的增加, 固-液界面移动速率增加, 速率系数的各向异性也增强.     

关于《非晶态金属的结构转变与动力学》一文的讨论

编辑同志: 对载于《物理》 1983年第 1期的《非晶态金属的结构转变与动力学》一文,其中关于TTT图所反映的非晶态金属的结晶行为,我觉得有一点需要补充. 该文认为,过冷液体沿文中图4中a淬火,避开了TTT曲线的“鼻端”,可以形成玻璃,而沿途径 b连续加热,则在Tx处开始结晶.仔细考查TTT图就会发现这种说法不够确切. TTT曲线是理论计算的结果[1].首先由实验求得过冷液体的粘度和相应晶体的熔融焓,然后由计算在给定过冷度下形成10-6晶体所需的时间.式中I和U分别为形核速度和晶体生长速度.计算中假设,熔体以无穷大的速度由熔点以上冷却到所讨…     

液体的过冷态演示

液体的实际结晶温度必须或多或少低于其固液二相平衡共存温度(即熔点),这种现象叫液体的过冷。也就是说,液态物质降温到熔点时并不立即凝固,而必须或多或少低于熔点才开始凝固。温度低于其熔点的液体叫做过冷液体。例如锑的熔点为631℃,而锑的溶液要过冷到590℃才开始结晶;很纯的水可以过冷到-40℃才结冰;镓的熔点是29.75℃,在静止时,它可以在低于溶点5°～15℃的情况下保持液态一年以上。     

冷速对液态金属Ga凝固过程中微观结构演变影响的模拟研究

采用分子动力学方法对液态金属Ga凝固过程中不同冷却速率对微观结构演变的影响进行了模拟跟踪研究. 运用HA键型指数法和原子成团类型指数法(CTIM)分析了金属原子Ga的成键类型和形成的基本原子团结构. 结果发现，冷却速率对凝固过程中的微观结构起着非常重要的作用. 在以1.0×10¹⁴，1.0×10¹³，1.0×10¹²K/s的速率冷却时，系统形成以与1311，1301键型相关的菱面体结构为主，夹杂着立方体、六角密集等其他团簇结构所构成的非晶态结构；在以1.0×10¹¹K/s的速率冷却时，系统明显发生结晶，其结晶转变温度T_c约为198K，且冷却速率越慢，结晶转变温度T_c越高，形成以与1421键型相关的斜方晶体(经可视化分析确认)为主要构型的晶态结构. 这将为研究液态金属的结晶转变过程提供一种新方法. 关键词： 液态金属Ga 凝固过程 微结构转变 分子动力学模拟     

金属基复合材料原位反应相场模型

原位反应法制备金属基复合材料具有增强体与基体间无杂质、无污染、颗粒分布均匀等优点,已成为制备金属基复合材料的一种重要方法,揭示其动力学机制及规律具有重要的理论及工业价值.然而,原位反应过程具有反应时间短、随机发生、温度高等特点,目前采用原位实验观测其反应过程仍存在较大困难.本文采用相场法模拟金属熔体内的原位反应过程,首先建立了能够描述双束金属熔体界面反应形核的相场模型,并采用该模型模拟了不同参数下相界反应形核过程.结果表明,形核率随着曲率半径及噪声强度的增大而增大,小曲率半径及强噪声条件下新相颗粒尺寸分布更加均匀,形核率随着过冷度的增大而先增大后减小.     

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.     

Kinetic details of crystallization in supercooled liquid Pb during the isothermal relaxation

The kinetic details of crystallization in supercooled liquid Pb during the isothermal relaxation process have been investigated by molecular dynamics simulations, and the microstructure evolution analyzed by the cluster-type index method (CTIM) and the tracing method. It has been found that, the dynamic features are consistently correlated with the microstructure evolution and the crystallization characteristics in the mean square displacement (MSD) and the non-Gaussian parameter (NGP): the β relaxation regime corresponds to the minor structural rearrangement because of the “cage effect”, and the atoms attempt to escape from the “cages”; the α relaxation regime is related to a more diffusive movement of atoms, and the appearance of the second plateau in MSD and the non-zero plateau in NGP corresponds to the completion of crystallization. In addition, three distinct stages of nucleation, growth of nuclei and coarsening of crystallites in the crystallization process have been clearly revealed.     

Atomic mechanism of glass formation in supercooled monatomic liquids

Atomic mechanism of glass formation in supercooled monatomic liquids is monitored via analyzing the spatial arrangement of solid-like atoms. The supercooled states are obtained by cooling from the melt using molecular dynamics (MD) simulation. Solid-like atoms, detected via Lindemann-like freezing criterion, are found throughout the liquid. Their number increases with decreasing temperature and they form clusters. In the deeply supercooled region, all solid-like atoms form a single percolation cluster which spans throughout the system. The number of atoms in this cluster increases steeply with further cooling. Glass formation in supercooled liquids occurs when a single percolation cluster of solid-like atoms involves the majority of atoms in the system to form a relatively rigid glassy solid. By analyzing the temperature dependence of static and dynamic properties, we identify three characteristic temperatures of glass formation in supercooled liquids including the Vogel–Fulcher temperature.     

New criterion in predicting glass forming ability of various glass-forming systems

It has been confirmed that glass-forming ability (GFA) of supercooled liquids is related to not only liquid phase stability but also the crystallization resistance. In this paper, it is found that the liquid region interval ($T_{\rm l}-T_{\rm g})$ characterized by the normalized parameter of $T_{\rm g}$/$T_{\rm l}$ could reflect the stability of glass-forming liquids at the equilibrium state, whilst the normalization of supercooled liquid region $\Delta T_{\rm x}$=($T_{\rm x}-T_{\rm g})$, i.e. $\Delta T_{\rm x}$/$T_{\rm x}$ (wherein $T_{\rm l}$ is the liquidus temperature, $T_{\rm g}$ the glass transition temperature, and $T_{\rm x}$ the onset crystallization temperature) could indicate the crystallization resistance during glass formation. Thus, a new parameter, defined as $\xi =T_{\rm g}$/$T_{\rm l}+\Delta T_{\rm x}$/$T_{\rm x}$ is established to predict the GFA of supercooled liquids. In comparison with other commonly used criteria, this parameter demonstrates a better statistical correlation with the GFA for various glass-forming systems including metallic glasses, oxide glasses and cryoprotectants.     

New criterion in predicting glass forming ability of various glass-forming systems

It has been confirmed that glass-forming ability （GFA） of supercooled liquids is related to not only liquid phase stability but also the crystallization resistance. In this paper, it is found that the liquid region interval （T1 - Tg） characterized by the normalized parameter of Tg/T1 could reflect the stability of glass-forming liquids at the equilibrium state, whilst the normalization of supercooled liquid region △Tx=（Tx - Tg）, i.e. △Tx/Tx （wherein T1 is the liquidus temperature, Tg the glass transition temperature, and Tx the onset crystallization temperature） could indicate the crystallization resistance during glass formation. Thus, a new parameter, defined as ζ = Tg/T1＋△Tx/Tx is established to predict the GFA of supercooled liquids. In comparison with other commonly used criteria, this parameter demonstrates a better statistical correlation with the GFA for various glass-forming systems including metallic glasses, oxide glasses and cryoprotectants.     

Crystallization evolution and relaxation behavior of high entropy bulk metallic glasses using microalloying process

The role of the microalloying process in relaxation behavior and crystallization evolution of Zr_(20) Cu_(20) Ni_(20) Ti_(20) Hf_(20) high entropy bulk metallic glass(HEBMG) was investigated. We selected Al and Nb elements as minor elements, which led to the negative and positive effects on the heat of mixing in the master HEBMG composition, respectively. According to the results, both elements intensified β relaxation in the structure; however, α relaxation remained stable. By using different frequencies in dynamic mechanical analysis, it was revealed that the activation energy of β relaxation for the Nb-added sample was much higher, which was due to the creation of significant structural heterogeneity under the microalloying process. Moreover, it was found that Nb addition led to a diversity in crystallization stages at the supercooled liquid region.It was suggested that the severe structural heterogeneity in the Nb-added sample provided multiple energy-level sites in the structure for enhancing the crystallization stages.     

Study of structural transition of nickel metal under temperature

A molecular dynamics (MD) simulation study has been performed to investigate the structural transition of nickel metal using Pak–Doyama potential. The local atomic structure was analyzed through the radial distribution function (RDF), coordination number distribution, simplex statistics and three-dimensional visualization. It was shown that the splitting of the second peak of RDF appears when the liquid transforms to amorphous solid. This feature originated from the transformation of simplexes from strongly to weakly distorted tetrahedron type. We found that the liquid state contains a significant number of nanocrystal nuclei which strongly depend on the temperature and MD time steps. Accordingly, the simulation shows that the crystallization originated from expanding of nanocrystal nucleation process. The thermodynamic properties are also estimated through the dependence of mean-squared displacement on the MD time steps.     

Ca70Mg30金属玻璃形成过程热力学、 动力学和结构特性转变机理的模拟研究

采用分子动力学方法对Ca₇₀Mg₃₀合金快速凝固玻璃形成过程进行了计算机模拟, 深入分析了液-固玻璃转变过程热力学、 动力学和结构特性的转变机理, 对不同方法所确立的玻璃转变温度之间的关系进行了探讨. 结果表明: 本模拟计算所获得的Ca₇₀Mg₃₀金属玻璃的结构因子和玻璃转变温度均与实验结果符合, 而且二十面体局域结构对Ca₇₀Mg₃₀金属玻璃的形成起决定性作用. 由于周围原子形成的瞬时"笼子效应", 过冷液体动力学特性逐渐偏离Arrhenius规律而满足模态耦合理论的幂指数规律. 动力学玻璃转变温度接近于微观结构玻璃转变温度, 但高于热力学玻璃转变温度; 而且它们与理想动力学玻璃转变温度之间满足Odagaki关系.     

Ice crystallization induced by optical breakdown

Ice crystallization in supercooled water has been initiated by focused Nd:YAG laser pulses at 1064 nm wavelength. The pulses of 8 ns duration and up to 2 mJ energy produce a bubble in the supercooled liquid after optical breakdown and plasma formation. The subsequent collapse and disintegration of the bubble into fragments was observed to be followed by ice crystal nucleation in many, but not all cases. Details of the crystallization events have been investigated by high-speed imaging, and nucleation statistics and crystal growth rates are given. It is argued that homogeneous nucleation in the compressed liquid phase is a plausible explanation of the effect.     

Oxidation of Zr 55 Cu 30 Al 10 Ni 5 bulk metallic glass in the glassy state and the supercooled liquid state

The oxidation behavior of Zr₅₅Cu₃₀Al₁₀Ni₅ bulk metallic glass in air in the glassy state and the supercooled liquid state was studied using a thermogravimetric analyzer, X-ray diffraction and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. It was found that the isothermal oxidation kinetics of the glass in both states follows a two-step parabolic law. The oxidation process is governed by the inward diffusion of oxygen and the outward diffusion of Cu, with the first being dominant. The faster diffusion of atoms in the supercooled liquid state led to a network precipitation of crystalline Cu, and the crystallization that occurred in this state in the later stage of oxidation caused a reduction in the rate of oxidation. Two types of Zr oxides, i.e. t-ZrO₂ and m-ZrO₂, were formed in the oxidation process in both the glassy and supercooled liquid states. t-ZrO₂ mainly formed in the outer layer of the oxide scale, while m-ZrO₂ tended to form in the inner layer. The formation of m-ZrO₂ is possibly activated by the crystallization of the glass near the interface of the oxide scale and the substrate alloy. In addition, oxidation also has a substantial effect on the formation of crystallized phases. The formation of a Cu-rich phase of Cu₁₀Zr₇ occurred in the oxidizing atmosphere. However, the formation of a Zr-rich phase of Zr₂(Ni, Cu) mainly took place in a vacuum environment. PACS 81.05.Kf; 81.65.Mq; 64.60.-i