过冷液体成核是材料科学领域极为重要的相变现象，过冷液体结构及其演化过程对于成核速率，材料的凝固组织乃至性能有十分重要的影响。本文研究了过冷液体的结构分析法，并利用分子动力学（MD）模拟了Lennard-Jones（LJ）流体的过冷液态结构及其在成核过程中的演化。对一些平衡晶体和亚稳液体的分子构型，分别使用局域结构序参量法，键对分析法和Voronoi分析法进行结构分析从而评估它们的识别能力。结果表明基于理想晶格结构序参量的理论值法在某些情况下会造成对体心立方（bcc）结构的误判，而键对分析法则依赖于近邻分子的定义。设计改进的迭代方案进一步修正了受扰变形的Voronoi多面体，从而改进了Voronoi分析法的结构识别能力。利用等温等压（NPT）系综MD模拟了一定温度范围内的过冷LJ液体结构随温度的变化关系。发现在此温度区，液体的亚稳结构与冷却速率无关，固有结构与冷却速率和温度都无关，液体结构随温度的变化不会对成核规律造成本质的影响。模拟结果还表明正二十面体（ico）结构在熔点以上的液体中就已经存在，并随温度的降低而增加。采用NPT系综MD模拟了温度处于上述区间内的LJ液体成核过程，并借助改进的Voronoi分析法追踪了初生相结构的演化。结果为ico短程序阻碍晶体成核提供了直接证据和详细图像，ico结构在空间上与晶核相互排挤，竞争生长，在时间上需要重组转化为其他结构形式才能参与晶体成核。研究证明晶体团簇中存在厚重的界面层，其结构介于外部的液体和内部的晶体之间。在所研究的过冷度下，多数晶体团簇首先形成的是面心立方（fcc）和密排六方（hcp）结构，当生长至一定阶段时，部分分子开始经过bcc结构成核，从而在团簇中出现不完整的bcc中间层。这体现出fcc，hcp和bcc结构成核的自由能垒相差不大，从而同时生长的复杂动力学规律，能较好地解释较浅和较深过冷度下结构演化的不同规律。模拟观察到团簇在生长过程中会以hcp面为交界，多次孪生形成具有五重轴的多取向晶核。论文还分析了五重轴上分子的结构，并从热力学和结构方面讨论了其形成机制。
Crystal nucleation is an important phenomenon in material science and technology. Liquid structure and its evolution are commonly believed to determine the solidification structure and performance of the products. With molecular dynamics (MD) simulation method, liquid structure and its evolution during nucleation in Lennard-Jones (LJ) supercooled liquids are studied in the present thesis.Three widely used methods for analyzing atomic structures are evaluated in crystalline solids and supercooled liquids. The local order parameter approach based on theoritical values of perfect crystals fails in some perturbed body-centered-cubic (bcc) environments, while the pair analysis method behaves approximate depending on the definition of the pairs. An improved procedure is designed to eliminate distorted Voronoi faces and edges in the Voronoi analysis method.Structures of the LJ liquids at different temperatures are simulated in isothermal- isobaric (NPT) ensembles. In the simulated states, the metastable structures are independent of cooling rates, and the inherent structures keep identical at different temperatures, thus the temperature effect of liquid structure seems not to alter the intrinsic nucleation rules. The icosaheral (ico) structures already exist in the liquids upon the melting points, and increase with the decrease of the temperature.The structure evolution during nucleation is traced in the proceeding studies. The ico structures compete with the crystalline clusters in space, and must be rearranged before they nucleated, therefore obstruct LJ liquid from crystal nucleation. The initial nucleated crystal is surrounded by heavy interface, and contains more face-centered-cubic (fcc) and hexagonal-close-packed (hcp) particles than bcc. However, the bcc structures grow during an intermediate stage and form an incomplete meso-layer in the clusters. This indicates a complicated dynamics in which many structures may grow simultaneously because of the comparable energy barriers between nucleation of these structures, and connects the differences between nucleation at lower and deeper undercoolings. The nucleated crystal forms multiply twinned particles with fivefold axes, bounded by hcp planes. The structures of the fivefold axes are characterized, and the discussion of their formation mechanism is also included.
