基于临界晶核被有效识别的快凝深过冷形核特征分析

In view of the extreme importance of nucleation and a long-standing mystery concerning crystalline nucleus in materials science, the critical nucleus emerged in the rapid solidification will be detected and identified by an inverse tracking method of atom trajectories, and the unique nucleation features under deep super-cooling will be also investigated by a serials of thermodynamic calculations. In this project, a computable modeling method capable of identifying embryos and nucleuses will be proposed firstly on the basis of the heredity of medium-range orders (MROs) in super-cooled liquid during the rapid solidification. And then a huge-scale molecular dynamics simulation (MD) for the rapid solidification of liquid metal silvers will be performed on the Tianhe-I supercomputer system situated in Hunan University. Subsequently, the research will focus: (1) to determine the critical size, atomic configuration and geometry as well as interfacial morphology of fcc-Ag mono-crystalline, poly-crystalline, hybrid-crystalline nucleuses under various super-cooled degrees in terms of the onset state of inheritable MROs at different cooling rates. (2) to explore the nucleation trigger mechanism of fcc phases and their nucleation competition with hcp and bcc as well as icosahedrons on the basis of a careful analysis for pregnant behaviors of numerous embryos and various nucleation paths from embryos to nucleuses with the help of first-principles calculation. (3) to investigate the impact of atomic configurations and interfacial morphologies of nucleuses on the energy barrier of homogenous nucleation by means of an exact calculation of solid-melt interfacial energies of fcc-Ag nucleuses with various configurations and interfacial morphologies. As a result, the accuracy and credibility of critical nucleuses identified by this project can be evaluated by experimentally measured homogenous nucleation rates, and some crucial factors associated with the energy barrier of homogenous nucleation under deep super-cooling will be also discovered.

鉴于形核在材料科学研究中的极端重要性，针对凝固晶核难识别这一具体科学问题与技术难题，通过完善过冷液体中中程序的结构表征方法及其遗传性跟踪技术，发展一种可识别和标度凝固晶胚晶核的团簇分析方法，并应用于快速凝固临界晶核的识别与形核特征分析。其中，超大尺寸体系液态金属凝固过程的MD模拟将在国家超算(长沙)中心天河I号上完成。主要研究内容为：① 选取激光焊料Ag为研究对象, 通过对快凝期间各种可遗传扩展团簇原子轨迹的连续逆向追踪，标度并区分fcc-Ag单晶/多晶/混晶晶核的临界尺寸、几何构型与界面形态。② 通过对不同类型晶胚孕育行为与形核路径的差异性分析，结合第一原理计算，研究深过冷下fcc相的形核触发机制及其与亚稳和非平衡相的形核竞争。③ 基于fcc单晶/多晶/混晶晶核的几何构型与界面形态，计算其与熔体原子间的液固界面能；然后，以文献报道的均匀形核速率为参照，检验模拟结果的精确度与可信性。

基于液态金属（团簇）在快凝过程中所呈现的（瞬态与连续）结构遗传性，通过完善过冷液体中fcc单晶团簇的结构表征方法及其遗传性跟踪技术，发展了一种可识别和标度晶胚与晶核的团簇分析方法，解决了凝固晶核难以识别的技术难题，并据此表征了快凝fcc-Al单晶晶核的几何构型、临界尺寸与界面形态，初步揭示了快速凝固的非平衡形核特征。结果表明：在∆T=0.42Tm时，fcc-Al单晶核的临界尺寸n*随冷凝温度T下降而减小，平均尺寸约为26个原子，芯部几何呈（非球）层状构型，液固界面为fcc/hcp/liquid多相结构；晶胚与晶核的生长除通过单原子吸附外，还存在不同晶胚的合并与分解；初始形核出现在热力学相变温度Tm之前，但形核率I(T)在0.45Tm时达到最大。应用于不同过冷度∆T下金属溶体的等温晶化分析，通过被识别临界晶核最初出现时间（即晶体形核时间），确定了Al熔体的均匀形核极限（即热力学调幅分解温度）为0.45Tm，以及形核能垒消失时对应的旋节点温度0.51Tm，并且证实Stokes-Einstein关系破裂不是体系达到均匀形核极限的必要条件；进一步基于对晶胚孕育与形核路径的仔细分析，定义了临界晶核的形核孕育时间τc，即τc包含晶胚孕育τe和晶胚生长τg两部分，揭示晶核区别于晶胚的主要特征是团簇瞬态可遗传时间成为了有效生长时间，并且发现：相对于从稳态形核速率I(T)导出的形核诱导时间τ，晶体形核孕育时间τc普遍很小，但全体临界晶核的平均形核孕育时间τc(T)与稳态形核率I(T)之间呈镜像对称。在∆T≈0.362Tm时，平均形核孕育时间τc(T)最小，稳态形核率I(T)最大。