论文总字数：26208字

摘 要

非晶合金的原子结构是揭示玻璃转变本质和理解其优异物理、化学和力学性能的基础，因而成为非晶领域的一个基本科学问题。同时非晶态处于亚稳态，在外界作用下，非晶合金会向相应晶态发生转变，形成性能优异的非晶/纳米晶。因此，研究非晶Cu形成及晶化过程的结构演变具有重要意义。

本论文采用分子动力学模拟方法，利用势能、体积、径向分布函数、键对分析、Voronoi多面体、瞬时构型、均方位移等结构、热力学和动力学方法来研究晶体Cu的熔化过程、熔体Cu的冷却过程和非晶Cu的等温晶化过程，进而揭示非晶Cu形成及晶化过程无序到有序的结构演变。

研究表明，冷速是影响熔体Cu凝固过程中结构转变及室温结构的重要因素，快速冷却形成非晶，慢速冷却形成晶体，形成非晶的临界冷却速率为6×10¹² K/s且玻璃转变温度随冷速的增大而升高。径向分布函数和键对分析表明，冷却速率对非晶Cu室温结构影响不大。非晶Cu形成过程中具有五次对称的二十面体结构在非晶和液态中占主导地位，且随着温度的降低二十面体含量逐渐升高。Voronoi多面体分析表明，非晶Cu中二十面体短程序占优势，二十面体团簇通过共点、共线、共面、嵌套四种连接方式扩展成二十面体中程序。非晶Cu纳米晶化过程中局域原子结构无序到有序的演变规律为长程无序结构调整原子位置首先形成具有一维周期的准有序结构，然后形成二维有序结构，最终形成晶体的三维有序排列。

关键字：非晶Cu 凝固过程 非晶晶化 分子动力学模拟

Structural evolution of amorphous Cu during the progress of glass formation and crystallization by molecular dynamics simulations

Abstract

The atomic structure is the basis to reveal the nature of glass transition and to understand their excellent physical, chemical and mechanical properties of amorphous alloys. Therefore, the amorphous structure is one of the most fundamental scientific problems in the field of amorphous alloy. Meanwhile, the amorphous alloy is metastable state thus the amorphous state can transform to the corresponding amorphous/nanocrystal composite with excellent properties under external effect. Therefore, it is very important to investigate the glass formation and crystallization of amorphous Cu.

In this thesis, the structural evolution during the crystal to liquid state for metal Cu, the liquid-solid transition for melting Cu and the crystallization for amorphous Cu have been systematically investigated by molecular dynamics (MD) simulations. The goal of this work is to reveal the rule of structural evolution during glass formation and crystallization of amorphous. The structural variation is characterized by various structural, thermodynamics and kinetic means of potential energy, molar volume, radial distribution function (RDF), bond-pair analysis, Voronoi tessellation analysis, atomic instantaneous configuration, mean square displacement.

The results indicate that the cooling rate plays a critical role in final solidification microstructures of the liquid Cu. The characteristics of liquid-to-glass phase transformation occur at the fast cooling rate, while the liquid transform to crystal Cu at the slow cooling rate. The critical cooling rate to form the amorphous state is determined to be about 6×10¹² K/s. Furthermore, the glass transition temperature increases with the increasing cooling rate.

The effect of cooling rate on the microstructure of amorphous Cu is small investigated by RDF and bond pair analysis methods. Furthermore, the result of bond-pair analysis demonstrates that the icosahedral short-range order (SRO) is dominant in amorphous and liquid state during the glass formation and the fraction of icosahedron increase during the liquid-solid transition. The Voronoi polyhedra analysis showed that the icosahedral-like with five-fold symmetry for is predominant in glassy Cu. Moreover, the icosahedral medium-range order (IMRO) was made up by ISRO via the linkage of vertex-, edge-, face-, and inter-crossed-shared atoms. The structural evolution from the disordered structure to crystal can be divided the following steps: a one dimension (1D) periodic structure is constructed from long-range disordered structure, and then a 2D periodic structure forms; finally a 3D periodic crystal structure develops.

Key words: Amorphous Cu; Solidification; Crystallization; Molecular dynamics simulation

目录

摘要 I

Abstract II

第一章 文献综述 1

1.1非晶合金的形成理论 1

1.1.1 非晶合金形成热力学 1

1.1.2非晶形成动力学 2

1.2 非晶合金的结构 3

1.2.1 非晶合金的结构特点 3

1.2.2 非晶合金的结构模型 4

1.3 非晶纳米晶形核机制 5

1.4非晶的晶化类型 6

1.4.1 多形型晶化 6

1.4.2 共晶型晶化 6

1.4.3 初晶型晶化 7

1.5 分子动力学在液体和非晶中的应用 7

1.5.1 金属液态结构 7

1.5.2高压相稳定性预测 7

1.5.3 凝固及玻璃化转变的模拟 7

1.6 研究目的与意义 7

第二章 分子动力学模拟的原理及表征手段 9

2.1 分子动力学模拟的基本原理 9

2.1.1 紧束缚势函数 9

2.1.2 系踪 10

2.1.3 分子动力学模拟的控温控压技术 10

2.1.4 积分步长 10

2.2 分子动力学模拟的结构和动力学分析技术 11

2.2.1 径向分布函数 11

2.2.2 配位数 11

2.2.3 键对分析 11

2.2.4 Voronoi多面体 11

2.2.5 均方位移 12

2.3 计算参数设置 12

2.4 研究内容 13

第三章 计算结果与数据分析 14

3.1 熔体Cu结构的获得 14

3.2 冷却速率对室温结构的影响 16

3.3冷却速率对玻璃转化温度的影响 19

3.4 非晶Cu形成过程中的结构演变 19

3.5非晶Cu的中短程有序结构 22

3.6非晶Cu等温晶化过程的结构演变 24

第四章 结论及展望 27

4.1 结论 27

4.2 展望 27

参考文献 28

致谢 30

第一章 文献综述

非晶合金（或金属玻璃）是近几十年来采用现代快速冷凝冶金技术获得的性能独特的新型结构材料，非晶态是指物质内部结构中原子呈长程无序排列的一种状态^[1]。非晶态既可以由熔融（液态）物质在冷却过程中不发生结晶而形成，也可以由物质原子通过气相沉积、离子束混合、机械合金化或强变形等方法获得。非晶态合金或金属玻璃与传统氧化物玻璃不同，合金中原子的结合是金属键，而不是共价键，所以许多与金属相关的特性被保留下来，非晶合金由于其长程无序而短程有序的结构特征使其具有一系列优异的性能。例如，非晶强度、硬度高，弹性应变极限高，具有良好的耐腐蚀性和耐磨性，此外还具有优异的软磁性能。从某种意义上讲，非晶态结构是无缺陷的，而不是像晶体材料那样有位错和晶界等。本课题所指的非晶合金指的是通过熔体快速冷凝方法得到的非晶材料。

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