【作者】 郑采星；

【导师】 徐仲榆； 刘让苏；

【作者基本信息】 湖南大学 ， 材料学， 2001， 博士

【摘要】 本文首次完成了对由50000个原子组成的液态(943K)Al原子系统及其快速凝固为非晶态过程的分子动力学模拟，建立了液态、非晶态的多面体及团簇结构模型。并且对液态金属由快速凝固向非晶态转变过程中微观结构的演变作了跟踪模拟研究。其主要结果为： 1．液态(943K)金属Al的结构从长程范围(宏观)来看，原子排列是不规则的，而在短程范围(微观)和瞬时来看，原子按键对、多面体排列存在短程有序。(1)从键型结构来分析，系统中1551、1541和1431键型大量存在，分别约为16.8％、15.8％、21.5％，总和达到54％；分别与fcc和hcp型晶体有关的键型1421、1422约占3.4％和7.0％；与菱面体结构相关的键型1321、1311、1301的成键数之和约为14％；bcc型晶体中大量出现的1441、1661键型则分别为4.8％和5.1％；近于游离态的键型1201、1211等键数之和约为13％。各种成键数之和在943K时约为95％；这说明系统在液态时，其真正处于游离状态的原子数还不到5％，即95％以上的原子仍然处在各种强弱不同的有序键合结构中。(2)系统中1551型与1441和1661型键按照欧拉多面体几何公式的要求组成了各种封闭的原子多面体结构，其中相当大的部分又以短程有序的(120120)型二十面体的形式存在，其数目在943k时为186，作为其缺陷结构，在二十面体的基体中还有许多不规则排列的具有配位数为12、13、14、15、16的三角面多面体，总数在943k时为462。(3)多面体外围原子还可能按一定的键型与邻近原子键合，或者各类原子多面体相互结合形成以原子多面体为核心的团簇结构。(4)液态金属的结构从微观上看，既有原子聚集的密集区即密排团簇结构：又有与游离态相一致的原子分布的稀疏区。(5)由于热力学涨落的作用，以上各数据略有起伏；多面体与团簇结构也不稳定。密集区与稀疏区的边界和大小随时都在改变。 2．系统在由液态Al快速凝固形成非晶态的过程中，经历了从液态、过冷液态到固态的转变，液态结构中出现的各种键型和原子多面体在快速凝固过程中具有不同的演变趋势。(1)当系统的温度下降到300K时，1551键型的成键数显著增加，其增幅竟达到75％，1541键型的数目变化不大，其增幅约为5.4％，而1431键型却减少了3.8％，三者共计占有总成键数的67％；与fcc和 摘要呻型晶脯关的键型1421、1422呈现下降趋势，两种键型的总成键数为7470；与菱面体结构相关的1321、1311键型的总成键数亦呈删趋势，总成键数为9．3N：与ho型晶脯关的IMI、lfol键型，则分别为3．在孤和SWh；近于游离态的键型lffil，1211的键数明显地渺，仅占1．6％；各种键型总数之和在3oK时接近删。从以上各种键型不同的演变趋势中可以看到：液态金属AI快速凝固形成非晶态AI的过程，不利于与各种晶体结构有关键型的生长，而大量增加的1551键型起着决定性的作用。Q）多面体结构中对系统微观结构变化起着关键作用的是用＊ 0 12 0）表示的H十面体。当温度降至3皿时，stoic个原子中的 19162个原子（含重复计数）酣 1551 键对组成了1474个二十面体；而由IMI，1551和lfol键的组合构成的一系列缺陷二十面体的总数也大幅增加至1392，即有2026个原于（含重复计数）舶于各种有缺陷的h十面体结构中。u）aM的p巳热力学起伏作用iE＃Jf＆gg，以原子多面体为核心的团簇结构尺寸加大，多面体与团簇结构趋于稳定。 N）由于非晶态中1551键的数目比其在液态中的数目提高了75％。并且有序度最高的二十面体和各种二十面体的缺陷结构，与液态金属相比，其数量分别增加了砌W和ZOyK）。因此非晶态的短椭序范围应该大于液态金属的短椭序范围。 3．快速凝固形成的多面脓内部空间间隙大于吐晶体中八面体空隙和四面体空隙，据此可以解释非晶态金属M比晶体M固溶度增加的实验事实。 4．根据二十面体数目随时间演变的关系曲线中所出现的转折点T，有可能得到一种确定金属玻璃转变温度＼的新方法。 5．各种有缺陷的原子多面体有同分异构赐，据此可以说明非晶态是gs的亚稳态。 6．系统中的大团簇结构是由小团簇（它由多个原子多面体结合组成）相互连接而成，而不是以某一个原子为中心按一定规则狮多个壳层。然而，这正是在腿凝固过程中所形成的团簇结构的特点，也是与由气相沉积、离子溅射等方法所获得的团簇结构的本质差别所在。 己提出的几种非晶态结构模型，并不能完全说明非晶态的微观结构和原子排列的细节。本文采用分子动力学模拟、键对分析方法、原子多面体及团簇结 I二 ／ ＞ 摘要构的概念剖析了非晶态AI的删结构及原子排列的细节?更多还原

【Abstract】 The molecular dynamics computer simulation study of Al liquid state (943 K) system constituted of 50,000 atoms and the rapid cooling process from liquid to amorphous state was first completed. The polyhedrals and cluster structure model of liquid state and amorphous state was established to study the change of the microstructure during the rapid solidification from liquid state to amorphous state. The following are the main results: 1. Though the atomic arrangement in liquid state (943K) Al is out of order in a long rang, yet it is instantaneously orderly in a short range, because bonded atoms construct polyhedrals. (1) The system of liquid state (943) Al abounds in the following bond types: 1551,1541 and 1431. The percentages of them are 16.8%, 15.2% and 21.5% respectively. The sum of them reaches 54%. The amounts of bond types 1421 and 1422 that relate to the fcc and hcp crystals are about 3.4% and 7.0% respectively. The amount of bond types 1321,1311 and 1301 that relate to the rhombohedron is about 14%. The amounts of bond types 1441 and 1661 that exist in bcc crystals are about 4.8% and 5.1% respectively. The sum of nearly ionized state bond types 1201 and 1211 is about 1.3%. The sum of various bond types is about 95% at 943K. It means that the amount of atoms that exist in nearly ionized state is less than 5%. Atoms above 95% exist in various structures with strong or weak orderly bonds in the system. (2) Bonds 1551,1441 and 1661 construct various closed polyhedral structures according to the requirements of Euler?polyhedral formula. The most of them exists in the short-range orderly form ?the icosahedron type (12 0 12 0). The number of them is 186 at 943K. As the defective structure of icosahedron, there are many atomic triangular polyhedrals that arrange irregularly with coordination number 12, 13, 14, 15 and 16. The total number of them is 462 at 943K. (3) The surrounding atoms in polyhedrals can bond with the neighboring atoms according to certain types. or the polyhedrals combine one another to form a cluster structure with one polyhedral as the nucleus. (4) In view of microstructure, there are dense regions of atomic aggregates and rare regions of ionized atoms. (5) The above data fluctuate due to the thermodnamic fluctuation. The structures of polyhedrals and clusters are unstable. The boundaries and the sizes of dense regions and rare regions are alwa s changing. 2. During the rapid solidification of liquid Al, the system undergoes a series of state transformations: liquid state, super cooling liquid state and solid state. Different the bond types and polyhedrals appeared in the liquid structure have different transformation tendency. (1) When the temperature of the liquid system decreases to 3 00K, the number of bond type 1551 increases 75%; the number of bond type 1541 increases only about 5.4%; but the number of bond type 1431 decreases 3.8%. The sum of these three types is 67% of the total number of various bond types. The number of bond types 1421 and 1422 that relate to fcc and hcp crystals decreases. The total number of them is 7.4% at 300K. The total number of bond types 1331 and 1311 that relate to rhombohedral structure decreases. The total number of them is 9.3% at 300K. The numbers of bond type 1441 and 1661 are 3.85% and 5.54% respectively. The numbers of bond type 1201 and 1211 that are nearly ionized decreases to 1.67%. The total number of various bond types is about to 98% at 3 00K. From the above transformation tendency, we can see that the rapid co更多还原