冷速对液态金属Pb凝固过程中微观团簇结构演变影响的模拟研究

采用分子动力学方法对六种不同冷却速率对液态金属Pb凝固过程中微观团簇结构演变的影响进行了模拟跟踪研究.采用双体分布函数、Honeycutt-Andersen键型指数法、原子团类型指数法(CTIM-2)、平均配位数等方法对凝固过程中微观团簇结构的演变进行了分析.结果表明：系统存在一个形成非晶态或晶态结构的临界冷速(介于5×10¹²与1×10¹²之间)，大于这个临界冷速时，系统将形成以1551，1541和1431键型为主的非晶态结构；当小于这个临界冷速时，系统将先形成以1441和1661键型或以bcc基本原子团(14 6 0 8 0 0)为主的晶态结构，并稳定存在一段时间，然后又迅速转变为以1421和1422键型为主或以fcc基本原子团(12 0 0 0 12 0)和hcp基本原子团(12 0 0 0 6 6)以一定比例并存的部分晶态结构；同时发现，冷速对系统中fcc结构和hcp结构的相对比例有明显的影响，冷速越低，fcc结构所占的比例越多，越倾向于形成完美的fcc晶态结构. 关键词： 液态金属铅 凝固过程 团簇结构演变 分子动力学模拟

Simulation study of effects of cooling rate on evolution of micro-cluster structures during solidification of liquid Pb

A tracing simulation has been performed for the solidification of liquid Pb at six different cooling rates by molecular dynamics method. The pair distribution function g(r) curves, the bond-type index method of Honeycutt-Andersen(HA), the cluster-type index method and average coordination numbers have been used to analyze the evolution of micro-cluster structures during solidification processes. The results show that there is a critical cooling rate (in the range of 1×10¹³ and 5×10¹² K·s^-1) for forming amorphous or crystal structure. When the cooling rate is higher than the critical cooling rate, amorphous structures are formed mainly with the 1551,1541 and 1431 bond-types. When the cooling rate is lower than the critical cooling rate, the crystal structures mainly with the 1441 and 1661 bond-types or the bcc basic cluster (14 6 0 8 0 0) are firstly formed, and keep stable for a period of time, and then rapidly transform to the partial crystalline structure mainly with the 1421 and 1422 bond-types, or the fcc basic cluster (12 0 0 0 12 0) and the hcp basics cluster (12 0 0 0 6 6) coexisting in a certain proportion . At the same time, it has been found that there are obvious effects of the cooling rate on the relative proportion of the fcc basic clusters to the hcp basic clusters, the smaller the cooling rate is, the bigger relative proportion of the fcc basic cluster, and the system tends to form highly perfect fcc crystal structure.