冷速对液态金属Na凝固过程中微观结构影响的模拟研究

Simulation study of effects of cooling rate on microstructure of liquid metal Na during solidification processes

采用分子动力学方法对液态金属Na在四种不同冷速下的快速凝固过程进行了模拟跟踪研究.采用双体分布函数g(r)曲线、Honeycutt_Andersen键型指数法和原子团类型指数法对凝固过程中微观结构的变化进行了分析.结果表明冷却速率对微结构的转变有决定性影响,当冷速为1.0×1014和1.0×1013K/s时,系统形成以1551和1541键型或以缺陷多面体基本原子团(131102)和二十面体基本原子团(120120)为主体的非晶态结构;当冷速为1.0×1012和1.0×1011K/s时,系统则形成以1441和1661键型或以体心立方基本原子团(14608)为主体的晶态结构.同时发现不同冷速对液态金属Na在液态和过冷态时微观结构的影响甚小;但不同冷速对其固态(非晶态利晶态)时的微观结构有显著的影响,且要在液-固转变点(分别在玻璃转变温度Tg和晶化起始温度Tc)附近或以后才能充分展现出来.根据这一特点,有可能建立另一种确定液态金属Tg和Tc的新方法.原子团类型指数法比键型指数法更有利于研究液态、非晶态等无序体系和一些晶化体系的具体结构特征.

A tracing simulation study has been performed for the solidification process of liquid metal Ni with four different cooling rates by means of molecular dynamics method. The pair distribution function g（r） curves, the bond-type index method of Honeycutt- Andersen （HA） and the cluster-type index method have been used to analyze the variations of microstructures during the solidification processes. The results show that the cooling rate plays a critical role in the transitions of microstructures. When the cooling rates are 1.0 × 10^14 K/s and 1.0 × 10^13 K/s, the amorphous structures are formed mainly with the 1551 and 1541 bondtypes or polyhedron basic cluster （ 13 1 10 2） and icosahedron cluster （ 12 0 12 0） in the system. When the cooling rates are 1.0 × 10^12 K/s and 1.0 × 10^11 K/s, the crystal structures are formed mainly with 1441 and 1661 bond-types or the bee basic cluster （14 6 0 8） in the system. At the same time, it has been found that the effects of different cooling rates on the microstructures of metal Na are very small in liquid and supercooled states, however, they are very remarkable in solid （ amorphous and crystal） states, and can be fully displayed only near the liquid-solid transition points, i. e., the glass transition temperature Ts and the crystallization temperature Tc , respectively. Accordingly, it possibly provides a new method to determine the Tg and Tc of liquid metals. The cluster-type index method would be more favorable than the bond-type index method for investigating the concrete structural characteristics of the disordering liquid, the amorphous and some crystallized systems.