中文摘要：

采用分子动力学方法对熔体初始温度热历史条件对液态金属Na凝固过程中微观结构的影响，进行了模拟研究，并采用双体分布函数g（r）曲线、键型指数法和原子团类型指数法对凝固过程中的微观结构进行了分析．结果表明：液态金属Na在不同熔体初始温度条件下以1×10^11K／s冷速凝固时，均形成晶化结构，其中1661和1441键型或体心立方基本原子团（14608）在凝固过程中对微观结构的转变起决定性作用．同时发现：熔体初始温度对凝固微结构有显著影响，而对液态和过冷态的微观结构影响并不明显，只有在晶化起始温度Tc附近才充分地展现出来．不同熔体初始温度对凝固结构的晶化程度有不同的影响，虽其影响程度是随着熔体初始温度的下降呈非线性变化关系的，但仍表明是可以通过改变熔体初始温度来加以控制的．原子团类型指数法（比键型指数法）更进一步表征了晶化体系中原子团的结构特征，将有利于对液态金属凝固过程中微观结构的转变机理进行更为深入的研究．

英文摘要：

Molecular dynamics simulation study has been performed on the effects of thermal history of initial melt temperature on microstructures during solidification of liquid metal Na. The pair distribution function g（r） curves, the bond-type index method and the cluster-type index method have been used to analyze the variations of microstructures during the solidification process. The results show that the solidification structures of liquid metal Na at the cooling rate of 1 × 10^11 K/s with different initial melt temperatures are always crystalline. The 1661 and 1441 bond-types or the bcc basic cluster （ 14 6 0 8） in the system play the critical role in the microstructure transitions. At the same time, it has been found that the effects of initial melt temperature on solidification microstructures are very remarkable, while they are not obvious in liquid and supercooled states, and the effects can be fully displayed only near the crystallization temperature Tc. The results also demonstrate that the effects of initial melt temperature on the crystallinity of solidification structures are different for different initial melt temperatures. Although the degree of influence is not linearly varying with the decrease of initial melt temperature, it still demonstrate that the influence degree can be controlled through the change of initial melt temperature. The cluster-type index method more clearly describes the cluster configurations in crystal system than the bond-type index method, so it is valuable to deeply investigate the evolution mechanisms of microstructures in liquid metal during solidification processes.