Simulation Study On Evolution And Description Of Microstructures During Solidification Processes Of Liquid Metal Sodium

In this thesis, with the molecular dynamics method, the solidification processes of liquid metal Na under different conditions are simulated. By means of the different microstructural description methods of the pair distribution function, bond-type index method, and cluster-type index method (CTIM), and the technique of tracing cluster based on the CTIM, the formation properties and evolution mechanisms of microstructures during the solidification processes of liquid metal under different conditions are deeply studied. According to the computer simulation results, some experimental phenomenons and solidification theories are explained on microstructural level.The solidification processes of liquid metal Na under different cooling rates and initial melt temperatures are simulated. The effects of different thermal history conditions on the microstructures during the solidification processes are investigated. The results show that the cooling rate plays a critical role in the transitions of microstructures. When the cooling rates are 1×10¹⁴ K/s and 1×10¹³ K/s, the amorphous structures are formed mainly with the icosahedron basic cluster and the defective icosahedron basic cluster. When the cooling rates are 1×10¹² K/s and 1×10¹¹K/s, the bcc crystal structures are formed. The critical cooling rate for the formation of amorphous Na is about 10¹³ K/s. The initial melt temperature evidently affects the crystallinity of solidification structures. The influence degree is not linearly varying with the decrease of initial melt temperature, and has the upper and lower limits. It is still demonstrated that the effects of different cooling rates and initial melt temperatures on the microstructures of metal Na are very small above the liquid-solid transition temperature (T_g and T_c), but they are fully displayed near the liquid-solid transition points. According to this feature, it possibly provides a new method to determine the T_g and T_c.The rapid solidification process of liquid metal Na with 50000 atoms is simulated. The formation properties and evolution mechanismes of cluster structures during the rapid solidification process are investigated. The results show that during the rapid solidification process of liquid metal Na and Al, there are some similar in their evolution features of cluster structures, meanwhile many differences. The CTIM can distinguish these differences more exactly than the H-A bond-type index method. The validity of CTIM in describing the cluster structures is confirmed, and it provides a new method of studing the formation properties of clusters during the solidification process. The formation and evolution of nano-clusters in the rapid solidification process have undergone a complicated evolution process: the small unstable cluster is formed in the liquid, through a middle-cluster in the supercooled liquid, and finally the nano-cluster is formed by combing several middle-clusters after T_g. The nano-clusters are formed more easily in liquid metal Na than in Al system, and the configurations of nano-clusters are also obviously different from those obtained by gaseous deposition, ionic spray methods and so on.The crystallization process during the solidification of liquid metal Na with 10000 atoms is simulated. The formation mechanismes of crystal nuclei are investigated by tracing the evolution of clusters. The results show that during the crystallization process of liquid metal Na, the microstructures transform from the icosahedron or defective icosahedron structure in the supercooled liquid, through the defective bcc structure near the crystallization temperature, and finally to the perfect bcc structure. The energies of clusters and their geometrical constraints interplay the favorable microstructures during the nucleation process. The formation of nucleus may go along many different pathways. The size of cluster and its internal structure both play a crucial role in determining whether it is a critical nucleus. It is also found that the critical nucleus is non-spherical and may include some metastable structures. And the deviation of critical sizes in our simulations from the prediction of classical nucleation theory is not too much. The evolution of the volume fraction of crystal phases during the isothermal crystallization of the supercooled liquid metal Na satisfies the Johnson-Mehl-Avrami (JMA) theory. This verifes the validity of basic solidification theory from the microstructural level, on the other hand, the validity of the technique of tracing cluster based on the CTIM on studying the crystallization process is displayed.The isothermal annealing process of amorphous metal Na with 10000 atoms is simulated. The formation and growth of nuclei in amorphous Na are traced. The results show the formations of nuclei undergo the shrink of clusters with icosahedron short-range order structure firstly, and then the bcc symmetric clusters are formed gradually. Only when the sizes of bcc symmetric clusters reach a critical size, they can turn into stable nuclei. The crystallization of amorphous Na exhibits three distinct stages three distinct stages of nucleation, subsequent growth of nuclei and coarsening of crystal grains. Through comparing the results obtained by different analysis methods, it is found that the CTIM can distinguish the different stages of crystallization processes. Our simulation results provide a reasonable explanation at atomic level for the experimental phenomenon that the DSC method cannot exactly reveal the crystallization stages of some amorphous metals.