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To deeply understand the formation mechanism of a critical nucleus during the nucleation process of liquid metal sodium, a system consisting of 10 000 Na atoms has been simulated by using molecular dynamics method. The evolutions of nuclei are traced directly, adopting the cluster-type index method. It is found that the energies of clusters and their geometrical constraints interplay to form the favorable microstructures during the nucleation process. The nucleus can be formed through many different pathways, and the critical size of the nucleus would be different for each pathway. It is also found that the critical nucleus is nonspherical and may include some metastable structures. Furthermore, the size of the cluster and its internal structure both play a crucial role in determining whether it is a critical nucleus, and this is in agreement with the simulations by computing the free energy of the Lennard-Jones system [D. Moroni, P. R. ten Wolde, and P. G. Bolhuis, Phys. Rev. Lett. 94, 235703 (2005)].

On the basis of the quantum Sutton-Chen potential, the rapid solidification processes of liquid silver have been studied by molecular dynamics simulation for four cooling rates. By means of several analysis methods, the competitions and transitions between microstructures during the cooling processes have been analyzed intensively. It is found that there are two phase transitions in all simulation processes. The first one is from liquid state to metastable (transitional) body-centered cubic (bcc) phase. The initial crystallization temperature T(ic) increases with the decrease of the cooling rate. The second one is from the transitional bcc phase to the final solid phase. This study validates the Ostwald's step rule and provides evidence for the prediction that the metastable bcc phase forms first from liquid. Further analyses reveal that the final solid at 273 K can be a mixture of hexagonal close-packed (hcp) and face-centered cubic (fcc) microstructures with various proportions of the two, and the slower the cooling rate is, the higher proportion the fcc structure occupies.

A molecular dynamics simulation has been performed on the rapid quenching processes of Ca7Mg3 alloy including 100 000 atoms. The structures of short-range order (SRO) and medium-range order (MRO) in Ca7Mg3 metallic glass are investigated by means of several structural analysis methods. It is found that the SRO in Ca7Mg3 metallic glass can be modeled by neither a uniquely prescribed stereo-chemical structure nor five Bernal polyhedra but rather various types of basic clusters in which the icosahedron is dominant. The local energy together with the geometrical constraint plays very important roles in the favorable local structure in metal glasses. The MRO in Ca7Mg3 metallic glass is characterized by certain types of extended icosahedral clusters combined by intercross-sharing atoms in the form of chains or dendrites, which is different from the fcc or icosahedral building schemes for the MRO in metallic glasses with significant chemical SRO. The size distributions of these MRO clusters present a magic number sequence of 19, 23, 25, 27, 29, 31, 33, 35, 37, 41, 43,…, and the magic clusters can be classed into three types according to their compactness.

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