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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.

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A molecular dynamics simulation study has been performed for a large system consisting of 100000 liquid metal Al atoms to investigate the formation and magic number characteristics of the cluster configurations formed during the rapid solidification processes. The cluster-type index method (CTIM) has been adopted to describe various types of cluster configurations. The results indicate that the icosahedral clusters (12 0 12 0) and their combinations play the most important role in the microstructure transitions during solidification processes; for the cluster configurations of different levels formed by various combinations with differing numbers of basic clusters, their size distributions possess obvious magic number sequence which is in turn as 13 (13), 19(21), 26-28(27), 32-33(32), 39-40, 43-44, 48…, (those in bracket are the corresponding value in liquid state); the magic numbers correspond to the peak value positions of the cluster numbers for corresponding level formed with various combinations by 1, 2, 3, 4, 5, 6, 7, …basic clusters, respectively. This magic number sequence is in good agreement with the experimental results obtained by Harris et al. At the same time, this simulation study also gives a scientific and reasonable explanation to these experimental results.

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