Simulation Of Molecular Dynamics Of Nucleation And Solid/Liquid Interface Energy Of Metals

The solid/liquid interface energy is an important physical parameter describing nucleation and growth process in solidification theory. It plays a central role in the determination of nucleation rate, growth rate and the morphologies of crystals. In order to comprehend the essences of the nucleation and the growth of crystal, it is necessary to achieve a further understanding of the solid/liquid interface energy. The present dissertation systematically carried out investigations on the undercooling and the solid/liquid interface energies of Ag, Cu and Ni using both experimental and simulation method. Additionally, the correlation between the subcritical nuclei and the crystal nuclei and the heterogeneous nucleation of Ni50Cu50 alloy has also been investigated. The main results and contributions of the paper are as follows:By means of combination method of melted glass denucleation and cyclical superheating, we have investigated the undercooling behavior of silver, copper and nickel. The maximum undercooling obtained for these metals are 263K,257K and 340K, respectively. From these undercoolings, the corresponding interface energy are predicted to be 0.1388,0.1939 and 0.27631Jm-2, which all agree well with the results from the interface energy model reported. The homogeneous undercoolings of these metals have been predicted according to interface energy model reported. The homogeneous undercoolings predicted are in very good agreement with our experimental results, indicating that homogeneous nucleations have occured in these metals. Additionally, homogeneous undercoolings of Ag, Cu and Ni have also been investiagated by using atomistic simulation. We find that the simulation results are in good accordance with the prediction from the interface energy model.Molecular dynamics simulation is carried out to investigate the effects of cooling rate on the final configurations of silver after rapid solidification. The cooling rate for the formation of a silver amorphous phase is determined by analyzing its pair distribution function, H-A bond index, and the largest crystal cluster. Further, the equilibrium structures of the subcritical nuclei and crystal clusters are studied. The results show that the solidified microstructure is composed of a mixture of crystal clusters and amorphous phases at a certain cooling rate range. The size of the largest crystal cluster decreases with the increasing cooling rate, and it completely disappears when the cooling rate exceeds a critical value. The structures of the subcritical nuclei and the largest crystal cluster are composed of lamellar structures of fcc and hcp atoms, indicating that the lamellar structure of fcc and hcp atoms in; the silver crystal originates from nucleation, and not from the growth of crystals.By adopting potentials with different interacting strength between alloy and the substrate, we have realized the atomistic simulation of homogeneous and heterogeneous nucleation of supercooled Ni50Cu50 melt. The result shows that the shape of the critical nuclei found at the heterogeneous nucleation is not the spherical cap. So the wetting angle in the classical nucleation theory(CNT) can't characterize the nucleation ability of the substrate. It is found a gap 8 between the substate and the critical nuclei, with which nucleation incubation time and atom numbers in the critical nuclei decrease. So we can characterize the nucleation ability of the substrate qualitatively with the parameterδ. We also find a linear relationship between the dimensionless atom number of critical nuclei and the logarithm of dimensionless nucleation incubation time. This indicates that CNT is proven true if we use dimensionless atom number instead of wetting angle.