The Structure And Kinetic Properties Of Ordered Alloy Solid-liquid Interface:a Molecular Dynamics Simulations

The properties of the solid-liquid interface is known to be fundamental important in understanding many phenomena and processes in material sciences, such as the crystal nucleation within melt, the near-equilibrium crystal growth, the interfacial solute segregation and the wetting of liquid droplets on solid substrates, etc. Due to the difficulties in performing direct measurements on solid-liquid interfaces, computer simulations, have become a mainstream in evaluating the interface properties. In this thesis, using molecular dynamics simulations, we have investigated the structure and kinetic properties of binary ordered alloy solid-liquid interface.Main results of the thesis are as following:(1) Using molecular dynamics simulations, we have investigated the structure and transport properties of solid-liquid interface for the (100),(110) and (111) interfacial orientations in the B2NiAl ordered alloy system. Our results show that all the studied interfaces are faceted in the vicinity of the melting temperature. The number density profiles exhibit evident orientation dependence. The oscillation in density is very similar to that for pure elements in NiAl(110), while for NiAl(100) and (111) the density oscillations with alternate Ni rich and Al rich keeps over a few layers into the liquid side. The two dimension structural analysis of the interfacial layers show that the transition from crystal to fluid occurs over a narrow regions of only a couple of layers and the atoms are forming ordered clusters in the interfacial layers. The diffusion constants show evident anisotropy around interface.(2) Disorder trapping associated with solidification is studied in the B2NiAl ordered alloy compound. At the high interface velocities studied we observe pronounced disorder and defect trapping, i.e., the formation of antisite defects and vacancies in the crystal at higher than equilibrium concentrations upon rapid solidification. The vacancies are located primarily on the Ni sublattice and the majority of antisite defects are Ni atoms on the A1sublattice, while the concentration of Al on the Ni sublattice is negligibly small. The defect concentration is found to increase in an approximately linear relationship with an increase of the interface velocity or undercooling. Further there is no significant anisotropy in the defect concentrations for different interface orientations. Our results also suggest that the currently available models of disorder trapping should be extended to include both antisite defects and vacancies.(3) Using molecular dynamics simulations, we have investigated the structure and kinetic properties of solid-liquid interface in a model ordered alloy. Our results show that the nature of the studied interface is faceted. Due to the coexistence of structural order and chemical order, the structure of this interface is remarkably different from heterogeneous or pure element solid-liquid interface. The number density oscillates in a complicated way along the interface normal direction, and this oscillation goes into liquid around30A. The two dimension structural analysis shows that the atoms are forming two-dimensional ordered clusters in the transition layer. The diffusion constants gradually increase from zero to a saturation value in the liquid side far from the interface. In the vicinity of the interface, the diffusion constants parallel to the interface direction are large than that along interface normal. Temperature gradients are found around the interface area during the solidification and melting process. The crystal growth from melt showed clearly the qualitative nature of the’layer by layer" growth mode for this faceted interface, namely, it is the same as the two-dimensional steps of growth of the single-element system faceted interface. The study found that the growth rate of the system is very slow, because the growth must go through two processes, first the adjustment of chemical composition, followed by the ordering of the structure. These two processes are not synchronized, which are both associated with atomic diffusion.(4) BOETTINGER and AZIZ (BA) model is the most widely used model of disorder trapping during the rapid solidification of ordered alloy, but this model has some limitations. At the high interface velocities, Our previous studies provide strong evidence that the ordered alloy will indeed produce disorder and defect trapping, in the form of antisite defects and vacancies. In this chapter we provide a model for trapping vacancies and antisite defect, which is a simple extension of BOETTINGER and AZIZ (B A) model and can be more universal description of the rapid solidification of ordered alloys systems.