The Defect Trapping And Dynamics Interface Structure During The Solidification Of Simple Metals

Interface is important research direction of condensed matter physics and material science.As a typical interface,the structure and properties of solid-liquid interfaces strongly affect the crystal growth,surface infiltration,and many other phenomenon or process.Because the experiment is difficult to directly obtain the information regarding to interface structure and dynamics properties,most of theoretical studies are carried out by computer simulations.Currently the structure and properties of solid-liquid interface at equilibrium are well investigated.However the structure associated with the moving interface remains to explore.In this thesis,by using the molecular dynamics simulation,we have studied the dynamic structure of moving interface and associated defect traping.The main results include:1st.By using molecular dynamics simulations,we have studied the solidification of simple metals(Ni and Al).We found that,for both metals at certain temperature(T*)the growth velocity reaches a maximum.Above this temperature,the two metals behavior similarly,the growth rate increases monotonously with the increase of undercooling temperature(defined as the difference between the melting temperature and the system temperature).Below this temperature,they are quite different,the growth velocity of Ni is almost constant,but the growth velocity of Al decreases to zero with the increase of undercooling temperature.Through theoretical analysis,we speculate that the existence of T*may be associated with the change of growth mechanism.Combining Broughton-Gilmer-Jackson(BGJ)model and Wilson-Frenkel(W-F)model,we develop a growth model,which can describe well the growth velocity via temperature in wide range of under cooling.2nd.We characterize the dynamic structure of solid-liquid interface during solidification of Ni and Al.We found,as the increase of under cooling,the atomic density near interface presents a minimium,which is even lower than that of liquid.For(100)orientation,two-dimension(2D)structure analysis of the solid-liquid interface indicates that,above T*,a gas-like layer exists at the interface.Below T*,this layer has the character of glass.The change in 2D structure of interface may be the refection of the change in growth mechanism.3rd.Using molecular dynamics simulations,we have systematically studied the defect trapping accompaning with solidification process of Ni and Al.The results show that,only vacancy is observed,and vacancy concentration always increases with the under cooling.Vacancy concentration has a strong dependence on growth velocity,but not a single value function of it.Before the interface growth velocity reaches the maximum,defect concentration increases with interface growth velocity increases,but after the interface growth velocity reached the maximum it is on the contrary.We have also discusse the prossable connection between defect concentration and growth velocity and diffusion coefficient.Diffusion process leads to transport atom to interface,but growth process eliminates liquid atoms near the interface,the competition between them eventually leads to the generation of vacancy.4th.We have also investigated the anisotropy of the dynamics structure of solid-liquid interface and defect trapping between(100)and(110)orientation.Results show an evident anisotropy.Especially,under the deep undercooling,2D radial distribution function is significant different.For the same undercooling temperature,the growth velocity of the(100)direction is always greater than that of(110)direction,and the defect concentration of the(100)direction is greater that of the(110)direction.