Molecular Dynamics Study On The Structure And Properties Of Solid-liquid Interface In Mg-Al Binary Alloy Melt

Magnesium and its alloys,which are called"green engineering metal materials in twenty-first Century",are promising to be applied in aerospace,military,and automotive industries,etc.However,the further development of magnesium alloys is also restricted owing to its drawbacks of bad ductility.In various processes,the control of solidification is one of the very important process to overcome the bad ductility and enhance the overall performance of magnesium alloys.In recent years,computer simulations including molecular dynamics(MD)method gradually become an important approach in the materials science research.The study on the solid-liquid interface of metal melts with molecular dynamics method can helps us deeply understand the mechanism of nucleation and crystal growth in the solidification process,and learn about those important information which cannot be obtained in the experiment.In this work,MD method was employed to investigate the structure and characteristics of the solid-liquid interfaces between pure Mg,Mg-3Al crystal and the Mg/Mg-xAl(x=3,6)melt.Firstly,the structure and characteristics of solid-liquid interfaces based on different crystal orientations in Mg-3Al alloy were investigated by using the MD method based on the EAM potential function.The melting point of Mg-3Al alloy was calculated.And the structure and characteristics of solid-liquid interfaces based on three lattice planes—(0001)、(~—1010)、(~—12~—10)were analyzed with characterization methods such as number density distribution,mean square displacement,diffusion coefficient,and two-dimensional radial distribution function.It is found that the three different solid-liquid interfaces of Mg-3Al alloy are rough interface,there are transition regions between solid phase and liquid phase,the characteristics of which are are different from and affected by the solid phase and liquid phase.The number density distribution,which is found to be anisotropy,along the normal vector perpendicular to the interface is analyzed.Moreover,the diffusion coefficient of the atomic layer near the solid-liquid interface in three directions is also anisotropy.and the diffusion in different directions at the same interface is also anisotropy,which is mainly parallel to the two-dimensional diffusion of the interface.According to two-dimensional radial distribution function and visual analysis,it can be found that the transition from solid phase to liquid phase occurs at several layers near the interface.And the two-dimensional radial distribution shows a gradual change from long-range order to short range order and long range disorder.Secondly,we studied the solid-liquid interfacial structure and properties of Mg-xAl(x=3,6)based pure Mg solid phase.A solid-liquid interface model was constructed,in which pure Mg crystal,Mg-3Al or Mg-6Al melt were selected as the solid phase and liquid phase,respectively,and a normal vector perpendicular to the(0001)plane was preset.The diffusion of atoms near the interface were investigated by using molecular dynamics method and characterization method such as density distribution,diffusion coefficient,radial distribution function and visualization analysis.Result shows that the melting point of Mg-6Al alloy is lower than that of Mg-3Al alloy,indicating the calculating result is match well with the fact.The analysis of the number density distributions of the two systems revealed that the number density distributions of the two systems are approximately the same.The Mg atoms in the solid phase near the interface will displace with the Mg atoms and Al atoms in the liquid phase.The concentration of Al atoms in the phase has a certain influence on the diffusion of Mg atoms,and the increase of the concentration of Al atoms will weaken the substitutional diffusion of Mg atoms to a certain extent.The analysis of the diffusion coefficient of the atomic layer and the mean square displacement of different atoms shows that from the solid phase to the liquid phase,the diffusion coefficient shows a monotonous increasing trend,the diffusion coefficient of Al atoms is larger,and the diffusion of Mg atoms is more regularity.The result by calculating the two-dimensional radial distribution function of the two systems shows that the degree of order of the crystal will cause a certain transformation of the liquid phase,and the degree of order from the solid phase to the liquid phase will gradually decrease,and the concentration of Al atoms may affect the number of interface transition layer.Through the visual analysis of several layers of atoms in the vicinity of the interface,the atom composition of the atomic layer shows that Mg atoms in the solid phase will undergo displacement-type diffusion with Mg atoms and Al atoms in the liquid phase.and the number of atomic transition layer is not the same at the solid-liquid interface composed of different liquid phases.A visual analysis of different moments in the same transitional atomic layer reveals that the concentration and position of different atoms in the atomic layer change with time,but overall it maintains the structural characteristics of the crystal.