Molecular Dynamics Simulation Of Solidification Characteristics And Solid-liquid Interface Structure Of Mg-Al Alloy

As 21 st century green engineering material has the most potential and application value with good chip processing performance, high strength, impact resistance and good damping properties,magnesium alloys are widely used in the aerospace materials, military, medicine and chemical industry, metal castings and automobile industry and other fields. But because of magnesium alloys have poor molding performance and poor plasticity, which restricts their application and development. However, with the continuous innovation and progress of science and technology, we have high requirement on material performance. So mainly by means of experimental research methods exist many limitations, which has been unable to meet the needs of the present stage of the research material. Therefore, through the computer simulation technology to explore new materials with excellent properties has become a trend. MD method gain popularity of materials science researchers because of it can observe the microstructure change of molten metal atom in the rapid solidification process, also has a high degree of accuracy in the calculation of macroscopic properties. In order to clarify the controversy, the solidification characteristics for Mg-Al alloy was studied in this paper, and the solid-liquid interface structure was also carried out by theoretical calculation and experimental research to reveal heterogeneous form mechanism of the solute atoms in magnesium melt.In this thesis, with the MD method, the structure evolution of the pure Mg, Mg-3Al alloy, Mg-6Al alloy and Mg-9Al alloy during solidification processes are simulated. And the RDF, PDF, MSD, total energy analysis of the system as well as VMD and Ovito MD visualization tools are used to the structure analysis. When the cooling rate is 1×1013,Mg-3Al alloy,Mg-6Al alloy and Mg-9Al alloy are all formed by the atomic arrangement of the ordered crystal structure. Except for the initial transition temperature is slightly different, Mg-3Al alloys present crystal structure characteristics at the temperature of 497 K, The liquid solid transition temperatures of Mg-6Al alloy is 647 K, and Mg-9Al alloy is 747 K. In addition, the Mg atom in Mg-Al alloy has a stronger tendency to bond with Al atom. By VMD molecular dynamics visualization analysis can be visually observed system crystallization of final system formed atomic arrangement ordered crystal structure, and with increasing aluminum content of the alloy system and system of the degree of crystallization is also improving.Using the MD method studied the Mg/Mg-3Al alloy’s solid-liquid interface structure. By means of visual software Ovito recorded the atomic positions at different moments, drawing the simulation system atomic position image, so as to qualitative analysis the arrangement of the atoms at the solid-liquid interface. The results showed that when temperature adjusted to 680 k, starting from 5000 steps, although Mg-3Al alloy atomic regions is still in a state of disorder, but close to the liquid region at the solid-liquid interface is beginning to show some order and found Al atoms show a tendency to spread. With the increase of the relaxation time, the matrix part near the solid-liquid interface of a few atomic layers there have been a certain degree of ordering, which also with a liquid attachment in the solid substrate, and Al atoms in the alloy are also in an ordered state. The system is in the phase equilibrium state after Solid-liquid coexistence system relaxation 1.2 million steps, and the atomic near the solid-liquid interface showed the obvious degree of orderUsing OM, SEM and EDS analysis test method studied the organization and composition distribution of magnesium alloy in the primary phase and eutectic phase interface, discussing Mg grains close packed plane orientation effect on eutectic growth and the distribution of Al element. The results showed that different Mg of close packed with Mg-9Al alloy formation interface of eutectic growth of not the same, and Mg-9Al alloy melt formed in pure Mg(0001)of close packed interface can promote grain growth and Al element in the liquid alloy diffusion along the solid-liquid interface and more active.