Molecular Dynamics Simulation Of Structure Evolution Of Pure Magnesium Melts During Solidification Process

In this thesis, with the molecular dynamics method, the structure evolution of the typical HCP metal Magnesium melt during solidification processes are simulated. Finnis-Sinclair many body inter-atomic potential is employed to describe inter-atomic interaction. And the radial distribution function, mean square displacement, total energy analysis of the system, H-A bond pair index method as well as VMD molecular dynamics visualization tools are used to the structural analysis. The entire analog cellular contains 500 atoms, using periodic boundary conditions and NVT ensemble, To control the constant temperature and press, Nose-Hoover method is employed.Through simulation, Mg melt's solidification process have been studied from 1293K to 293K with four different cooling rates of 5×1014K/s,5×1013K/s,1×1013K/s,1×1012K/s, according to the simulation results, we have explain a number of important experimental phenomena and solidification theory at the micro level. The results show that:the F-S potential function that my simulation selected can describe the interaction force between the Mg atoms accurately, so that other conclusions obtained by this simulation test in micro level are also trusted; the cooling rate play a key role in the microstructures transformation, as the cooling rate being 5×1014K/s, the amorphous structures would be formed with 1541 bond-types as the main body in the system, and the glass transition temperature is 693K; as the cooling rate being 5x1013K/s, although the system form a crystalline structure finally, but the degree of crystallization is very low. So, we can figure out that 5×1013K/s is Mg melt's critical cooling rate of the formation of amorphous. With the reduction of the cooling rate, the increase in coagulation time, as the cooling rates being 1×1013K/s,1×1012K/s, the system finally formed the HCP crystal structure with the increasing degree of crystallization and based on 1421 and 1422 bond-types on the main body; durning the formation of crystal structure at the cooling rates of 5×1013K/s,1×1013K/s,1×1012K/s, the crystallizing points were 543K.643K,743K respectively, that is, the slower cooling rate, the crystallization temperature is closer to the quasi-static freezing point of Mg, the higher degree of crystallinity; At various cooling rates, solidification process have emerged in the 1441 and 1661 bong-type, in the process of amorphous formation, most of 1441 and 1661 bong-types are retained, However, in the process of crystal structure formation, most of 1441 and 1661 bong-types evolve into the HCP structure;In the end, by way of VMD molecular dynamics visualization tools, we can intuitive to see that the atoms finally in a disordered state of arrangement at the cooling rate of 5×1014K/s; and at the cooling rate of 5×1013K/s, atomic arrangement is generally orderly, but the ordering is not high; at the cooling rates of 1×10 K/s,1×10 K/s, the system has a high degree of order, and the highest degree of order belongs to the cooling rate of 1×1012K/s; this proves that the pure magnesium melt finally formed disordered amorphous at the cooling rate of 5x1014K/s, and that, the pure magnesium melt ultimately form an orderly crystalline structure at the cooling rates of 5×1013K/s,1×1013K/s,1×1012K/s, moreover, with the cooling rate decreased gradually increasing degree of order.