Study On The Structure Of Metal Melt And Solidification Behavior By Molecular Dynamic Simulation Method

This study investigates the influence of thermal history condition on diffusion coefficient of Au and Al3Mg2 by using the method of molecular dynamics simulation, as well as the influence of thermal history condition on homogeneous nucleation undercooling and microstructure evolutions for solidification of fcc-Au and fcc-Al3Mg2 metals. The results obtained are as follows:The diffusion coefficient of metal melt increases with increasing super-heated temperature, with increasing the holding time then tends to a constant. The diffusion coefficient increases with the super-heated temperature of the Au melt is heated from 1450K to 1900K. The diffusion coefficient of Au melt increases gradually with increasing the holding time. When the holding time more than 2 ps, diffusion coefficient do not changes as well as the energy, the analysis shows that the change of diffusion coefficient depends on the energy. The diffusion coefficient increases with the super-heated temperature of the Al3Mg2 melt is heated from 800K to 1000K. The diffusion coefficient of Al3Mg2 melt increases gradually with increasing holding time. When the holding time is more than 3 ps, the diffusion coefficient approaches a constant.The metal melt homogeneous nucleation undercooling increases gradually with increasing the initial temperature then tends to a constant, with increasing the cooling rate. When the Au melt solidifies at the cooling rate of 1×1011.0K/s, with increasing the super-heated temperature, the homogeneous nucleation undercooling increases gradually and tends to a constant as the initial temperature is higher than a certain value. The maximum homogeneous nucleation undercooling for Au melt reaches 406 K. When the Au melt solidifies at different cooling rates, the homogeneous nucleation undercooling increases gradually with increasing cooling rates, the result consists with the theoretical calculation. When the Al3Mg2 melt solidifies at the cooling rate of 1×1011.0K/s, the homogeneous nucleation undercooling is 61K with increasing the super-heated temperature. When the Al3Mg2 melt solidifies at different cooling rates, the homogeneous nucleation undercooling increases gradually with increasing cooling rates.The solidification structure of the metal melt is related to the cooling rate. The solidification structure is crystalline structure when the cooling rate is smaller, while the solidification structure gradually transforms from the crystalline structure to amorphous structure with increasing of cooling rate. The Au melt is cooled from 1700 K to 100 K under different cooling rates and relaxed at 100 K. When the cooling rate is less than 1×1011.0K/s, a mixed crystalline of FCC and HCP structures can be found in the as-solidified microstructure. As the cooling rate is larger than 1×1013.5K/s, amorphous structure is obtained. Additionally, a mixture structure of crystalline and amorphous structures is found at a cooling speed ranging from 1×1011.0K/s to 1×1013.5K/s. Thus, the critical cooling rate to form ideal amorphous of Au melt is estimated to be 1×1013.5K/s, which consists with the theoretical calculation. The simulation of Al3Mg2 melt cooled from 1000 K to 200 K under different cooling rates and relaxed at 200 K, which indicates that amorphous structure is formed with the cooling rate is larger than 1×1012.0K/s.