Effect Of Melt Thermal History On The Solidification Behaviors Of Metals

Thermal history condition plays a key role in affecting the solidification undercooling and cluster structure of metals. Thus, in view of theory and engineering points, it is of great importance to explore the relationship between thermal history condition and the solidification processing of metals. In this thesis, with EAM multi-body potential function and the molecular dynamics method, the solidification process of aluminum, silver and nickel melt under different thermal history condition have been simulated. Classical solidification theory has been conducted to study the number of atoms within the maximum cluster as a function of initial temperature of aluminum, silver and nickel melts. Main conclusions can be drawn as follows.Simulation results show that the crystal structure is obtained with the cooling rates lower than1012.7K/s,1011.1K/s and1011.8K/s for aluminum, silver and nickel melts. Respectively, the homogeneous nucleation undercoolings of aluminum, silver and nickel melts predicted from the theoretical model under different cooling rates agree well with the simulated results. The solidification processes of aluminum, silver and nickel melt at a constant cooling rate of1011.0K/s have been simulated and the homogeneous nucleation undercoolings increase with increasing of the melt temperature. When the melt temperature is greater than a certain value, the undercooling tends to be a constant. The relationship between the atom number (nmax) of the maximum cluster in the melt and the melt temperature of the metals were predicted theoretically. Moreover, the calculation results reveal that the atom number of the maximum cluster in the melt decreases with increasing of melt temperature.The microstructure envolutions of nickel melt solidified from the initial temperature of2278K at different cooling rate have been studied. Simulation results show that the crystal structure is obtained at a cooling rate lower than1011K/s; while the mixed structures of crystal and amorphous structure are obtained with the cooling rate from1011K/s to1014.5K/s. The solidified crystal of nickel is of FCC structure when the cooling rate is lower than1010K/s, while it changes to crystal structure composed of FCC and HCP with the cooling rate from1010K/s and1014.5K/s. By analyzing the calculation and simulation results, it is determined that the critical cooling rate for nickel melt to form ideal metallic glass is1014.5K/s. Moreover, it is found that the structures of the subcritical nuclei (the cooling rate is higher than1014.5K/s), critical nuclei (the cooling rate is1014.5K/s), and the growing crystal (the cooling rate is lower than1014.5K/s) are the lamellar FCC and HCP clusters, which indicates that the subcritical nuclei, critical nuclei and the growing crystal have the same structure.