Extraordinary Conditions, The Melt Structure Evolution

The molecular dynamics simulations with EAM and TB potential functions have been performed to study the liquid structure under non-routine environments, such as high pressure and shear stress, as well as the dynamic functions of supercooled liquid.We presented MD simulations on liquid Au and NiAl alloy within the framework of empirical TB and EAM potentials respectively to simulate the structural transition under different pressures during the rapid cooling processes. The simulation results show that, the applied pressures strongly affect the crystallization or glass transition processes of liquid melts, which can be clearly revealed through the pair correlation function curves. The atomic nearest distance is shortened and the types and numbers of the atom clusters have been changed under different pressures. Pressure favors glass formation under a certain pressure, moreover, the glass transition temperature increases with the pressure. Further structural analysis shows that a great deal of 1551, 1441 bonded pairs which are the characteristic pairs of icosahedral clusters have been detected in the glass. While the possibility of crystallization for the system is enhanced with increasing pressure. The liquid crystallize into crystal, and the crystallization temperature increases with increasing pressure. The liquid structure becomes more complicated under the action of pressure.The structural properties as functions of temperature and shear rate for liquid Al have been studied using MD simulation. The metallic interaction is based on the EAM potential of J.Mei. The temperature decreases from 1500K to 400K with a cooling rate of 6.25×10~12K/s. The sudden drop in the continuous energy-temperature curves dependence of shear rate reveals that crystal structure is formed under low temperature a nd c rystallization t emperature i ncreases a s t he s hear r ate i s e nhanced. The fact attributes to the structural change which is a result of the decrease of local energy barrier at the transition point. The pair correlation function and pair analysis technology are applied to explore the microstructural order of the liquid state as a function of shear rate. The addition of shear stress enhances the disorder of liquid athigher temperature and makes the atomic nearest distance shortened. For normal liquid metal, our simulations show that the 1421 bonded pair corresponding to FCC structure has a significant presence. However, the 1661+1441 bonded pairs which are the characteristic pairs of BCC crystalline order are in a predominant position at T <800K with the addition of the shear stress, as well as some icosahedral order in the sheared system. The structural transition caused by the action of shear stress could be a good explanation for the sudden change in energy-temperature curve.MD simulations on the dynamic functions of liquid Ni during two different cooling processes are performed, as well as a primary discussion on the process of relaxation, nucleation and glass transition for supercooled liquid. The system gets a crystal structure at the cooling rate of 1.9xlO12K/s, and solidification temperature Ts is about 990K; while an amorphous structure is formed when the cooling rate is increased to 5.7xlO12K/s, and glass transition temperature Tg is about 670K. Threetime-correlation functions MSD> Fs(k,t) and NGP are calculated, and accordinglythe primary relaxation process in the course of cooling can be described. Relaxation, nucleation and glass transition are shown to be closely correlated with each other. During the course of glass transition, the liquid is in a metastable state through thediffusive rearrangements of atoms, and /? -relaxation regime has an important effecton process of glass forming, in which the dynamics of liquid is stagnated and the system exhibits a more pronounced heterogeneity of the relaxations. The systemundergoes J3 -relaxation regime and a -relaxation regime in a short time when thecooling rate is slower, and a massive structural rearrangement leading to incipient nucleation for crystallization occurs shortly after entering a -relaxation regime.