The Establishment Of Interatomic Potentials And Heterogeneous Properties For Metallic Liquids

During the quenching process, dynamical and structural heterogeneity coexist in the liquid metals or metallic glasses, according to experimental and theoretical studies. The researches on the heterogeneity are of great significance on the development of rapid solidification theory and on the preparation of glass-forming materials. Computer simulation is an effective tool to gain insight into the time dependence of the structure and dynamics of supercooled liquids and glasses. The simulation results based on the proper interatomic potentials for liquid metals can make good predictions on the physical properties of the melts, and provide theoretical guides for experiments.The main investigation results obtained by computer simulations, as well as the main contents in this thesis, have been listed as follow:The pair potential of liquid metals requires far long-range oscillatory interactions resulting from the presence of conduction electrons. We shall modify the long-range attractive part of the traditional Lennard-Jones (LJ) potential by adding an additional parameterθ, and our modified LJ potential is as follows:Based on the dense gas-like model of viscosity, we have obtained the modified LJ potentials suitable for studying the liquid structure of Al, Cu and Ag. Experimental data considered in deriving the potentials include the liquid density, viscosity and pair correlation function. The energy and structural properties of liquid Al, Cu and Ag in cooling processes have been studied via molecular dynamics simulations, exhibiting correct trends as a function of temperature. Calculated results are comparable with those derived by J. Mei or Johnson's embedded-atom method (EAM) and effective pair potential. The differences between potential energy derived by pair potentials and calculations from EAM model may be attributed to the neglect of the electron background energy in pair potentials. The results also reveal that the uncertainty of experimental data affects greatly the accuracy of the pair potential.Based on the modified LJ potential for pure metals, the modification on this LJ model has been done to make it suitable to describe the properties of the binary alloy Al₇₅CU₂₅. The enthalpy of mixing of unlike atoms is involved when the total activation energy of the alloy is calculated. Experimental data involving viscosity and pair correlation function are used to derive the potential parameters. MD simulations on the structure factor S(Q), based on the modified LJ potential for Al-Cu alloy, are in good agreement with the experimental data. When a cooling process was performed with the cooling rate of 0.2K/ps, crystal peaks can be detected in the pair correlation of the Al₇₅Cu₂₅ alloy at the temperature of 400K; moreover, the first peak in g_Al-Cu(r) is very intense, manifesting the tendency for Al atoms to be preferentially surrounded by Cu atoms as the nearest neighbors. Results on the pair analysis parameters reveal that there is large numbers of FCC structure existing in the final state. The calculated bond orientational order parameters W₆ and Q₆ are comparable with experimental results.The thermodynamic, dynamic and structural relaxations of supercooled liquid andglassy Ni have been investigated using molecular dynamics simulations based on theembedded atom method. The glass transition temperature T_g is estimated as 800K bycharacterizing pair correlation function, and the critical temperature T_c is 840Kaccording to the calculated diffusion constant. Three temperature regions can beobserved as the temperature decreases: (a) in the temperature range of 840K≤T≤1000K, crystallization of supercooled liquid occurs when the characteristic relaxationtime is long enough to reach the incubation time r; (b) a complicated region composedof partial crystal and glass will appear within the range of 810K≤T≤840K; (c) andthe glass crystallization can be detected as the temperature is below 810K. Results fordynamic and structural relaxations indicate that not only the crystallization ofsupercooled liquid, but that of the glass derive from the nucleation and growth ofcrystal nuclei which relates closely to the structural rearrangements of atoms. Atemperature-time-transformation schematic plot has been constructed based ontemperature dependence of the characteristic relaxation times.The NiZr₂ entire amorphous ribbons were prepared by melt spinning. A spot of oxygen in the amorphous ribbon have been found by the electron-probe, implying that the existence of oxygen makes for the formation of NiZr₂ entire amorphous. The results of X-ray diffraction (XRD) and differential scanning calorimetry (DSC) have shown that the crystallization of NiZr₂ amorphous alloy has two phases. The process of glass crystallization can be express as follows: amorphous→amorphous + ZrO→ZrO + NiZr₂, and the final produces are ZrO phase and NiZr₂ phase. Based on the experimental results, molecular dynamics simulation is performed to investigate the glass formation of liquid NiZr₂ alloy with the interatomic potential proposed by Hausleitner-Hafner. Some relevant features of the pair correlation functions are in good agreement with those obtained by experiment. The pair analysis parameters are calculated, suggesting that there exist icosahedral ordering, four-fold symmetrical bipyramid and triangular-faced polyhedral units in the amorphous NiZr₂ structure. The result is beneficial to open avenues toward the understanding of fundamental theoretical problems of glass formation of simple binary alloys.