Analysis of semi-empirical interatomic potentials appropriate for simulation of crystalline and liquid Al and Cu

We investigate the application of embedded atom method (EAM) interatomic potentials in the study of crystallization kinetics from deeply undercooled melts, focusing on the fcc metals Al and Cu. For this application, it is important that the EAM potential accurately reproduces melting properties and liquid structure, in addition to the crystalline properties most commonly fit in its development. To test the accuracy of previously published EAM potentials and to guide the development of new potential in this work, first-principles calculations have been performed and new experimental measurements of the Al and Cu liquid structure factors have been undertaken by X-ray diffraction. We demonstrate that the previously published EAM potentials predict a liquid structure that is too strongly ordered relative to measured diffraction data. We develop new EAM potentials for Al and Cu to improve the agreement with the first-principles and measured liquid diffraction data. Furthermore, we calculate liquid-phase diffusivities and find that this quantity correlates well with the liquid structure. Finally, we perform molecular dynamics simulations of crystal nucleation from the melt during quenching at constant cooling rate. We find that EAM potentials, which predict the same zero-temperature crystal properties but different liquid structures, can lead to quite different crystallization kinetics. More interestingly, we find that two potentials predicting very similar equilibrium solid and liquid properties can still produce very different crystallization kinetics under far-from-equilibrium conditions characteristic of the rapid quenching simulations employed here.     

Structure of liquid Al and Al67Mg33 alloy: comparison between experiment and simulation

We report data on the structure of liquid Al and an Al₆₇Mg₃₃ alloy obtained from state-of-the-art X-ray diffraction experiments and ab initio molecular dynamics (AIMD) simulations. To facilitate a direct comparison between these data, we develop a method to elongate the AIMD pair correlation function in order to obtain reliable AIMD structure factors. The comparison reveals an appreciable level of discrepancy between experimental and AIMD liquid structures, with the latter being consistently more ordered than the former at the same temperature. The discrepancy noted in this study is estimated to have significant implications for simulation-based calculations of liquid transport properties and solid–liquid interface kinetic properties.     

Atomic Structure and Properties of the (310) Symmetrical Tilt Grain Boundary (STGB) in SrTiO3. Part I: Atomistic Simulations

The relaxed structure and energy of the (310) symmetrical tilt grain boundary (STGB) in SrTiO₃ have been calculated using static lattice energy minimization methods. In principle, the (310) GB plane can either be a cation-rich, positively charged SrTiO plane or a negatively charged oxygen plane, and both scenarios have been considered in this report. The effect of point-defect reconstruction at the GB core region, manifested either as completely missing columns or as half-filled columns of ions as suggested by experiments, has been analyzed. The results indicate that while Schottky defects are very strongly preferred energetically at the GB core, there is not significant gain in energy by having half-filled columns, as opposed to fully-dense and fully-empty columns, at the GB core. The simulation results have been analyzed in the context of Pauling's rules of crystal chemistry and bicrystallography. The results form the basis for an objective comparison with experimental studies in Part II of the paper.     

Surface diffusion of Si,Ge and C adatoms on Si （001） substrate studied the molecular dynamics simulation

Depositions of Si, Ge and C atoms onto a preliminary Si (001) substrate at different temperatures are investigated by using the molecular dynamics method. The mechanism of atomic self-assembling occurring locally on the flat terraces between steps is suggested. Diffusion and arrangement patterns of adatoms at different temperatures are observed. At 900 K, the deposited atoms are more likely to form dimers in the perpendicular [110] direction due to the more favourable movement along the perpendicular [110] direction. C adatoms are more likely to break or reconstruct the dimers on the substrate surface and have larger diffusion distances than Ge and Si adatoms. Exchange between C adatoms and substrate atoms are obvious and the epitaxial thickness is small. Total potential energies of adatoms and substrate atoms involved in the simulation cell are computed. When a newly arrived adatom reaches the stable position, the potential energy of the system will decrease and the curves turns into a ladder-like shape. It is found that C adatoms can lead to more reduction of the system energy and the potential energy of the system will increase as temperature increases.     

深过冷液态Ni2TiAl合金热物理性质的分子动力学模拟

采用嵌入原子法对深过冷条件下Ni₂TiAl合金的焓和密度进行了分子动力学模拟，模拟获得了比热和密度在1200K至2000K范围内的温度依赖关系. 结果表明过冷条件下Ni₂TiAl合金的比热、密度和温度之间存在着线性关系，它们都随着温度的降低而降低. 比热的大小与依据Neumann-Kopp法则估算的结果相近，同时模拟过程中的尺度效应经验证可以忽略. 关键词： 2TiAl合金')" href="#">Ni₂TiAl合金 比热 密度 分子动力学模拟     

Computer simulation of the structure of MZr2 liquid and amorphous alloys

In the present study, we compare the structure of NiZr₂ and CuZr₂ disordered (liquid and amorphous) alloys. While Cu and Ni have similar atomic radii, the formation heats of these alloys differ by more than a factor of two. Moreover, the most stable crystal phase in the CuZr₂ alloy has the C11b lattice while the most stable phase in the NiZr2 alloy has the C16 lattice. Comparing these two alloys allows us to directly explore how the chemistry and atomic size affect the disordered phase structure. We find that all differences in the structures of the disordered alloys are readily explained by the smaller Ni–Zr separation driven by the higher heat of mixing and the structure of the disordered alloys does not necessarily resemble the local packing structure of their crystalline counterparts. Comparison of the disordered alloys and their most stable crystal phase structures explains the better glass formability of the CuZr₂ alloy.     

Study of structural and magnetic properties of Fe_(80)P-_9B_(11) amorphous alloy by ab initio molecular dynamic simulation

The structural and magnetic properties of Fe_(80)P_9B_(11) amorphous alloy are investigated through ab initio molecular dynamic simulation. The structure evolution of Fe_(80)P_9B_(11) amorphous alloy can be described in the framework of topological fluctuation theory, and the fluctuation of atomic hydrostatic stress gradually decreases upon cooling. The left sub peak of the second peak of Fe–B partial pair distribution functions(PDFs) becomes pronounced below the glass transition temperature, which may be the major reason why B promotes the glass formation ability significantly. The magnetization mainly originates from Fe 3d states, while small contribution results from metalloid elements P and B. This work may be helpful for developing Fe-based metallic glasses with both high saturation flux density and glass formation ability.     

Computation of some thermodynamics, transport, structural properties, and new equation of state for fluid methane using two-body and three-body intermolecular potentials from molecular dynamics simulation

Molecular dynamics simulation has been performed to obtain pressure, radial distribution function, and self-diffusion coefficient of fluid methane using one site OPLS (optimized potentials for liquid simulations), five sites OPLS-SITE, and two-body HFD (Hartree-Fock dispersion)-like potentials. To take higher-body forces into account, three-body potential of Hauschild and Prausnitz (1993) has been used with the two-body HFD-like potential. The significance of this work is that the three-body potential of Hauschild and Prausnitz extended as a function of density and temperature and used with the HFD-like potential to improve the prediction of the results of pressure of fluid methane without requiring an expensive three-body calculation. The molecular dynamics simulation of methane has been also used to determine a new equation of state. The results are in a good agreement with experimental and theoretical values.