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Register a new userMolecular Dynamics Computer Simulation of Crystal Growth and Melting in Al50Ni50
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Authors
Ali Kerrache
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The melting and crystallization of Al50Ni50} are studied by means of molecular dynamics computer simulations, using a potential of the embedded atom type to model the interactions between the particles. Systems in a slab geometry are simulated where the B2 phase of AlNi in the middle of an elongated simulation box is separated by two planar interfaces from the liquid phase, thereby considering the (100) crystal orientation. By determining the temperature dependence of the interface velocity, an accurate estimate of the melting temperature is provided. The value k=0.0025 m/s/K for the kinetic growth coefficient is found. This value is about two orders of magnitude smaller than that found in recent simulation studies of one-component metals. The classical Wilson-Frenkel model is not able to describe the crystal growth kinetics on a quantitative level. We argue that this is due to the neglect of diffusion processes in the liquid-crystal interface.
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A novel Stochastic Event-Driven Molecular Dynamics (SEDMD) algorithm is developed for the simulation of polymer chains suspended in a solvent. The polymers are represented as chains of hard spheres tethered by square wells and interact with the solvent particles with hard core potentials. The algorithm uses Event-Driven Molecular Dynamics (EDMD) for the simulation of the polymer chain and the interactions between the chain beads and the surrounding solvent particles. The interactions between the solvent particles themselves are not treated deterministically as in event-driven algorithms, rather, the momentum and energy exchange in the solvent is determined stochastically using the Direct Simulation Monte Carlo (DSMC) method. The coupling between the solvent and the solute is consistently represented at the particle level, however, unlike full MD simulations of both the solvent and the solute, the spatial structure of the solvent is ignored. The algorithm is described in detail and applied to the study of the dynamics of a polymer chain tethered to a hard wall subjected to uniform shear. The algorithm closely reproduces full MD simulations with two orders of magnitude greater efficiency. Results do not confirm the existence of periodic (cycling) motion of the polymer chain.
Melting and freezing transitions in two dimensional systems are known to show highly unusual characteristics. Most of the earlier studies considered atomic systems; the melting behavior in two dimensional molecular solids is still largely unexplored. In order to understand the role of multiple energy and length scales present in molecular systems on nature of melting transition, here we report computer simulation studies of melting of a two dimensional Mercedes-Benz (MB) system. We find that the interplay between the strength of isotropic and anisotropic interactions can give rise to rich phase diagram. The computed solid-liquid phase diagram consists of isotropic liquid and two crystalline phases - honeycomb and oblique. In contradiction to the celebrated KTHNY theory, we observe strongly one step first order melting transitions for both the honeycomb and oblique solids. The defects in both solids and liquids near the transition are more complex compared to the atomic systems.
We introduce a new criterion--based on multipole dynamical correlations calculated within Reptation Quantum Monte Carlo--to discriminate between a melting vs. freezing behavior in quantum clusters. This criterion is applied to small clusters of para-hydrogen molecules (both pristine and doped with a CO cromophore), for cluster sizes around 12 molecules. This is a magic size at which para-hydrogen clusters display an icosahedral structure and a large stability. In spite of the similar geometric structure of CO@(pH2)_12 and (pH2)_13, the first system has a rigid, crystalline, behavior, while the second behaves more like a superfluid (or, possibly, a supersolid).
We report a molecular dynamics simulation of melting of tungsten (W) nanoparticles. The modified embedded atom method (MEAM) interatomic potentials are used to describe the interaction between tungsten atoms. The melting temperature of unsupported tungsten nanoparticles of different sizes are found to decrease as the size of the particles decreases. The melting temperature obtained in the present study is approximately a decreasing function of inverse radius, in a good agreement with the predictions of thermodynamic models. We also observed that the melting of a W nanoparticle is preceded by the premelting of its outer skin at a temperature lower than its melting temperature.
In this study, the crack propagation of the pre-cracked mono-crystal nickel with the voids and inclusions has been investigated by molecular dynamics simulations. Different sizes of voids, inclusions and materials of inclusions are used to fully study the effect of the voids and inclusions during the crack propagation process. The dislocations evolution, stress distribution and crack length are analyzed as the associated mechanical properties. The results indicate that the voids and inclusions can change the path of crack propagation of the pre-cracked mono-crystal nickel. Moreover, the results show that the voids and inclusions can lead a better resistance to plastic deformation of the mono-crystal and the inclusions can make the system more difficult to fracture.
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