Theory, Modeling and Numerical Simulation

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Authors: Qian Chen, X.Y. Liu, S.B. Biner
Abstract: In this study, the role of solute segregation on the strength and the evolution behavior of dislocation junctions is studied by utilizing kinetic Monte Carlo and 3D dislocation dynamics simulations. The different solute concentrations and the character of the junctions are all included in the simulations in an effort to make a parametric investigation. The results indicate that solute segregation can lead to both strengthening and weakening behavior depending upon the evolution of the dislocation junctions.
Authors: Iván Santos, Luis Alberto Marqués, Lourdes Pelaz, Pedro Lopez, María Aboy
Abstract: We focus this work on multi-scale modeling of the ion-beam-induced amorphization and recrystallization in Si, although our scheme can be applied to other materials. We use molecular dynamics to study the formation mechanisms of amorphous regions. We have observed that along with energetic ballistic collisions that generate Frenkel pairs, low energy interactions can produce damage through the melting and quenching of target regions. By quantifying these results, we have developed an improved binary collision approximation model which gives a damage description similar to molecular dynamics. We have successfully applied our model to ion and cluster implantations. In order to define the energetic of defects in a more computationally efficient Kinetic Monter Carlo code, we have used molecular dynamics results related to the recrystallization behavior of local amorphous regions. The combination of all these simulation tools, molecular dynamics (fundamental studies of damage formation and recrystallization), improved binary collisions (including ballistic and melting-related damage) and Kinetic Monte Carlo (for efficient defect kinetics modeling during the implantation and the subsequent annealing), allows us to model the effect of ion mass, beam current and implant temperature on the amount and morphology of residual defects in Si.
Authors: Yun Feng Shi, Donald W. Brenner
Abstract: We present results from molecular dynamics simulations of shock-induced hydrodynamic void collapse in a model energetic crystal. During void collapse, hotspot formation is observed that leads to subsequent detonation. The hotspot formation mechanism is identified as shock energy focusing via jetting. There is another initiation mechanism that arises from the interaction of reflected shock waves with the rigid piston, which is considered to be an artifact. Such artifact can be eliminated by altering the location of the void. The detonation threshold as a function of the velocity of the driven piston is determined for various void geometries. It is found that a system containing a void has a lower detonation threshold than that of a perfect energetic crystal. The amount of reduction of the detonation threshold depends on the geometry of the void. For square voids, there exists a minimum size above which reduction of the detonation threshold occurs. Among voids that have an equal volume, the void that is elongated along the shock direction gives the lowest detonation threshold.
Authors: Zhi Liang Pan, Yu Long Li, Qiu Ming Wei
Abstract: Using molecular dynamics (MD) simulation, we have investigated the mechanical properties and the microstructural evolution of nanocrystalline tantalum (NC-Ta, grain size from 3.25 nm to ~13.0 nm) under uniaxial tension. The results show the flow stress at a given offset strain decreases as the grain size is decreased within the grain size regime studied, implying an inverse Hall-Petch effect. A strain rate sensitivity of ~0.14, more than triple that of coarse-grain Ta, is derived from the simulation results. Twinning is regarded to be a secondary deformation mechanism based on the simulations. Similar to nanocrystalline iron, stress-induced phase transitions from body-centered cubic (BCC) to face-centered cubic (FCC) and hexagonal close-packed (HCP) structures take place locally during the deformation process, The maximum fraction of FCC atoms varies linearly with the tensile strength. We can thus conclude that a critical stress exists for the phase transition to occur. It is also observed that the higher the imposed strain rate, the further delayed is the phase transition. Such phase transitions are found to occur only at relatively low simulation temperatures, and are reversible with respect to stress.
Authors: Mikhail D. Starostenkov, Dmitry V. Sinyaev, Roman Y. Rakitin, Gennady M. Poletaev
Abstract: The paper is devoted to the research of diffusion mechanism in Ni3Al intermetallide over grain boundaries of tilt <111> and <100> by the method of molecular dynamics. It was found that grain boundary diffusion represented the combination of three main mechanisms: migration of atoms along the nuclei of grain boundary dislocations, cyclic mechanism near the nuclei and the formation of the chain of displaced atoms from one nucleus of dislocation to the nucleus of the other one. Diffusion mobility of Ni atoms was essentially bigger than the mobility of Al atoms. Grain boundary slipping and intergrain slipping were also observed during the deformation of the bicrystal.
Authors: Damian Burch, Gogi Singh, Gerbrand Ceder, Martin Z. Bazant
Abstract: A general continuum model has recently been proposed for the dynamics of ion intercalation in a single crystal of rechargeable-battery electrode materials [1]. When applied to strongly phase-separating, highly anisotropic materials such as LiFePO4, phase-transformation waves are predicted between the lithiated and unlithiated portions of a crystal. In this paper, we extend the analysis of the wave dynamics, and we describe a new mechanism for current capacity fade through the interactions of these waves with defects in the material.
Authors: Byoung Min Lee, Shinji Munetoh, Teruaki Motooka, Yeo Wan Yun, Kyu Mann Lee
Abstract: The structural properties of SiO2 liquid during cooling have been investigated by molecular dynamics simulations. The interatomic forces acting on the particles are calculated by the modified Tersoff potential parameters. The glass transition temperature and structural properties of the resulting SiO2 system at various temperatures have been investigated. The fivefold coordinations of Si and threefold coordinations of O atoms were observed, and the coordination defects of system decrease with decreasing temperature up to 17 % at 300 K. The self-diffusion coefficients for Si and O atoms drop to almost zero below 3000 K. The structures were distorted at high temperatures, but very stable atomic network persisted up to high temperature in the liquid state.
Authors: Frédéric Soisson, Chu Chun Fu
Abstract: We present Monte Carlo simulations of the first stages of the coherent precipitation of Cu in α-Fe during thermal ageing and under irradiation. The simulations are based on a diffusion model by vacancy and self-interstitial jumps, the parameters of which are fitted on ab initio calculations. The simulations of precipitation kinetics during thermal ageing are compared with experimental ones. They reveal that precipitates containing up to several tens of atoms can be much more mobile than individual copper atoms, due to the trapping of vacancies in Cu. Monte Carlo simulations are also used to analyze the coupling between fluxes of point defects and Cu which occurs under irradiation and the possible resulting radiation induced segregation phenomena.
Authors: Sudip Chakraborty, G. Sashidhar, S.V. Ghaisas, V. Sundararajan
Abstract: Motivated by the possibility of scaling down of various electronic devices at the nanolevel, we have chosen a simple p-n junction like device in silicon structure. An aggregation of seventy-eight silicon atoms, passivated by oxygen is considered. We compute the electronic structure of such a cluster and then the density of states and the optical spectra for this aggregate are compared with a modified one. The modification is introduced by substituting phosphorus and boron atom within this cluster of silicon atoms in both sides in order to make a p-n junction like situation. A variant of this p-n junction like structure is introduced by adding a layer of oxygen between the phosphorus and boron regions. Comparison of the electronic structures of all these systems reveals several interesting properties.
Authors: Ulmas Gafurov
Abstract: A modeling method is suggested on the basis of measurable structural and deformation parameters to estimate the interrelation between the interconnecting chains over amorphous segments conformation structure distribution and the elastic modulus in tension experimental value for a loaded oriented linear amorphous- crystalline polymer. Macromolecule chain pulling out value of polymer crystallite was defined using the Frenkel-Kontorowa’s dislocation model .

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