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.
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.
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.
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.
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
Abstract: A general continuum model has recently been proposed for the dynamics of ion
intercalation in a single crystal of rechargeable-battery electrode materials . 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.
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
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.
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.
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 .