1,164 research outputs found
Direct prediction of the solute softening-to-hardening transition in W-Re alloys using stochastic simulations of screw dislocation motion
Interactions among dislocations and solute atoms are the basis of several
important processes in metals plasticity. In body-centered cubic (bcc) metals
and alloys, low-temperature plastic flow is controlled by screw dislocation
glide, which is known to take place by the nucleation and sideward relaxation
of kink pairs across two consecutive \emph{Peierls} valleys. In alloys,
dislocations and solutes affect each other's kinetics via long-range stress
field coupling and short-range inelastic interactions. It is known that in
certain substitutional bcc alloys a transition from solute softening to solute
hardening is observed at a critical concentration. In this paper, we develop a
kinetic Monte Carlo model of screw dislocation glide and solute diffusion in
substitutional W-Re alloys. We find that dislocation kinetics is governed by
two competing mechanisms. At low solute concentrations, nucleation is enhanced
by the softening of the Peierls stress, which overcomes the elastic repulsion
of Re atoms on kinks. This trend is reversed at higher concentrations,
resulting in a minimum in the flow stress that is concentration and temperature
dependent. This minimum marks the transition from solute softening to
hardening, which is found to be in reasonable agreement with experiments
A rigorous sequential update strategy for parallel kinetic Monte Carlo simulation
The kinetic Monte Carlo (kMC) method is used in many scientific fields in
applications involving rare-event transitions. Due to its discrete stochastic
nature, efforts to parallelize kMC approaches often produce unbalanced time
evolutions requiring complex implementations to ensure correct statistics. In
the context of parallel kMC, the sequential update technique has shown promise
by generating high quality distributions with high relative efficiencies for
short-range systems. In this work, we provide an extension of the sequential
update method in a parallel context that rigorously obeys detailed balance,
which guarantees exact equilibrium statistics for all parallelization settings.
Our approach also preserves nonequilibrium dynamics with minimal error for many
parallelization settings, and can be used to achieve highly precise sampling
Diffuse-interface polycrystal plasticity: Expressing grain boundaries as geometrically necessary dislocations
The standard way of modeling plasticity in polycrystals is by using the
crystal plasticity model for single crystals in each grain, and imposing
suitable traction and slip boundary conditions across grain boundaries. In this
fashion, the system is modeled as a collection of boundary-value problems with
matching boundary conditions. In this paper, we develop a diffuse-interface
crystal plasticity model for polycrystalline materials that results in a single
boundary-value problem with a single crystal as the reference configuration.
Using a multiplicative decomposition of the deformation gradient into lattice
and plastic parts, i.e. F(X,t) = F^L(X,t) F^P(X,t), an initial stress-free
polycrystal is constructed by imposing F^L to be a piecewise constant rotation
field R^0(X), and F^P = R^0(X)^T, thereby having F(X,0) = I, and zero elastic
strain. This model serves as a precursor to higher order crystal plasticity
models with grain boundary energy and evolution.Comment: 18 pages, 7 figure
Formation of Nanotwin Networks during High-Temperature Crystallization of Amorphous Germanium
Germanium is an extremely important material used for numerous functional
applications in many fields of nanotechnology. In this paper, we study the
crystallization of amorphous Ge using atomistic simulations of critical
nano-metric nuclei at high temperatures. We find that crystallization occurs by
the recurrent transfer of atoms via a diffusive process from the amorphous
phase into suitably-oriented crystalline layers. We accompany our simulations
with a comprehensive thermodynamic and kinetic analysis of the growth process,
which explains the energy balance and the interfacial growth velocities
governing grain growth. For the crystallographic
orientation, we find a degenerate atomic rearrangement process, with two
zero-energy modes corresponding to a perfect crystalline structure and the
formation of a twin boundary. Continued growth in this direction
results in the development a twin network, in contrast with all other growth
orientations, where the crystal grows defect-free. This particular mechanism of
crystallization from amorphous phases is also observed during solid-phase
epitaxial growth of semiconductor crystals, where growth is
restrained to one dimension. We calculate the equivalent X-ray diffraction
pattern of the obtained nanotwin networks, providing grounds for experimental
validation
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