56 research outputs found
Mesoscale model of dislocation motion and crystal plasticity
A consistent, small scale description of plastic motion in a crystalline
solid is presented based on a phase field description. By allowing for
independent mass motion given by the phase field, and lattice distortion, the
solid can remain in mechanical equilibrium on the timescale of plastic motion.
Singular (incompatible) strains are determined by the phase field, to which
smooth distortions are added to satisfy mechanical equilibrium. A numerical
implementation of the model is presented, and used to study a benchmark
problem: the motion of an edge dislocation dipole in a hexagonal lattice. The
time dependence of the dipole separation agrees with classical elasticity
without any adjustable parameters.Comment: 5 pages, 3 figure
Rotation induced grain growth and stagnation in phase field crystal models
We consider the grain growth and stagnation in polycrystalline
microstructures. From the phase field crystal modelling of the coarsening
dynamics, we identify a transition from a grain-growth stagnation upon deep
quenching below the melting temperature to a continuous coarsening at
shallower quenching near . The grain evolution is mediated by local grain
rotations. In the deep quenching regime, the grain assembly typically reaches a
metastable state where the kinetic barrier for recrystallization across
boundaries is too large and grain rotation with subsequent coalescence is
infeasible. For quenching near , we find that the grain growth depends on
the average rate of grain rotation, and follows a power law behavior with time
with a scaling exponent that depends on the quenching depth.Comment: (4 pages, 4 figures
Intermittent dislocation density fluctuations in crystal plasticity from a phase-field crystal model
Plastic deformation mediated by collective dislocation dynamics is
investigated in the two-dimensional phase-field crystal model of sheared single
crystals. We find that intermittent fluctuations in the dislocation population
number accompany bursts in the plastic strain-rate fluctuations. Dislocation
number fluctuations exhibit a power-law spectral density at high
frequencies . The probability distribution of number fluctuations becomes
bimodal at low driving rates corresponding to a scenario where low density of
defects alternate at irregular times with high population of defects. We
propose a simple stochastic model of dislocation reaction kinetics that is able
to capture these statistical properties of the dislocation density fluctuations
as a function of shear rate
Classical analogies for the force acting on an impurity in a Bose-Einstein condensate
We study the hydrodynamic forces acting on a small impurity moving in a
two-dimensional Bose-Einstein condensate at non-zero temperature. The
condensate is modelled by the damped-Gross Pitaevskii (dGPE) equation and the
impurity by a Gaussian repulsive potential coupled to the condensate. For weak
coupling, we obtain analytical expressions for the forces acting on the
impurity, and compare them with those computed through direct numerical
simulations of the dGPE and with the corresponding expressions for classical
forces. For non-steady flows, there is a time-dependent force dominated by
inertial effects and which has a correspondence in the Maxey-Riley theory for
particles in classical fluids. In the steady-state regime, the force is
dominated by a self-induced drag. Unlike at zero temperature, where the drag
force vanishes below a critical velocity, at low temperatures the impurity
experiences a net drag even at small velocities, as a consequence of the energy
dissipation through interactions of the condensate with the thermal cloud. This
dissipative force due to thermal drag is similar to the classical Stokes' drag.
There is still a critical velocity above which steady-state drag is dominated
by acoustic excitations and behaves non-monotonically with impurity's speed.Comment: 21 pages, 4 figures. Supplementary movies available at:
https://cloud.ifisc.uib-csic.es/nextcloud/index.php/s/AFzw6JxNW77DT6
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