4,098 research outputs found
The phonon drag force acting on a mobile crystal defect: full treatment of discreteness and non-linearity
Phonon scattering calculations predict the drag force acting on defects and
dislocations rises linearly with temperature, in direct contradiction with
molecular dynamics simulations that often finds the drag force to be
independent of temperature. Using the Mori-Zwanzig projection technique, with
no recourse to elasticity or scattering theories, we derive a general Langevin
equation for a crystal defect, with full treatment of discreteness and
non-linearity in the defect core. We obtain an analytical expression for the
drag force that is evaluated in molecular statics and molecular dynamics,
extracting the force on a defect directly from the inter-atomic forces. Our
results show that a temperature independent drag force arises because
vibrations in a discrete crystal are never independent of the defect motion, an
implicit assumption in any phonon-based approach. This effect remains even when
the Peierls barrier is effectively zero, invalidating qualitative explanations
involving the radiation of phonons. We apply our methods to an interstitial
defect in tungsten and solitons in the Frenkel-Kontorova model, finding very
good agreement with trajectory-based estimations of the thermal drag force.Comment: 20 pages, 8 figure
Hybrid mean field and real space model for vacancy diffusion-mediated annealing of radiation defects
In a fusion or advanced fission reactor, high energy neutrons induce the
formation of extended defect clusters in structural component materials,
degrading their properties over time. Such damage can be partially recovered
via a thermal annealing treatment. Therefore, for the design and operation of
fusion and advanced fission nuclear energy systems it is critical to estimate
and predict the annealing timescales for arbitrary configurations of defect
clusters. In our earlier paper [I. Rovelli, S. L. Dudarev, and A. P. Sutton, J.
Mech. Phys. Solids 103, 121 (2017)] we extended the Green function formulation
by Gu, Xiang et al. [Y. Gu, Y. Xiang, S. S. Quek, and D. J. Srolovitz, J. Mech.
Phys. Solids 83, 319 (2015)] for the climb of curved dislocations, to include
the evaporation and growth of cavities and vacancy clusters, and take into
account the effect of free surfaces. In this work, we further develop this
model to include the effect of radiation defects that are below the
experimental detection limit, via a mean field approach coupled with an
explicit treatment of the evolution of discrete defect clusters distributed in
real space. We show that randomly distributed small defects screen diffusive
interactions between larger discrete clusters. The evolution of the coupled
system is modelled self-consistently. We also simulate the evolution of defects
in an infinite laterally extended thin film, using the Ewald summation of
screened Yukawa-type diffusive propagators
High energy collision cascades in tungsten: dislocation loops structure and clustering scaling laws
Recent experiments on in-situ high-energy self-ion irradiation of tungsten
(W) show the occurrence of unusual cascade damage effects resulting from single
ion impacts, shedding light on the nature of radiation damage expected in the
tungsten components of a fusion reactor. In this paper, we investigate the
dynamics of defect production in 150 keV collision cascades in W at atomic
resolution, using molecular dynamics simulations and comparing predictions with
experimental observations. We show that cascades in W exhibit no subcascade
break-up even at high energies, producing a massive, unbroken molten area,
which facilitates the formation of large defect clusters. Simulations show
evidence of the formation of both 1/2 and interstitial-type
dislocation loops, as well as the occurrence of cascade collapse resulting in
vacancy-type dislocation loops, in excellent agreement with experimental
observations. The fractal nature of the cascades gives rise to a scale-less
power law type size distribution of defect clusters.Comment: 6 pages, 3 figure
Theory and Simulation of the diffusion of kinks on dislocations in bcc metals
Isolated kinks on thermally fluctuating (1/2) screw, edge and
(1/2) edge dislocations in bcc iron are simulated under zero stress
conditions using molecular dynamics (MD). Kinks are seen to perform stochastic
motion in a potential landscape that depends on the dislocation character and
geometry, and their motion provides fresh insight into the coupling of
dislocations to a heat bath. The kink formation energy, migration barrier and
friction parameter are deduced from the simulations. A discrete
Frenkel-Kontorova-Langevin (FKL) model is able to reproduce the coarse grained
data from MD at a fraction of the computational cost, without assuming an a
priori temperature dependence beyond the fluctuation-dissipation theorem.
Analytic results reveal that discreteness effects play an essential r\^ole in
thermally activated dislocation glide, revealing the existence of a crucial
intermediate length scale between molecular and dislocation dynamics. The model
is used to investigate dislocation motion under the vanishingly small stress
levels found in the evolution of dislocation microstructures in irradiated
materials
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