412 research outputs found
First principles determination of the Peierls stress of the shuffle screw dislocation in silicon
The Peierls stress of the a/2 screw dislocation belonging to the shuffle
set is calculated for silicon using density functional theory. We have checked
the effect of boundary conditions by using two models, the supercell method
where one considers a periodic array of dislocations, and the cluster method
where a single dislocation is embedded in a small cluster. The Peierls stress
is underestimated with the supercell and overestimated with the cluster. These
contributions have been calculated and the Peierls stress is determined in the
range between 2.4 x 10-2 and 2.8 x 10-2 eV {\AA}-3. When moving, the
dislocation follows the {111} plane going through a low energy metastable
configuration and never follows the 100 plane, which includes a higher energy
metastable core configuration
Explanation of the discrepancy between the measured and atomistically calculated yield stresses in body-centered cubic metals
We propose a mesoscopic model that explains the factor of two to three
discrepancy between experimentally measured yield stresses of BCC metals at low
temperatures and typical Peierls stresses determined by atomistic simulations
of isolated screw dislocations. The model involves a Frank-Read type source
emitting dislocations that become pure screws at a certain distance from the
source and, owing to their high Peierls stress, control its operation. However,
due to the mutual interaction between emitted dislocations the group consisting
of both non-screw and screw dislocations can move at an applied stress that is
about a factor of two to three lower than the stress needed for the glide of
individual screw dislocations.Comment: 4 pages, 2 figures; RevTex4; submitted to PR
Inertial and retardation effects for dislocation interactions
A new formulation for the equation of motion of interacting dislocations is
derived. From this solution it is shown that additional coupling forces, of
kinetic and inertial origin, should be considered in Dislocation Dynamics (DD)
simulations at high strain rates. A heuristic modification of this general
equation of motion enables one to introduce retardation into inertial and
elastic forces, in accordance with a progressive rearrangement of fields
through wave propagation. The influence of the corresponding coupling terms and
retardation effects are then illustrated in the case of dislocation dipolar
interaction and coplanar annihilation. Finally, comparison is made between the
modified equation of motion and a precise numerical solution based on the
Peierls-Nabarro Galerkin method. Good agreement is found between the
Peierls-Nabarro Galerkin method and the EoM including retardation effects for a
dipolar interaction. For coplanar annihilation, it is demonstrated that an
unexpected mechanism, involving a complex interplay between the core of the
dislocations and kinetics energies, allows a renucleation from the completely
annihilated dislocations. A description of this phenomenon that could break the
most favourable reaction between dislocations is proposed
Interplay between elastic fields due to gravity and a partial dislocation for a hard-sphere crystal coherently grown under gravity: driving force for defect disappearance
We previously observed that an intrinsic staking fault shrunk through a glide
of a Shockley partial dislocation terminating its lower end in a hard-sphere
crystal under gravity coherently grown in by Monte Carlo simulations
[Mori et al., Molec. Phys. 105, 1377 (2007)]; it was an answer to a one-decade
long standing question why the stacking disorder in colloidal crystals reduced
under gravity [Zhu et al., Nature 387, 883 (1997)]. Here, we present an elastic
energy calculation; in addition to the self-energy of the partial dislocation
[Mori et al., Prog. Theor. Phys. Suppl. 178, 33 (2009)] we calculate the
cross-coupling term between elastic field due to gravity and that due to a
Shockley partial dislocation. The cross term is a increasing function of the
linear dimension R over which the elastic field expands, showing that a driving
force arises for the partial dislocation moving toward the upper boundary of a
grain.Comment: 8pages, 4figures, to be published in Molecular Physic
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Topological model of type II deformation twinning in NiTi martensite
© 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group. A topological model of the formation of type II twins in NiTi martensite is presented. Disconnection dipoles are generated initially and expand on the rational k 1 = (011) plane, subsequently accumulating to form a tilt wall perpendicular to k 1 . Equilibrium occurs when the strain and rotational distortion fields of the constituent defects are equally partitioned between the adjacent crystals. The resultant interface is parallel to the irrational conjugate plane, (Formula presented.), consistent with the classical geometric theory of deformation twinning and previously published experimental observations. It is proposed that the formation of type II twins on k 2 occurs in this manner, rather than formation of type I twins on k 1 , because the disconnections have limited mobility on k 1 even though they are readily nucleated on this plane. We support this conjecture by showing that the Burgers vector of the defects has a small magnitude, implying easy nucleation, but their motion along k 1 is accompanied by complicated atomic shuffling
Nucleation and growth of platelets in hydrogen-ion-implanted silicon
H ion implantation into crystalline Si is known to result in the precipitation of planar defects in the form of platelets. Hydrogen-platelet formation is critical to the process that allows controlled cleavage of Si along the plane of the platelets and subsequent transfer and integration of thinly sliced Si with other substrates. Here we show that H-platelet formation is controlled by the depth of the radiation-induced damage and then develop a model that considers the influence of stress to correctly predict platelet orientation and the depth at which platelet nucleation density is a maximum. © 2005 American Institute of Physics
Void Growth in BCC Metals Simulated with Molecular Dynamics using the Finnis-Sinclair Potential
The process of fracture in ductile metals involves the nucleation, growth,
and linking of voids. This process takes place both at the low rates involved
in typical engineering applications and at the high rates associated with
dynamic fracture processes such as spallation. Here we study the growth of a
void in a single crystal at high rates using molecular dynamics (MD) based on
Finnis-Sinclair interatomic potentials for the body-centred cubic (bcc) metals
V, Nb, Mo, Ta, and W. The use of the Finnis-Sinclair potential enables the
study of plasticity associated with void growth at the atomic level at room
temperature and strain rates from 10^9/s down to 10^6/s and systems as large as
128 million atoms. The atomistic systems are observed to undergo a transition
from twinning at the higher end of this range to dislocation flow at the lower
end. We analyze the simulations for the specific mechanisms of plasticity
associated with void growth as dislocation loops are punched out to accommodate
the growing void. We also analyse the process of nucleation and growth of voids
in simulations of nanocrystalline Ta expanding at different strain rates. We
comment on differences in the plasticity associated with void growth in the bcc
metals compared to earlier studies in face-centred cubic (fcc) metals.Comment: 24 pages, 12 figure
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