1,323 research outputs found
Structure identification methods for atomistic simulations of crystalline materials
We discuss existing and new computational analysis techniques to classify
local atomic arrangements in large-scale atomistic computer simulations of
crystalline solids. This article includes a performance comparison of typical
analysis algorithms such as Common Neighbor Analysis, Centrosymmetry Analysis,
Bond Angle Analysis, Bond Order Analysis, and Voronoi Analysis. In addition we
propose a simple extension to the Common Neighbor Analysis method that makes it
suitable for multi-phase systems. Finally, we introduce a new structure
identification algorithm, the Neighbor Distance Analysis, that is designed to
identify atomic structure units in grain boundaries
Thermally-activated Non-Schmid Glide of Screw Dislocations in W using Atomistically-informed Kinetic Monte Carlo Simulations
Thermally-activated \small{\nicefrac{1}{2}} screw dislocation motion
is the controlling plastic mechanism at low temperatures in body-centered cubic
(bcc) crystals. Motion proceeds by the nucleation and propagation of
atomic-sized kink pairs susceptible of being studied using molecular dynamics
(MD). However, MD's natural inability to properly sample thermally-activated
processes as well as to capture screw dislocation glide calls for the
development of other methods capable of overcoming these limitations. Here we
develop a kinetic Monte Carlo (kMC) approach to study single screw dislocation
dynamics from room temperature to and at stresses
, where and are the melting point and
the Peierls stress. The method is entirely parameterized with atomistic
simulations using an embedded atom potential for tungsten. To increase the
physical fidelity of our simulations, we calculate the deviations from Schmid's
law prescribed by the interatomic potential used and we study single
dislocation kinetics using both projections. We calculate dislocation
velocities as a function of stress, temperature, and dislocation line length.
We find that considering non-Schmid effects has a strong influence on both the
magnitude of the velocities and the trajectories followed by the dislocation.
We finish by condensing all the calculated data into effective stress and
temperature dependent mobilities to be used in more homogenized numerical
methods
Interface-controlled creep in metallic glass composites
In this work we present molecular dynamics simulations on the creep behavior
of metallic glass composites. Surprisingly, all composites
exhibit much higher creep rates than the homogeneous glass. The glass-crystal
interface can be viewed as a weak interphase, where the activation barrier of
shear transformation zones is lower than in the surrounding glass. We observe
that the creep behavior of the composites does not only depend on the interface
area but also on the orientation of the interface with respect to the loading
axis. We propose an explanation in terms of different mean Schmid factors of
the interfaces, with the amorphous interface regions acting as preferential
slip sites.Comment: 11 pages, 13 figure
From Practice to Policy to Practice
Recent molecular dynamics simulation results have increased conceptual understanding of the grazing and the ploughing friction at elevated temperatures, particularly near the substrate's melting point. In this commentary we address a major constraint concerning its experimental verification
A scalable parallel Monte Carlo algorithm for atomistic simulations of precipitation in alloys
We present an extension of the semi-grandcanonical (SGC) ensemble that we
refer to as the variance-constrained semi-grandcanonical (VC-SGC) ensemble. It
allows for transmutation Monte Carlo simulations of multicomponent systems in
multiphase regions of the phase diagram and lends itself to scalable
simulations on massively parallel platforms. By combining transmutation moves
with molecular dynamics steps structural relaxations and thermal vibrations in
realistic alloys can be taken into account. In this way, we construct a robust
and efficient simulation technique that is ideally suited for large-scale
simulations of precipitation in multicomponent systems in the presence of
structural disorder. To illustrate the algorithm introduced in this work, we
study the precipitation of Cu in nanocrystalline Fe.Comment: 12 pages; 10 figure
Solute effects on edge dislocation pinning in complex alpha-Fe alloys
Reactor pressure vessel steels are well-known to harden and embrittle under neutron irradiation, mainly because of the formation of obstacles to the motion of dislocations, in particular, precipitates and clusters composed of Cu, Ni, Mn, Si and P. In this paper, we employ two complementary atomistic modelling techniques to study the heterogeneous precipitation and segregation of these elements and their effects on the edge dislocations in BCC iron. We use a special and highly computationally efficient Monte Carlo algorithm in a constrained semi-grand canonical ensemble to compute the equilibrium configurations for solute clusters around the dislocation core. Next, we use standard molecular dynamics to predict and analyze the effect of this segregation on the dislocation mobility. Consistently with expectations our results confirm that the required stress for dislocation unpinning from the precipitates formed on top of it is quite large. The identification of the precipitate resistance allows a quantitative treatment of atomistic results, enabling scale transition towards larger scale simulations, such as dislocation dynamics or phase field.Fil: Pascuet, Maria Ines Magdalena. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comision Nacional de Energia Atomica. Centro Atomico Constituyentes. Departamento de Materiales; ArgentinaFil: Martínez, E.. Los Alamos National High Magnetic Field Laboratory; Estados UnidosFil: Monnet, G.. EDF–R&D; FranciaFil: Malerba, L.. SCK•CEN. Structural Materials Expert Group. Nuclear Materials Institute; Bélgic
Global transition path search for dislocation formation in Ge on Si(001)
Global optimization of transition paths in complex atomic scale systems is
addressed in the context of misfit dislocation formation in a strained Ge film
on Si(001). Such paths contain multiple intermediate minima connected by
minimum energy paths on the energy surface emerging from the atomic
interactions in the system. The challenge is to find which intermediate states
to include and to construct a path going through these intermediates in such a
way that the overall activation energy for the transition is minimal. In the
numerical approach presented here, intermediate minima are constructed by
heredity transformations of known minimum energy structures and by identifying
local minima in minimum energy paths calculated using a modified version of the
nudged elastic band method. Several mechanisms for the formation of a 90{\deg}
misfit dislocation at the Ge-Si interface are identified when this method is
used to construct transition paths connecting a homogeneously strained Ge film
and a film containing a misfit dislocation. One of these mechanisms which has
not been reported in the literature is detailed. The activation energy for this
path is calculated to be 26% smaller than the activation energy for half loop
formation of a full, isolated 60{\deg} dislocation. An extension of the common
neighbor analysis method involving characterization of the geometrical
arrangement of second nearest neighbors is used to identify and visualize the
dislocations and stacking faults
Anomalous compliance and early yielding of nanoporous gold
We present a study of the elastic and plastic behavior of nanoporous gold in
compression, focusing on molecular dynamics simulation and inspecting
experimental data for verification. Both approaches agree on an anomalously
high elastic compliance in the early stages of deformation, along with a quasi
immediate onset of plastic yielding even at the smallest load. Already before
the first loading, the material undergoes spontaneous plastic deformation under
the action of the capillary forces, requiring no external load. Plastic
deformation under compressive load is accompanied by dislocation storage and
dislocation interaction, along with strong strain hardening.
Dislocation-starvation scenarios are not supported by our results. The
stiffness increases during deformation, but never approaches the prediction by
the relevant Gibson-Ashby scaling law. Microstructural disorder affects the
plastic deformation behavior and surface excess elasticity might modify elastic
response, yet we relate the anomalous compliance and the immediate yield onset
to an atomistic origin: the large surface-induced prestress induces elastic
shear that brings some regions in the material close to the shear instability
of the generalized stacking fault energy curve. These regions are elastically
highly compliant and plastically weak
Minimum energy path for the nucleation of misfit dislocations in Ge/Si(001) heteroepitaxy
A possible mechanism for the formation of a 90{\deg} misfit dislocation at
the Ge/Si(001) interface through homogeneous nucleation is identified from
atomic scale calculations where a minimum energy path connecting the coherent
epitaxial state and a final state with a 90{\deg} misfit dislocation is found
using the nudged elastic band method. The initial path is generated using a
repulsive bias activation procedure in a model system including 75000 atoms.
The energy along the path exhibits two maxima in the energy. The first maximum
occurs as a 60{\deg} dislocation nucleates. The intermediate minimum
corresponds to an extended 60{\deg} dislocation. The subsequent energy maximum
occurs as a second 60{\deg} dislocation nucleates in a complementary, mirror
glide plane, simultaneously starting from the surface and from the first
60{\deg} dislocation. The activation energy of the nucleation of the second
dislocation is 30% lower than that of the first one showing that the formation
of the second 60{\deg} dislocation is aided by the presence of the first one.
The simulations represent a step towards unraveling the formation mechanism of
90{\deg} dislocations, an important issue in the design of growth procedures
for strain released Ge overlayers on Si(100) surfaces, and more generally
illustrate an approach that can be used to gain insight into the mechanism of
complex nucleation paths of extended defects in solids
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