508 research outputs found
Dislocation transport and line length increase in averaged descriptions of dislocations
Crystal plasticity is the result of the motion and interaction of
dislocations. There is, however, still a major gap between microscopic and
mesoscopic simulations and continuum crystal plasticity models. Only recently a
higher dimensional dislocation density tensor was defined which overcomes some
drawbacks of earlier dislocation density measures. The evolution equation for
this tensor can be considered as a continuum version of dislocation dynamics.
We use this evolution equation to develop evolution equations for the total
dislocation density and an average curvature which together govern a faithful
representation of the dislocation kinematics without having to use extra
dimensions
Screened empirical bond-order potentials for Si-C
Typical empirical bond-order potentials are short ranged and give ductile
instead of brittle behavior for materials such as crystalline silicon or
diamond. Screening functions can be used to increase the range of these
potentials. We outline a general procedure to combine screening functions with
bond-order potentials that does not require to refit any of the potential's
properties. We use this approach to modify Tersoff's [Phys. Rev. B 39, 5566
(1989)], Erhart & Albe's [Phys. Rev. B 71, 35211 (2005)] and Kumagai et al.'s
[Comp. Mater. Sci. 39, 457 (2007)] Si, C and Si-C potentials. The resulting
potential formulations correctly reproduce brittle materials response, and give
an improved description of amorphous phases
Stress correlations of dislocations in a double-pileup configuration: a continuum dislocation density approach – complas XII
Dislocation motion in the crystal lattice of materials is the basis for macroscopic plasticity. While continuum models for describing the role of dislocations in plasticity have existed for decades, only recently have the mathematical tools become available to describe ensembles of moving, oriented lines. These tools have allowed for the creation of a Continuum Dislocation Dynamics (CDD) theory describing a second-order dislocation density tensor, a higher order analog of the classical dislocation density tensor, and its evolution in time. In order to reduce the computational complexity of the theory, a simplified theory has also been developed, which more readily allows for a numerical implementation, useful for describing larger systems of dislocations. In order to construct a self-consistent implementation, several issues have to be resolved including calculation of the stress field of a system of dislocations, coarse graining, and boundary values. The present work deals with the implementation including treatment of the near- and far-field stresses caused by the dislocation density tensor as well as boundary value considerations. The implementation is then applied to a few simple benchmark problems, notably the double pileup of dislocations in 1D. Applications to more general problems are considered, as well as comparisons with analytical solutions to classical dislocation problems. Focus is placed on problems where analytical solutions as well as simulations of discrete dislocations are known which act, along with experimental results, as the basis of comparison to determine the validity of the results
Fracture of complex metallic alloys: An atomistic study of model systems
Molecular dynamics simulations of crack propagation are performed for two
extreme cases of complex metallic alloys (CMAs): In a model quasicrystal the
structure is determined by clusters of atoms, whereas the model C15 Laves phase
is a simple periodic stacking of a unit cell. The simulations reveal that the
basic building units of the structures also govern their fracture behaviour.
Atoms in the Laves phase play a comparable role to the clusters in the
quasicrystal. Although the latter are not rigid units, they have to be regarded
as significant physical entities.Comment: 6 pages, 4 figures, for associated avi file, see
http://www.itap.physik.uni-stuttgart.de/~frohmut/MOVIES/C15.LJ.011.100.av
Dynamic fracture of icosahedral model quasicrystals: A molecular dynamics study
Ebert et al. [Phys. Rev. Lett. 77, 3827 (1996)] have fractured icosahedral
Al-Mn-Pd single crystals in ultrahigh vacuum and have investigated the cleavage
planes in-situ by scanning tunneling microscopy (STM). Globular patterns in the
STM-images were interpreted as clusters of atoms. These are significant
structural units of quasicrystals. The experiments of Ebert et al. imply that
they are also stable physical entities, a property controversially discussed
currently. For a clarification we performed the first large scale fracture
simulations on three-dimensional complex binary systems. We studied the
propagation of mode I cracks in an icosahedral model quasicrystal by molecular
dynamics techniques at low temperature. In particular we examined how the shape
of the cleavage plane is influenced by the clusters inherent in the model and
how it depends on the plane structure. Brittle fracture with no indication of
dislocation activity is observed. The crack surfaces are rough on the scale of
the clusters, but exhibit constant average heights for orientations
perpendicular to high symmetry axes. From detailed analyses of the fractured
samples we conclude that both, the plane structure and the clusters, strongly
influence dynamic fracture in quasicrystals and that the clusters therefore
have to be regarded as physical entities.Comment: 10 pages, 12 figures, for associated avi files, see
http://www.itap.physik.uni-stuttgart.de/~frohmut/MOVIES/emitted_soundwaves.avi
and
http://www.itap.physik.uni-stuttgart.de/~frohmut/MOVIES/dynamic_fracture.av
Diffraction microstrain in nanocrystalline solids under load - heterogeneous medium approach
This is an account of the computation of X-ray microstrain in a polycrystal
with anisotropic elasticity under uniaxial external load. The results have been
published in the article "Microstrain in nanocrystalline solids under load by
virtual diffraction", at Europhysics Letters 89, 66002 (2010). The present
information was submitted to Europhysics Letters as part of the manuscript
package, and was available to the reviewers who recommended the paper for
publication.Comment: Supporting online material for J. Markmann, D. Bachurin, L.-H. Shao,
P. Gumbsch, J. Weissm\"uller, Microstrain in nanocrystalline solids under
load by virtual diffraction, Europhys. Lett. 89, 66002 (2010
Atomistically enabled nonsingular anisotropic elastic representation of near-core dislocation stress fields in -iron
The stress fields of dislocations predicted by classical elasticity are known
to be unrealistically large approaching the dislocation core, due to the
singular nature of the theory. While in many cases this is remedied with the
approximation of an effective core radius, inside which ad hoc regularizations
are implemented, such approximations lead to a compromise in the accuracy of
the calculations. In this work, an anisotropic non-singular elastic
representation of dislocation fields is developed to accurately represent the
near-core stresses of dislocations in -iron. The regularized stress
field is enabled through the use of a non-singular Green's tensor function of
Helmholtz-type gradient anisotropic elasticity, which requires only a single
characteristic length parameter in addition to the material's elastic
constants. Using a novel magnetic bond-order potential to model atomic
interactions in iron, molecular statics calculations are performed, and an
optimization procedure is developed to extract the required length parameter.
Results show the method can accurately replicate the magnitude and decay of the
near-core dislocation stresses even for atoms belonging to the core itself.
Comparisons with the singular isotropic and anisotropic theories show the
non-singular anisotropic theory leads to a substantially more accurate
representation of the stresses of both screw and edge dislocations near the
core, in some cases showing improvements in accuracy of up to an order of
magnitude. The spatial extent of the region in which the singular and
non-singular stress differ substantially is also discussed. The general
procedure we describe may in principle be applied to accurately model the
near-core dislocation stresses of any arbitrarily shaped dislocation in
anisotropic cubic media.Comment: Appearing in Phys. Rev.
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