1,657 research outputs found
Coincidence site modules in 3-space
The coincidence site lattice (CSL) problem and its generalization to
Z-modules in Euclidean 3-space is revisited, and various results and
conjectures are proved in a unified way, by using maximal orders in quaternion
algebras of class number 1 over real algebraic number fields.Comment: 25 page
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
Magnetic hopfions in solids
Hopfions are an intriguing class of string-like solitons, named according to
a classical topological concept classifying three-dimensional direction fields.
The search of hopfions in real physical systems is going on for nearly half a
century, starting with the seminal work of Faddeev. But so far realizations in
solids are missing. Here, we present a theory that identifies magnetic
materials featuring hopfions as stable states without the assistance of
confinement or external fields. Our results are based on an advanced
micromagnetic energy functional derived from a spin-lattice Hamiltonian.
Hopfions appear as emergent particles of the classical Heisenberg model.
Magnetic hopfions represent three-dimensional particle-like objects of
nanometre-size dimensions opening the gate to a new generation of spintronic
devices in the framework of a truly three-dimensional architecture. Our
approach goes beyond the conventional phenomenological models. We derive
material-realistic parameters that serve as concrete guidance in the search of
magnetic hopfions bridging computational physics with materials science
The influence of solute diffusion and anisotropic elasticity on the deformation of iron: a study using planar discrete dislocation plasticity
Plastic deformation of crystalline metals is facilitated by crystallographic defects called dislocations. In certain alloys at certain temperatures and loading rates, the motion of these dislocations can become chaotic and unstable. This is a phenomenon related to dynamic strain ageing called the Portevin–Le Châtelier effect. The effect occurs when impurities, such as carbon and nitrogen, segregate to dislocations and obstruct their motion, leading to reduced ductility, hardening, and in the worst case, mechanical failure. In this thesis, I model plasticity with planar discrete dislocation plasticity. I extend planar discrete dislocation plasticity using anisotropic elasticity and introduce discrete diffusing solutes. This is used to understand the deformation of iron; and the onset and progression of the Portevin–
Le Châtelier effect. Results from the model show that the nature of the diffusing solute field significantly affects the manifestation of the Portevin–Le Châtelier effect. As the solute concentration increases, the iron specimen’s proof stress, flow stress and solute strengthening increases; and the serrations recorded in a stress-strain curve (symptomatic of the Portevin–Le Châtelier effect) become smaller and occur at a slower frequency. Conversely, as the solute diffusivity increases, the serrations become larger and occur at a faster frequency. Furthermore, I show that anisotropic elasticity alters the mechanisms of plastic deformation of α-iron at elevated temperatures; specifically, that a type edge dislocations, which are energetically unfavourable at moderate temperatures, are nucleated in increasing numbers. This agrees with experimental observations found in the literature but cannot be predicted by isotropic elastic models. The increase in a edge dislocations is symptomatic of the precipitous decrease in the yield strength of iron. My work has important consequences for the application of steels in elevated temperature environments such as generation IV very high temperature nuclear fission reactors.Open Acces
Periodic minimal surfaces of cubic symmetry
A survey of cubic minimal surfaces is presented, based on the concept of fundamental surface patches and their relation to the asymmetric units of the space groups. The software Surface Evolver has been used to test for stability and to produce graphic displays. Particular emphasis is given to those surfaces that can be generated by a finite piece bounded by straight
lines. Some new varieties have been found and a systematic nomenclature is introduced, which provides a symbol (a ‘gene’) for each triply-periodic minimal surface that specifies the surface unambiguously
Similar sublattices of the root lattice
Similar sublattices of the root lattice are possible, according to a
result of Conway, Rains and Sloane, for each index that is the square of a
non-zero integer of the form . Here, we add a constructive
approach, based on the arithmetic of the quaternion algebra and the existence of a particular involution of the
second kind, which also provides the actual sublattices and the number of
different solutions for a given index. The corresponding Dirichlet series
generating function is closely related to the zeta function of the icosian
ring.Comment: 17 pages, 1 figure; revised version with several additions and
improvement
Structural trends in clusters of quadrupolar spheres
The influence of quadrupolar interactions on the structure of small clusters
is investigated by adding a point quadrupole of variable strength to the
Lennard-Jones potential. Competition arises between sheet-like arrangements of
the particles, favoured by the quadrupoles, and compact structures, favoured by
the isotropic Lennard-Jones attraction. Putative global potential energy minima
are obtained for clusters of up to 25 particles using the basin-hopping
algorithm. A number of structural motifs and growth sequences emerge, including
star-like structures, tubes, shells and sheets. The results are discussed in
the context of colloidal self-assembly.Comment: 8 pages, 6 figure
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Stochastic Geometry, Spatial Statistics and Statistical Physics
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