4,693,049 research outputs found
Effects of solute content on grain refinement in an isothermal melt
This is the port-print version of the article. The official published version can be obtained from the link below - Copyright @ 2011 Acta Materialia Inc. Published by Elsevier LtdIt is well accepted in the literature that for effective grain refinement some solute is required in the melt to restrict the growth of the solid even if potent nucleating particles with a favourable physical nature are present. In this paper we investigate the effect of the solute on grain initiation in an isothermal melt, and an analytical model is developed to account for the effect of solute elements on grain size. This study revealed that the solute elements in the liquid ahead of the growing crystals reduce the growth velocity of the nucleated crystals and increase the maximum undercooling achievable before recalescence. This allows more particles to be active in nucleation and, consequently, increases the number density of active particles, giving rise to a finer grain size. The analytical model shows that the final grain size can be related to the maximum undercooling, average growth velocity and solid fraction at the moment of recalescence. Further analysis using the free growth model and experimental data in the literature revealed that for a given alloy system solidified under similar conditions the grain size can be empirically related to 1/Q (Q is the growth restriction factor) to a power of 1/3, which is considerably different from the empirical linear relationship in the literature. It is demonstrated that the 1/3 power law can describe the experimental data more accurately than a linear relationship.The EPSRC is gratefully acknowledged for providing financial support under Grant EP/H026177/1
Comparative study of dimer vacancies and dimer-vacancy lines on Si(001) and Ge(001)
Although the clean Si(001) and Ge(001) surfaces are very similar, experiments
to date have shown that dimer-vacancy (DV) defects self-organize into vacancy
lines (VLs) on Si(001), but not on Ge(001). In this paper, we perform
empirical-potential calculations aimed at understanding the differences between
the vacancies on Si(001) and Ge(001). We identify three energetic parameters
that characterize the DVs on the two surfaces: the formation energy of a single
DV, the attraction between two DVs in adjacent dimer rows, and the strain
sensitivity of the formation energy of DVs and VLs. At the empirical level of
treatment of the atomic interactions (Tersoff potentials), all three parameters
are favorable for the self-assembly of DVs on the Si(001) surface rather than
on Ge(001). The most significant difference between the defects on Si(001) and
on Ge(001) concerns the formation energy of single DVs, which is three times
larger in the latter case. By calculating the strain-dependent formation
energies of DVs and VLs, we propose that the experimental observation of
self-assembly of vacancies on clean Ge(001) could be achieved by applying
compressive strains of the order of 2%.Comment: 3 tables, 4 figures, to appear in Surface Scienc
Constraining the regular Galactic Magnetic Field with the 5-year WMAP polarization measurements at 22 GHz
[ABRIDGED] The knowledge of the regular component of the Galactic magnetic
field gives important information about the structure and dynamics of the Milky
Way, as well as constitutes a basic tool to determine cosmic rays trajectories.
It can also provide clear windows where primordial magnetic fields could be
detected. We want to obtain the regular (large scale) pattern of the magnetic
field distribution of the Milky Way that better fits the polarized synchrotron
emission as seen by the 5-year WMAP data at 22 GHz. We have done a systematic
study of a number of Galactic magnetic field models: axisymmetric, bisymmetric,
logarithmic spiral arms, concentric circular rings with reversals and
bi-toroidal. We have explored the parameter space defining each of these models
using a grid-based approach. In total, more than one million models are
computed. The model selection is done using a Bayesian approach. For each
model, the posterior distributions are obtained and marginalised over the
unwanted parameters to obtain the marginal 1-D probability distribution
functions. In general, axisymmetric models provide a better description of the
halo component, although attending to their goodness-of-fit, the rest of the
models cannot be rejected. In the case of disk component, the analysis is not
very sensitive for obtaining the disk large scale structure, because of the
effective available area (less than 8% of the whole map and less than 40% of
the disk). Nevertheless, within a given family of models, the best-fit
parameters are compatible with those found in the literature. The family of
models that better describes the polarized synchrotron halo emission is the
axisymmetric one, with magnetic spiral arms with a pitch angle of ~24 degrees,
and a strong vertical field of 1 microG at z ~ 1 kpc. When a radial variation
is fitted, models require fast variations.Comment: 14 pages, 9 figures. Accepted for publication in A&
Effects of lattice mismatch on interfacial structures of liquid and solidified Al in contact with hetero-phase substrates: MD simulations
Published under licence in IOP Conference Series: Material Science and Engineering by IOP Publishing Ltd. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.In this study, the effects of the misfit on in-plane structures of liquid Al and interfacial structure of solidified Al in contact with the heterophase substrates have been investigated, using molecular dynamics (MD) simulations. The MD simulations were conducted for Al/fcc (111) substrates with varied misfits. The order parameter and atomic arrangement indicated that the in-plane ordering of the liquid at the interface decreases significantly with an increase of the misfit, i.e., solid-like for small misfit and liquid-like for large misfit. Further, our MD simulation results revealed that a perfect orientation relationship forms at the interface between the substrate and the solidified Al for a misfit of less than -3% and the boundary is coherent. With an increase in the misfit, Shockley partial and extended dislocations form at the interface, and the boundary becomes a semi-coherent or low-angle twist boundary.EPSR
Climate change and rising energy costs: a threat but also an opportunity for a healthier future?
Health problems caused by overconsumption, growing inequalities and diminished well-being are issues that have been attributed to the prioritization of economic growth as the central purpose of society. It is also known that climate change and rising energy prices will inevitably bring changes to the globe's economic models. Doctors and the wider public health community have campaigned successfully in the past on issues such as the threat of nuclear war. Is it now time for this constituency to make its distinctive contribution to these new threats to health
Multicriteria VMAT optimization
Purpose: To make the planning of volumetric modulated arc therapy (VMAT)
faster and to explore the tradeoffs between planning objectives and delivery
efficiency.
Methods: A convex multicriteria dose optimization problem is solved for an
angular grid of 180 equi-spaced beams. This allows the planner to navigate the
ideal dose distribution Pareto surface and select a plan of desired target
coverage versus organ at risk sparing. The selected plan is then made VMAT
deliverable by a fluence map merging and sequencing algorithm, which combines
neighboring fluence maps based on a similarity score and then delivers the
merged maps together, simplifying delivery. Successive merges are made as long
as the dose distribution quality is maintained. The complete algorithm is
called VMERGE.
Results: VMERGE is applied to three cases: a prostate, a pancreas, and a
brain. In each case, the selected Pareto-optimal plan is matched almost exactly
with the VMAT merging routine, resulting in a high quality plan delivered with
a single arc in less than five minutes on average.
VMERGE offers significant improvements over existing VMAT algorithms. The
first is the multicriteria planning aspect, which greatly speeds up planning
time and allows the user to select the plan which represents the most desirable
compromise between target coverage and organ at risk sparing. The second is the
user-chosen epsilon-optimality guarantee of the final VMAT plan. Finally, the
user can explore the tradeoff between delivery time and plan quality, which is
a fundamental aspect of VMAT that cannot be easily investigated with current
commercial planning systems
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