2,466 research outputs found
Kinetics of natural aging in Al-Mg-Si alloys studied by positron annihilation lifetime spectroscopy
The process of natural aging in pure ternary Al-Mg-Si alloys was studied by
positron annihilation lifetime spectroscopy in real time in order to clarify
the sequence and kinetics of clustering and precipitation. It was found that
natural aging takes place in at least five stages in these alloys, four of
which were directly observed. This is interpreted as the result of complex
interactions between vacancies and solute atoms or clusters. One of the early
stages of positron lifetime evolution coincides with a clustering process
observed by differential scanning calorimetry (DSC) and involves the formation
of a positron trap with \sim 0.200 ns lifetime. In later stages, a positron
trap with a higher lifetime develops in coincidence with the DSC signal of a
second clustering reaction. Mg governs both the kinetics and the lifetime
change in this stage. Within the first 10 min after quenching, a period of
nearly constant positron lifetime was found for those Mg-rich alloys that later
show an insufficient hardness response to artificial aging, the so-called
"negative effect." The various processes observed could be described by two
effective activation energies that were found by varying the aging temperature
from 10 to 37\degree C.Comment: arXiv admin note: same as v2, to correct mistaken v
Condensed Multiwalled Carbon Nanotubes as Super Fibers
The ultra-low intershell shear strength in carbon nanotubes (CNTs) has been
the primary obstacle to applications of CNTs as mechanical reinforcements. In
this paper we propose a new CNT-system composed of comprising of coaxial
cylindrical shells of sp2-bonded carbons with condensed intershell spacings.
Our atomistic calculations show that such condensed multiwalled carbon
nanotubes (CMWNTs) can greatly enhance intershell shear strengths by several
orders, and can simultaneously generate higher tensile strengths and moduli
respectively than those of ordinary CNTs. It has further shown that CMWNTs can
maintain thermally stable up to 2,000 K. By taking advantage of the primary
enhancement mechanism of CMWNTs, a method of producing CMWNTs is therefore
proposed tentatively. It is believed that CMWNTs featured with those properties
can be taken as excellent candidates of super fibers for creating space
elevators
Depletion of density of states near Fermi energy induced by disorder and electron correlation in alloys
We have performed high resolution photoemission study of substitutionally
disordered alloys Cu-Pt, Cu-Pd, Cu-Ni, and Pd-Pt. The ratios between alloy
spectra and pure metal spectra are found to have dips at the Fermi level when
the residual resistivity is high and when rather strong repulsive
electron-electron interaction is expected. This is in accordance with Altshuler
and Aronov's model which predicts depletion of density of states at the Fermi
level when both disorder and electron correlation are present.Comment: 1 tex file and 4 ps file
Electrical conductivity measured in atomic carbon chains
The first electrical conductivity measurements of monoatomic carbon chains
are reported in this study. The chains were obtained by unraveling carbon atoms
from graphene ribbons while an electrical current flowed through the ribbon
and, successively, through the chain. The formation of the chains was
accompanied by a characteristic drop in the electrical conductivity. The
conductivity of carbon chains was much lower than previously predicted for
ideal chains. First-principles calculations using both density functional and
many-body perturbation theory show that strain in the chains determines the
conductivity in a decisive way. Indeed, carbon chains are always under varying
non-zero strain that transforms its atomic structure from cumulene to polyyne
configuration, thus inducing a tunable band gap. The modified electronic
structure and the characteristics of the contact to the graphitic periphery
explain the low conductivity of the locally constrained carbon chain.Comment: 21 pages, 9 figure
Non-hexagonal-ring defects and structures induced by strain in graphene and in functionalized graphene
We perform {\textit ab initio} calculations for the strain-induced formation
of non-hexagonal-ring defects in graphene, graphane (planar CH), and graphenol
(planar COH). We find that the simplest of such topological defects, the
Stone-Wales defect, acts as a seed for strain-induced dissociation and
multiplication of topological defects. Through the application of inhomogeneous
deformations to graphene, graphane and graphenol with initially small
concentrations of pentagonal and heptagonal rings, we obtain several novel
stable structures that possess, at the same time, large concentrations of
non-hexagonal rings (from fourfold to elevenfold) and small formation energies
Study of ageing in Al-Mg-Si alloys by positron annihilation spectroscopy
In many common Al-Mg-Si alloys (6000 series) intermediate storage at or near
'room temperature' after solutionising leads to pronounced changes of the
precipitation kinetics during the ensuing artificial ageing step at \approx
180{\deg}C. This is not only an annoyance in production, but also a challenge
for researchers. We studied the kinetics of natural 'room temperature' ageing
(NA) in Al-Mg-Si alloys by means of various different techniques, namely
electrical resistivity and hardness measurement, thermoanalysis and positron
lifetime and Doppler broadening (DB) spectroscopy to identify the stages in
which the negative effect of NA on artificial ageing might appear. Positron
lifetime measurements were carried out in a fast mode, allowing us to measure
average lifetimes in below 1 minute. DB measurements were carried out with a
single detector and a 68Ge positron source by employing high momentum analysis.
The various measurements show that NA is much more complex than anticipated and
at least four different stages can be distinguished. The nature of these stages
cannot be given with certainty, but a possible sequence includes vacancy
diffusion to individual solute atoms, nucleation of solute clusters, Mg
agglomeration to clusters and coarsening or ordering of such clusters. Positron
lifetime measurements after more complex ageing treatments involving storage at
0{\deg}C, 20{\deg}C and 180{\deg}C have also been carried out and help to
understand the mechanisms involved
The Influence of Alloy Composition and Liquid Phase on Foaming of AlâSiâMg Alloys
The foaming behaviour of aluminium alloys processed by the powder compaction technique depends crucially on the exact alloy composition. The AlSi8Mg4 alloy has been in use for a decade now, and it has been claimed that this composition lies in an âisland of good foamingâ. We investigated the reasons for this by systematically studying alloys around this composition by varying the Mg and Si content by a few percent. We applied in situ X-ray 2D and 3D imaging experiments combined with a quantitative nucleation number and expansion analysis, X-ray tomography of solid foams to assess the pore structure and pore size distribution, and in situ diffraction experiments to quantify the melt fraction at any moment. We found a correlation between melt fraction and expansion height and verified that the âisland of good foamingâ actually exists, and foams outside a preferred range for the liquid fractionâjust above TS and between 40â60%âshow a poorer expansion performance than the reference alloy AlSi8Mg4. A very slight increase of Si and decrease of Mg content might further improve this foam
Electrical transport properties of bulk NiFe alloys and related spin-valve systems
Within the Kubo-Greenwood formalism we use the fully relativistic,
spin-polarized, screened Korringa-Kohn-Rostoker method together with the
coherent-potential approximation for layered systems to calculate the
resistivity for the permalloy series NiFe. We are able to
reproduce the variation of the resistivity across the entire series; notably
the discontinuous behavior in the vicinity of the structural phase transition
from bcc to fcc. The absolute values for the resistivity are within a factor of
two of the experimental data. Also the giant magnetoresistance of a series of
permalloy-based spin-valve structures is estimated; we are able to reproduce
the trends and values observed on prototypical spin-valve structures.Comment: 6 pages, ReVTeX + 4 figures (Encapsulated Postscript), submitted to
PR
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