36,901 research outputs found
Effect of iron on the microstructure and mechanical property of Al-Mg-Si-Mn and Al-Mg-Si diecast alloys
This article is made available through the Brunel Open Access Publishing Fund. Copyright @ 2012 Elsevier B.V.This article has been made available through the Brunel Open Access Publishing Fund.AlāMgāSi based alloys can provide super ductility to satisfy the demands of thin wall castings in the application of automotive structure. In this work, the effect of iron on the microstructure and mechanical properties of the AlāMgāSi diecast alloys with different Mn concentrations is investigated. The CALPHAD (acronym of Calculation of Phase Diagrams) modelling with the thermodynamic properties of the multi-component AlāMgāSiāMnāFe and AlāMgāSiāFe systems is carried out to understand the role of alloying on the formation of different primary Fe-rich intermetallic compounds. The results showed that the Fe-rich intermetallic phases precipitate in two solidification stages in the high pressure die casting process: one is in the shot sleeve and the other is in the die cavity, resulting in the different morphologies and sizes. In the AlāMgāSiāMn alloys, the Fe-rich intermetallic phase formed in the shot sleeve exhibited coarse compact morphology and those formed in the die cavity were fine compact particles. Although with different morphologies, the compact intermetallics were identified as the same Ī±-AlFeMnSi phase with typical composition of Al24(Fe,Mn)6Si2. With increased Fe content, Ī²-AlFe was found in the microstructure with a long needle-shaped morphology, which was identified as Al13(Fe,Mn)4Si0.25. In the AlāMgāSi alloy, the identified Fe-rich intermetallics included the compact Ī±-AlFeSi phase with typical composition of Al8Fe2Si and the needle-shaped Ī²-AlFe phase with typical composition of Al13Fe4. Generally, the existence of iron in the alloy slightly increases the yield strength, but significantly reduces the elongation. The ultimate tensile strength maintains at similar levels when Fe contents is less than 0.5 wt%, but decreases significantly with the further increased Fe concentration in the alloys. CALPHAD modelling shows that the addition of Mn enlarges the Fe tolerance for the formation of Ī±-AlFeMnSi intermetallics and suppresses the formation of Ī²-AlFe phase in the AlāMgāSi alloys, and thus improves their mechanical properties.EPSRC and JL
Diffusion of a liquid nanoparticle on a disordered substrate
We perform molecular dynamic simulations of liquid nanoparticles deposited on
a disordered substrate. The motion of the nanoparticle is characterised by a
'stick and roll' diffusive process. Long simulation times (),
analysis of mean square displacements and stacking time distribution functions
demonstrate that the nanoparticle undergoes a normal diffusion in spite of long
sticking times. We propose a phenomenological model for the size and
temperature dependence of the diffusion coefficient in which the activation
energy scales as .Comment: Accepted for publication in Phys. Rev.
Effect of iron on the microstructure and mechanical property of Al-Mg-Si-Mn and Al-Mg-Si diecast alloys
This article is made available through the Brunel Open Access Publishing Fund. Copyright @ 2012 Elsevier B.V.This article has been made available through the Brunel Open Access Publishing Fund.AlāMgāSi based alloys can provide super ductility to satisfy the demands of thin wall castings in the application of automotive structure. In this work, the effect of iron on the microstructure and mechanical properties of the AlāMgāSi diecast alloys with different Mn concentrations is investigated. The CALPHAD (acronym of Calculation of Phase Diagrams) modelling with the thermodynamic properties of the multi-component AlāMgāSiāMnāFe and AlāMgāSiāFe systems is carried out to understand the role of alloying on the formation of different primary Fe-rich intermetallic compounds. The results showed that the Fe-rich intermetallic phases precipitate in two solidification stages in the high pressure die casting process: one is in the shot sleeve and the other is in the die cavity, resulting in the different morphologies and sizes. In the AlāMgāSiāMn alloys, the Fe-rich intermetallic phase formed in the shot sleeve exhibited coarse compact morphology and those formed in the die cavity were fine compact particles. Although with different morphologies, the compact intermetallics were identified as the same Ī±-AlFeMnSi phase with typical composition of Al24(Fe,Mn)6Si2. With increased Fe content, Ī²-AlFe was found in the microstructure with a long needle-shaped morphology, which was identified as Al13(Fe,Mn)4Si0.25. In the AlāMgāSi alloy, the identified Fe-rich intermetallics included the compact Ī±-AlFeSi phase with typical composition of Al8Fe2Si and the needle-shaped Ī²-AlFe phase with typical composition of Al13Fe4. Generally, the existence of iron in the alloy slightly increases the yield strength, but significantly reduces the elongation. The ultimate tensile strength maintains at similar levels when Fe contents is less than 0.5 wt%, but decreases significantly with the further increased Fe concentration in the alloys. CALPHAD modelling shows that the addition of Mn enlarges the Fe tolerance for the formation of Ī±-AlFeMnSi intermetallics and suppresses the formation of Ī²-AlFe phase in the AlāMgāSi alloys, and thus improves their mechanical properties.EPSRC and JL
Electronic bandstructure and optical gain of lattice matched III-V dilute nitride bismide quantum wells for 1.55 m optical communication systems
Dilute nitride bismide GaNBiAs is a potential semiconductor alloy for near-
and mid-infrared applications, particularly in 1.55 m optical
communication systems. Incorporating dilute amounts of Bismuth (Bi) into GaAs
reduces the effective bandgap rapidly, while significantly increasing the
spin-orbit-splitting energy. Additional incorporation of dilute amounts of
Nitrogen (N) helps to attain lattice matching with GaAs, while providing a
route for flexible bandgap tuning. Here we present a study of the electronic
bandstructure and optical gain of the lattice matched
GaNBiAs/GaAs quaternary alloy quantum well (QW) based on the
16-band kp model. We have taken into consideration the interactions
between the N and Bi impurity states with the host material based on the band
anticrossing (BAC) and valence band anticrossing (VBAC) model. The optical gain
calculation is based on the density matrix theory. We have considered different
lattice matched GaNBiAs QW cases and studied their energy dispersion curves,
optical gain spectrum, maximum optical gain and differential gain; and compared
their performances based on these factors. The thickness and composition of
these QWs were varied in order to keep the emission peak fixed at 1.55 m.
The well thickness has an effect on the spectral width of the gain curves. On
the other hand, a variation in the injection carrier density has different
effects on the maximum gain and differential gain of QWs of varying
thicknesses. Among the cases studied, we found that the 6.3 nm thick
GaNBiAs lattice matched QW was most suited for 1.55
m (0.8 eV) GaAs-based photonic applications.Comment: Accepted in AIP Journal of Applied Physic
ROAM: a Radial-basis-function Optimization Approximation Method for diagnosing the three-dimensional coronal magnetic field
The Coronal Multichannel Polarimeter (CoMP) routinely performs coronal
polarimetric measurements using the Fe XIII 10747 and 10798 lines,
which are sensitive to the coronal magnetic field. However, inverting such
polarimetric measurements into magnetic field data is a difficult task because
the corona is optically thin at these wavelengths and the observed signal is
therefore the integrated emission of all the plasma along the line of sight. To
overcome this difficulty, we take on a new approach that combines a
parameterized 3D magnetic field model with forward modeling of the polarization
signal. For that purpose, we develop a new, fast and efficient, optimization
method for model-data fitting: the Radial-basis-functions Optimization
Approximation Method (ROAM). Model-data fitting is achieved by optimizing a
user-specified log-likelihood function that quantifies the differences between
the observed polarization signal and its synthetic/predicted analogue. Speed
and efficiency are obtained by combining sparse evaluation of the magnetic
model with radial-basis-function (RBF) decomposition of the log-likelihood
function. The RBF decomposition provides an analytical expression for the
log-likelihood function that is used to inexpensively estimate the set of
parameter values optimizing it. We test and validate ROAM on a synthetic test
bed of a coronal magnetic flux rope and show that it performs well with a
significantly sparse sample of the parameter space. We conclude that our
optimization method is well-suited for fast and efficient model-data fitting
and can be exploited for converting coronal polarimetric measurements, such as
the ones provided by CoMP, into coronal magnetic field data.Comment: 23 pages, 12 figures, accepted in Frontiers in Astronomy and Space
Science
The Nullity of Bicyclic Signed Graphs
Let \Gamma be a signed graph and let A(\Gamma) be the adjacency matrix of
\Gamma. The nullity of \Gamma is the multiplicity of eigenvalue zero in the
spectrum of A(\Gamma). In this paper we characterize the signed graphs of order
n with nullity n-2 or n-3, and introduce a graph transformation which preserves
the nullity. As an application we determine the unbalanced bicyclic signed
graphs of order n with nullity n-3 or n-4, and signed bicyclic signed graphs
(including simple bicyclic graphs) of order n with nullity n-5
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