The Influence of Secondary Processing Conditions on the Mechanical Properties and Microstructure of a Particle Reinforced Aluminium Metal Matrix Composite

The influence of secondary processing conditions on an aluminium metal matrix composite, comprising of an AA2124 matrix and 3 Jlm particulate SiC reinforcement at 25 volume percent was investigated. The metal matrix composite (MMC) was extruded at three different temperatures, 350???????C, 450???????C and 550???????C, at a ratio of20:1 and at three different ratios, 5:1, 10:1 and 20:1, at a temperature of 450???????C. It was subsequently solution heat treated and naturally aged. A mechanical property assessment was carried out using standard tensile and rotating bend fatigue test methods to determine the properties of the material extruded under each condition. A novel technique using a Focussed Ion Beam (FIB) Microscope was developed to prepare polished specimens and microtextural analysis was performed by FIB imaging. Additionally, techniques were successfully established, through the use of FIB milling and polishing, to provide site-specific electron transparent films, permitting detailed examination ofthe microstructure with a transmission electron microscope. Material extruded at 550???????C exhibited a lower yield strength than material extruded at 350???????C and 450???????C, which was attributed to grain coarsening and recrystallisation. Evidence of recrystallisation was found during texture analysis by X-Ray diffraction, where there was a reduction in the intensity of the fibre texture in the extrusion direction. The phenomenon was also observed during irticrostructural analysis work, where recrystallised grains at grain boundaries were observed. Higher extrusion ratios offered a small improvement in tensile properties, due to an enhanced fibre texture within the microstructure. Microtextural examination gave evidence of the existence of both high angle grain and low angle grain boundaries for the material extruded at 350???????C. It is believed that a subgrain structure was partially transformed during extrusion, through subgrain rotation, leading to the formation of high angle grain boundaries. This'microstructure was found to offer the optimum mechanical properties.Imperial Users onl

Glow in the Dark Matter: Observing galactic halos with scattered light

We consider the observation of diffuse halos of light around the discs of spiral galaxies, as a probe of the interaction cross section between Dark Matter and photons. Using the galaxy M101 as an example, we show that for a scattering cross section at the level of 10^(-23) x (m/GeV) cm^2 or greater Dark Matter in the halo will scatter light out from the more luminous centre of the disc to larger radii, contributing to an effective increased surface brightness at the edges of the observed area on the sky. This allows us to set an upper limit on the DM-photon cross section using data from the Dragonfly instrument. We then show how to improve this constraint, and the potential for discovery, by combining the radial profile of DM-photon scattering with measurements at multiple wavelengths. Observation of diffuse light presents a new and potentially powerful way to probe the interactions of Dark Matter with photons, which is complimentary to existing searches.Comment: 6 pages, 3 figures: v2 matches version accepted to PRL, with an extended discussion of potential background

Shocking Signals of Dark Matter Annihilation

We examine whether charged particles injected by self-annihilating Dark Matter into regions undergoing Diffuse Shock Acceleration (DSA) can be accelerated to high energies. We consider three astrophysical sites where shock acceleration is supposed to occur, namely the Galactic Centre, galaxy clusters and Active Galactic Nuclei (AGN). For the Milky Way, we find that the acceleration of cosmic rays injected by dark matter could lead to a bump in the cosmic ray spectrum provided that the product of the efficiency of the acceleration mechanism and the concentration of DM particles is high enough. Among the various acceleration sources that we consider (namely supernova remnants (SNRs), Fermi bubbles and AGN jets), we find that the Fermi bubbles are a potentially more efficient accelerator than SNRs. However both could in principle accelerate electrons and protons injected by dark matter to very high energies. At the extragalactic level, the acceleration of dark matter annihilation products could be responsible for enhanced radio emission from colliding clusters and prediction of an increase of the anti-deuteron flux generated near AGNs.Comment: 9 pages, 4 figure

Thermal and Mechanical Performance of a Carbon/Carbon Composite Spacecraft Radiator

Carbon-carbon composite materials offer greater thermal efficiency, stiffness to weight ratio, tailorability, and dimensional stability than aluminum. These lightweight thermal materials could significantly reduce the overall costs associated with satellite thermal control and weight. However, the high cost and long lead-time for carbon-carbon manufacture have limited their widespread usage. Consequently, an informal partnership between government and industrial personnel called the Carbon-Carbon Spacecraft Radiator Partnership (CSRP) was created to foster carbon-carbon composite use for thermally and structurally demanding space radiator applications. The first CSRP flight opportunity is on the New Millennium Program (NMP) Earth Orbiter-1 (EO-1) spacecraft, scheduled for launch in late 1999. For EO-1, the CSRP designed and fabricated a Carbon-Carbon Radiator (CCR) with carbon-carbon facesheets and aluminum honeycomb core, which will also serve as a structural shear panel. While carbon-carbon is an ideal thermal candidate for spacecraft radiators, in practice there are technical challenges that may compromise performance. In this work, the thermal and mechanical performance of the EO-1 CCR is assessed by analysis and testing. Both then-nal and mechanical analyses were conducted to predict the radiator response to anticipated launch and on-orbit loads. The thermal model developed was based on thermal balance test conditions. The thermal analysis was performed using SINDA version 4.0. Structural finite element modeling and analysis were performed using SDRC/1-DEAS and UAI/NASTRAN, respectively. In addition, the CCR was subjected to flight qualification thermal/vacuum and vibration tests. The panel meets or exceeds the requirements for space flight and demonstrates promise for future satellite missions

Garlanding as sexual invitation: Indian Buddhist evidence

Asian Studie

The Victorian creation of Buddhism

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42939/1/10781_2005_Article_BF01061755.pd

Observational Constraints on the Dependence of Radio-Quiet Quasar X-ray Emission on Black Hole Mass and Accretion Rate

In this work we use a sample of 318 radio-quiet quasars (RQQ) to investigate the dependence of the ratio of optical/UV flux to X-ray flux, alpha_ox, and the X-ray photon index, Gamma_X, on black hole mass, UV luminosity relative to Eddington, and X-ray luminosity relative to Eddington. Our sample is drawn from the SDSS, with X-ray data from ROSAT and Chandra, and optical data mostly from the SDSS; 153 of these sources have estimates of Gamma_X from Chandra. We estimate M_BH using standard estimates derived from the Hbeta, Mg II, and C IV broad emission lines. Our sample spans a broad range in black hole mass (10^6 < M_BH / M_Sun < 10^10) and redshift (z < 4.8). We find that alpha_ox increases with increasing M_BH and L_UV / L_Edd, and decreases with increasing L_X / L_Edd. In addition, we confirm the correlation seen in previous studies between Gamma_X and M_BH and both L_UV / L_Edd and L_X / L_Edd; however, we also find evidence that the dependence of Gamma_X of these quantities is not monotonic, changing sign at M_BH ~ 3 x 10^8 M_Sun. We argue that the alpha_ox correlations imply that the fraction of bolometric luminosity emitted by the accretion disk, as compared to the corona, increases with increasing accretion rate relative to Eddington. In addition, we argue that the Gamma_X trends are caused by a dependence of X-ray spectral index on accretion rate. We discuss our results within the context of accretion models with comptonizing corona, and discuss the implications of the alpha_ox correlations for quasar feedback. To date, this is the largest study of the dependence of RQQ X-ray parameters on black hole mass and related quantities, and the first to attempt to correct for the large statistical uncertainty in the broad line mass estimates.Comment: Accepted by ApJ, 23 pages, 15 figures, emulateapj styl