Engineers constantly seek advancements in the performance of aircraft and power generation engines, including, lower costs and emissions, and improved fuel efficiency. Nickel-base superalloys are the material of choice for turbine discs, which experience some of the highest temperatures and stresses in the engine. Engine performance is proportional to operating temperatures. Consequently, the high-temperature capabilities of disc materials limit the performance of gas-turbine engines. Therefore, any improvements to engine performance necessitate improved alloy performance. In order to take advantage of improvements in high-temperature capabilities through tailoring of alloy microstructure, the overall objectives of this work were to establish relationships between alloy processing and microstructure, and between microstructure and mechanical properties. In addition, the project aimed to demonstrate the applicability of neural network modeling to the field of Ni-base disc alloy development and behavior. A full program of heat-treatment, microstructural quantification, mechanical testing, and neural network modeling was successfully applied to next generation Ni-base disc alloys. Mechanical testing included hot tensile, hot hardness, creep deformation, creep crack growth, and fatigue crack growth. From this work the mechanisms of processing-structure and structure-property relationships were studied. Further, testing results were used to demonstrate the applicability of machine-learning techniques to the development and optimization of this family of superalloys.

Dissertation (Ph.D.)--University of Toledo, 2009. / Typescript. "Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Physics ." Bibliography: leaves 71-74.

Thesis (Ph. D.)--University of California, Riverside, 2008. / Includes abstract. Available via ProQuest Digital Dissertations. Title from first page of PDF file (viewed March 10, 2010). Includes bibliographical references (p. 96-104). Also issued in print.

Thesis (Ph. D.)--University of California, San Diego, 2001. / Vita. Includes bibliographical references.

The Magnetorotational Instability (MRI) is believed to be the primary mechanism for angular momentum transfer in astrophysical accretion disks. This instability, which exists in ionized disks in the presence of weak magnetic fields, can either transfer angular momentum directly, or give rise to anomalous viscosity via non-linear turbulence. While many previous analytical treatments are concerned with the local theory of the MRI, when the length scale of rotation shear is comparable to the length scale of the most unstable modes, a global analysis is necessary. In this dissertation we investigate the global theory of the linear MRI. In particular, we show how rotation shear can localize global modes and how the global growth rates can differ signicantly from the local approximation in certain cases. Changes in the equilibrium density are considered. In addition, the effects of Hall Magnetohydrodynamics on the MRI are studied in both the local and global cases. / text

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Jouvence of FLUOR (JouFLU) is a major overhaul of the FLUOR (Fiber Linked Unit for Optical Recombination) beam combiner built by the Laboratoire d’études spatiales et d’instrumentation en astrophysique (LESIA) and installed at the CHARA Array. These upgrades improve the precision, observing efficiency, throughput, and integration of FLUOR with the CHARA Array as well as introduce new modes of operation to this high-precision instrument for interferometry. Such high precision observations with FLUOR have provided the first unambiguous detections of hot dust around main sequence stars, showing an unexpectedly dense population of (sub)micrometer dust grains close to their sublimation temperature, 1400 K. Competing models exist to explain the persistence of this dust; some of which suggest that dust production is a punctuated and chaotic process fueled by asteroid collisions and comet infall that would show variability on timescales of a few years. By re-observing stars from the exozodiacal disks survey we have searched for variations in the detected disks. We have found evidence that for some stars the amount of circumstellar flux from these previously detected exozodiacal disks, or exozodis, has varied. The flux from some exozodis has increased, for some the flux has decreased, and for a few the amount has remained constant. These results are intriguing and will be no doubt useful for future modeling of this phenomenon. Furthermore, long-term monitoring is suggested for some of these objects to confirm detections and determine the rate of variation.

published_or_final_version / Physics / Master / Master of Philosophy

In this thesis, RXTE observations of 4U 1907+09 are presented. Timing analysis of these data sets have yielded quasi periodic oscillations (QPOs) at orbital phases corresponding to the two flares in every orbital period. Known continuous spin down trend and QPO behaviour at the flares strongly suggest that a transient accretion disk occurs at the flares. Our findings strongly suggested that neutron star passes through the equatorial wind of Be companion star. During these passages a transient disk forms around Be neutron star.

A dual channel polarimeter was incorporated into the Hokupa'a adaptive optics system mounted on the Gemini North telescope to enhance sensitivity to detecting the light scattered by circumstellar material. The technique suppressed noise introduced by non-repeatable variations of the point spread function which limit the sensitivity of non-simultaneous adaptive optics imaging. Polarimetric images of the classical T-Tauri star environments around GG Tauri Aab, TW Hydrae, LkCa 15, LkHα 242, GM Aurigae, and SR24 N/S were observed to establish the instrument's sensitivity. A survey of nearby ( d < 25 pc), young ( age < 1 Gyr), solar-analog stars was undertaken with the polarimeter to search for collisionally active debris disks analogous to our young solar system. Of the 24 stars sampled, none were found to have obvious scattered light signatures. Isotropic and Mie scattering model images of debris disks were used to constrain the amount of material around the survey stars to no more than M dust ∼ 10 -2 M Moon of 1-10μ m sized dust contained between 5-50 AU from the sample stars. Particle lifetimes under the influence of the Poynting Robertson Drag, radiation pressure, and solar wind drag are calculated as a function of central star spectral type. The corpuscular drag from stellar winds shorten dust lifetimes by an amount inversely proportional to the stellar wind mass-loss rate. This translates into dust lifetimes 100-1000 times shorter around young solar analog stars compared to the present day. This effect, cam significantly reduce the near-IR detectability of debris disks around these chromospherically active stars.