Thesis (Ph. D.)--Boston University, 2003. / In this thesis, studies which concern two different subjects related to phase transitions in fluids and crystalline solids are presented. Condensed matter formation, structure, and phase transitions are modeled using molecular dynamics simulations of simple discontinuous potentials with attractive and repulsive interactions. Novel phase diagrams are proposed for quasicrystals, crystals, and liquids. In the first part of the thesis, the formation of a quasicrystal in a two dimensional monodisperse system is investigated using molecular dynamics simulations of hard sphere particles interacting via a two-dimensional square-well potential. It is found that for certain values of the square-well parameters more than one stable crystalline phase can form. By quenching the liquid phase at a very low temperature, an amorphous phase is obtained. When this the amorphous phase is heated, a quasicrystalline structure with five-fold symmetry forms. From estimations of the Helmholtz potentials of the stable crystalline phases and of the quasicrystal, it is concluded that within a specific temperature range, the observed quasicrystal phase can be the stable phase. The second part of the thesis concerns a study of the liquid-liquid phase transition for a single-component system in three dimensions, interacting via an isotropic potential with a repulsive soft-core shoulder at short distance and an attractive well at an intermediate distance. The potential is similar to potentials used to describe [TRUNCATED}

<p> An X-ray study of the various phases of anhydrous sodium pyrophosphate has been carried out. The system began with an ordered structure in orthorhombic space group P212121 and ended with a completely disordered structure in hexagonal space group P63/mmc, All the intermediate phases were partially disordered.</p> <p> The crystal structure of the first two phases of the system were determined while the rest were only partially solved due to the complexity of the disorder involved. A discussion on the symmetry aspects of the phase transformations was given and a model for the disorder of the hexagonal phase was proposed.</p> <p> As an integral part of the study the room-temperature phase of sodium diarsenate was also investigated.</p> / Thesis / Doctor of Philosophy (PhD)

No description available.

Medium-manganese steels (5-12 wt%) are candidates for third-generation advanced high strength steel (AHSS). Potential applications for these steels are centered around the automotive industry due to their combination of high tensile strength, high tensile ductility, and low alloying cost. Previous studies at VT have been primarily focused on the effect of chemistry on mechanical properties with only a minor emphasis on microstructure. This led to a detailed investigation into the effect of carbon content on the microstructure of Fe8Mn2AlSiC alloys. Six different chemistries with carbon contents of 0.30, 0.34, 0.39, 0.44, 0.49 and 0.52 wt% were produced at the Kroehling Advanced Materials Foundry. After a variety of heat treatments, the samples were characterized using x-ray diffraction (XRD), electron backscatter diffraction (EBSD), electron probe microanalysis (EPMA), optical microscopy, and hardness testing. This thesis will discuss how the microstructure and hardness of these medium manganese steels is influenced by the carbon content. / Master of Science / This research will be used to help design steel alloys that might one day be used in automotive applications. These steels need to be tough and ductile so they can absorb impact without fracturing. This is especially important in the event of a car crash, in which the steel needs to deform without breaking and causing injury to the driver or passenger. In order to achieve such qualities today, expensive elements are often added to the steel which increases cost. Medium manganese steels hope to alleviate this issue by providing a less expensive alternative with similar deformation properties. The properties of steel can be correlated with its microstructure, and more specifically, the different phases that make up the microstructure. These phases give rise to the macroscopic properties that make steel so useful. Microstructure can be controlled through chemistry and through thermomechanical processes. This research focuses on the effects of carbon and on heat treatments. This research is unique in that it keeps the chemistry consistent between all of the samples, making the effect of carbon or of the heat treatment identifiable. A total of six different carbon contents were tested over eight different heat treatment conditions. After creating the samples, the hardness was measured. The samples were then characterized to understand the microstructure. The results of this research showed there is a direct connection between heat treatment and chemistry to the microstructure.

Yu Wei-jian. / Thesis submitted in: November 2000. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2001. / Includes bibliographical references (leaves 70-71). / Text in English; abstracts in English and Chinese. / Yu Wei-jian. / TITLE PAGE --- p.i / THESIS COMMITTEE --- p.ii / ABSTRACT (English) --- p.iii / ABSTRACT (Chinese) --- p.iv / ACKNOWLEDGEMENTS --- p.v / TABLE OF CONTENTS --- p.vi / LIST OF FIGURES --- p.viii / LIST OF TABLES --- p.ix / Chapter CHAPTER 1 --- INTRUDUCTION --- p.1 / Chapter Section 1.1 --- Background of the BN System --- p.1 / Chapter 1.1.1 --- Desirable Properties of c-BN --- p.1 / Chapter 1.1.2 --- Phases in the BN System --- p.2 / Chapter 1.1.3 --- Phase Diagram of BN --- p.4 / Chapter 1.1.4 --- Synthesis Techniques for c-BN Films --- p.5 / Chapter 1.1.5 --- Characterization of BN Films --- p.6 / Chapter Section 1.2 --- Background of Theory --- p.7 / Chapter Section 1.3 --- Objectives --- p.9 / Chapter 1.3.1 --- Determination of Stable State of BN --- p.9 / Chapter 1.3.2 --- Phonon-dispersion Relations of BN --- p.9 / Chapter 1.3.3 --- "Phase (p, T) Diagram" --- p.10 / Chapter 1.3.4 --- Transformation Paths in Direct Compressions --- p.10 / Chapter Section 1.4 --- Roadmap --- p.11 / Chapter CHAPTER 2 --- METHODS --- p.12 / Chapter Section 2.1 --- Density Functional Theory (DFT) for E0 Calculation --- p.12 / Chapter Section 2.2 --- Direct Force-constant Method --- p.16 / Chapter Section 2.2 --- Quasi-harmonic Approximation --- p.26 / Chapter CHAPTER 3 --- RESULTS --- p.27 / Chapter Section 3.1 --- Stable State of BN --- p.27 / Chapter Section 3.2 --- Phonon-dispersion Relations --- p.29 / Chapter Section 3.3 --- "Phase (p, T) Diagram of BN" --- p.36 / Chapter Section 3.4 --- Transformation Paths via Direct Compression --- p.44 / Chapter 3.4.1 --- Direct Compression of h-BN and r-BN --- p.46 / Chapter 3.4.2 --- Direct Compression of t-BN --- p.50 / Chapter Section 3.5 --- Energy Barriers in the Transformation of h-BN to c-BN --- p.52 / Chapter CHAPTER 4 --- DISCUSSION --- p.58 / Chapter Section 4.1 --- Transition States in the Transformation of h-BN to c-BN --- p.58 / Chapter Section 4.2 --- Phonon-dispersion Relations --- p.60 / Chapter Section 4.3 --- Phase Diagrams --- p.62 / Chapter Section 4.4 --- Future Studies --- p.63 / Chapter 4.4.1 --- Cubic BN Films Formation --- p.63 / Chapter 4.4.1.1 --- Nanoarches Nucleation --- p.64 / Chapter 4.4.1.2 --- Growth: Interfaces Between h-BN {0001} and c-BN Planes --- p.64 / Chapter 4.4.2 --- Transformation Paths of w-BN to c-BN and h-BN to r-BN --- p.65 / Chapter CHAPTER 5 --- CONCLUSION --- p.66 / APPENDIX: Mechanistic Models in c-BN Films Formation --- p.67 / REFERENCES --- p.70

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE / Vita. / Includes bibliographical references. / by Lawrence Maxwell Krauss. / Ph.D.

Thesis (Ph. D.)--University of Chicago, Dept. of the Geophysical Sciences, August 1999. / Includes bibliographical references. Also available on the Internet.

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 174-187). Available also in a digital version from Dissertation Abstracts.

Thesis (Ph. D.)--University of Washington, 2001. / Vita. Includes bibliographical references (leaves 133-141).

Thesis (Ph.D)--Physics, Georgia Institute of Technology, 2009. / Committee Chair: Li Mo; Committee Member: Chou Meiyin; Committee Member: First Phillip; Committee Member: Nguyen Toan; Committee Member: Zangwill Andrew. Part of the SMARTech Electronic Thesis and Dissertation Collection.