Homogenization of alloying elements is desired during sintering of powder metallurgy components. The redistribution processes such as penetration of liquid phase into the interparticle/grain boundaries of solid particles and subsequent solid-state  diffusion of alloy element(s) in the base powder, are important for the effective homogenization of alloy element(s) during liquid phase sintering of the mixed powders. The aim of this study is to increase the understanding of alloy element redistribution processes and their effect on the dimensional properties of the compact by means of numerical and experimental techniques. The phase field model coupled with Navier-Stokes equations is used for the simulations of dynamic wetting of millimeter- and micrometer-sized metal drops and liquid phase penetration into interparticle boundaries. The simulations of solid particle rearrangement under the action of capillary forces exerted by the liquid phase are carried out by using the equilibrium equation for a linear elastic material. Thermodynamic and kinetic calculations are performed to predict the phase diagram and the diffusion distances respectively. The test materials used for the experimental studies are three different powder mixes; Fe-2%Cu, Fe-2%Cu-0.5%C, and Fe-2%(Cu-2%Ni-1.5%Si)-0.5%C. Light optical microscopy, energy dispersive X-ray spectroscopy and dilatometry are used to study the microstructure, kinetics of the liquid phase penetration, solid-state diffusion of the Cu, and the dimensional changes during sintering. The wetting simulations are verified by matching the spreading experiments of millimeter-sized metal drops and it is observed that wetting kinetics is much faster for a micrometer-sized drop compared to the millimeter-sized drop. The simulations predicted the liquid phase penetration kinetics and the motion of solid particles during the primary rearrangement stage of liquid phase sintering in agreement with the analytical model. Microscopy revealed that the C addition delayed the penetration of the Cu rich liquid phase into interparticle/grain boundaries of Fe particles, especially into the grain boundaries of large Fe particles, and consequently the Cu diffusion in Fe is also delayed. We propose that the relatively lower magnitude of the sudden volumetric expansion in the master alloy system could be due to the continuous melting of liquid forming master alloy particles. / <p>QC 20140515</p>

Communication delays are common in many complex systems. It has been shown that these delays cannot be neglected when they are long enough compared to other timescales in the system. In systems of coupled phase oscillators discrete delays in the coupling give rise to effects such as multistability of steady states. However, variability in the communication times inherent to many processes suggests that the description with discrete delays maybe insufficient to capture all effects of delays. An interesting example of the effects of communication delays is found during embryonic development of vertebrates. A clock based on biochemical reactions inside cells provides the periodicity for the successive and robust formation of somites, the embryonic precursors of vertebrae, ribs and some skeletal muscle. Experiments show that these cellular clocks communicate in order to synchronize their behavior. However, in cellular systems, fluctuations and stochastic processes introduce a variability in the communication times. Here we account for such variability by considering the effects of distributed delays. Our approach takes into account entire intervals of past states, and weights them according to a delay distribution. We find that the stability of the fully synchronized steady state with zero phase lag does not depend on the shape of the delay distribution, but the dynamics when responding to small perturbations about this steady state do. Depending on the mean of the delay distribution, a change in its shape can enhance or reduce the ability of these systems to respond to small perturbations about the phase-locked steady state, as compared to a discrete delay with a value equal to this mean. For synchronized steady states with non-zero phase lag we find that the stability of the steady state can be altered by changing the shape of the delay distribution. We conclude that the response to a perturbation in systems of phase oscillators coupled with discrete delays has a sharper functional dependence on the mean delay than in systems with distributed delays in the coupling. The strong dependence of the coupling on the mean delay time is partially averaged out by distributed delays that take into account intervals of the past.

Positioning an underwater object with respect to a reference point is required in diverse areas in ocean scientific and engineering undertakings, such as marine habitat monitoring, study of sedimentation processes, underwater searching and mapping, data collection, instrument placement and retrieval, and so on. Underwater acoustic positioning systems, including long baseline (LBL) systems, short baseline (SBL) systems, and ultra-short baseline (USBL) systems, are designed to operate from a reference point and employ external transducers or transducer arrays as aids for positioning. Traditional positioning methods rely on measuring of time-of-flight of an acoustic signal travelling from the target to the reference platform by means of the cross-correlation method. The positioning accuracy of LBL systems varies from a few centimeters to a few meters, depending on the operating range and working frequency. LBL systems provide a uniform positioning accuracy for a given transponder array setup, but they suffer the time-consuming instrument deployment on the seafloor, as well as the complicated operating procedures. SBL and USBL systems have relatively simple configurations. But their positioning accuracy is a function of water depth and operating range. To obtain absolute position accuracy, additional sensors such as the ship's gyro or a surface navigation system are needed. In this thesis, a novel positioning method is proposed which takes advantages of a tether cable between the reference platform and the target. This method conducts positioning via continuous phase measurement between a reference signal and the acoustic signal transmitted by the target to the reference platform. It is named the Positioning-based-on-PHase-Measurement method or PPHM method in short. Every 2π change in the phase difference between these two signals corresponds to a one-wavelength range increment along the radial direction from the target’s initial position to its new position. If a receiver array is used, with at least two hydrophones, the target’s bearing information can be also calculated by measuring the phases of the output signals from each of the array hydrophones. Under ideal conditions, the positioning error of the PPHM method is proportional to the phase measurement error. The PPHM method is very sensitive to changes in the underwater medium, such as sound speed variations, ocean currents and multipath interferences. Environmental fluctuations will degrade the positioning performance. These problems will be investigated and solutions will be proposed to minimize their effects. The PPHM method can be used to position an underwater moving object such as a remotely operated vehicle (ROV) or a bottom crawler. Also, it can be used to monitor the ocean currents speed variations over a path, or to monitor the movements of tectonic plates. The last two applications will be addressed in detail in this thesis, whereas the first one is very challenging and needs more work.

Solid-state amorphization of a crystalline solid to an amorphous phase is extensively studied as a first order phase transition at low temperature for almost thirty years. In this dissertation, we report the recent progress on phenomenological models employed for thermodynamic description of macroscopic systems and fluctuations and nucleation of mesoscopic inhomogeneous systems in binary solid solutions under polymorphic constraints with no long-range diffusion involved. Based on our understanding on atomic picture of solid-state amorphization in binary solid solutions, we propose a Landau free energy to describe amorphization as the first order phase transition. The order parameter is defined which represents the loss of long-range translational order. The elastic strain field induced by composition disorder plays the important role through the bilinear coupling with the order parameter. Elastic softening and amorphization happen simultaneously. From the similarity between the melting and amorphization, we use the temperature and composition as two external variables and treat solid-state amorphization as low temperature melting under polymorphic constraints. For homogeneous system, the phase diagrams for endothermic melting and exothermic melting are built separately and the corresponding thermodynamic quantities are presented. A microscopic homogeneous nucleation mechanism is proposed conceptually in binary solid solutions under polymorphic constraints. The formation of an amorphous embryo is initiated from the composition modulation in the crystal state and a subsequent polymorphous nucleation within the as-formed heterophase fluctuation. This homogeneous nucleation path is thought to be associated with the nonlinear energy localization mechanism connected with the localized large-amplitude excitations of atoms, which are induced by nonlinear and disorder. A Landau-Ginzburg free energy is constructed to describe the critical nucleus and the growth of the new phase in one-dimensional systems. Analytical and numerical methods contribute to the understanding the fluctuations and nucleation processes. Size-dependent melting and amorphization in nanosolids are investigated. Two models are proposed for nanocrystalline solid solutions to glass transformations. Based on the thin film model with finite thickness, we build one-dimensional Landau-Ginzburg approach, which includes surface contribution and size dependence, and numerical results do show similarity with experimentsâ results qualitatively.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Thesis (M.S.)--State University of New York at Binghamton, Department of Chemistry, 2006. / Includes bibliographical references.

Zugl.: Aachen, Techn. Hochsch., Diss., 2008.

Thesis (Ph. D.)--University of Washington, 1997. / Vita. Includes bibliographical references (p. [148]-178).

Mémoire (M.Eng.)--Université du Québec à Chicoutimi, 1998. / Document électronique également accessible en format PDF. CaQCU

Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Dr. Seyed M. Ghiaasiaan; Committee Member: Dr. Marc K. Smith; Committee Member: Dr. Sheldon M. Jeter