Kinetics of Aluminization and Homogenization in Wrought H-X750 Nickel-Base Superalloy

In sub-millimeter sheets of wrought H-X750 Nickel-base superalloy, aluminum-rich coatings are bonded to matrix with a vapor phase aluminization process. If an appropriate amount of aluminum is bonded to matrix with homogenization treatment, the resulting diffusion couple will diffuse into coherent (g/g’) heterogeneous phases creating matrix that is both precipitation and solid solution strengthened. The diffusional mechanisms for solid solution mass transport involved with the growth and dispersion of bonded aluminum-rich coatings in the aluminization process only differ from the no external mass flow homogenization process with annealing treatment in that the boundary conditions are different. In each case these forces that activate diffusion at the macroscopic level are connected to the activation energies of random walks of atoms on a wide scale at the angstrom level. An overview of wrought Nickel-base superalloy is presented. Starting with thin sheets the alloy will be aluminized and homogenized. The research from this study will determine the parameters for the movement of the phase boundaries, mass transport, and the time variant concentration fields for both the aluminization and homogenization processes. This is predictable for both single dimension fluxes assuming the interdiffusivities and fluxes at the phase boundaries are known. Because mass-transport is related to the movement of the phase boundaries through density, an investigation into the less dense aluminum-rich coatings and resultant matrix is also included

Thermophysical Phenomena in Metal Additive Manufacturing by Selective Laser Melting: Fundamentals, Modeling, Simulation and Experimentation

Among the many additive manufacturing (AM) processes for metallic materials, selective laser melting (SLM) is arguably the most versatile in terms of its potential to realize complex geometries along with tailored microstructure. However, the complexity of the SLM process, and the need for predictive relation of powder and process parameters to the part properties, demands further development of computational and experimental methods. This review addresses the fundamental physical phenomena of SLM, with a special emphasis on the associated thermal behavior. Simulation and experimental methods are discussed according to three primary categories. First, macroscopic approaches aim to answer questions at the component level and consider for example the determination of residual stresses or dimensional distortion effects prevalent in SLM. Second, mesoscopic approaches focus on the detection of defects such as excessive surface roughness, residual porosity or inclusions that occur at the mesoscopic length scale of individual powder particles. Third, microscopic approaches investigate the metallurgical microstructure evolution resulting from the high temperature gradients and extreme heating and cooling rates induced by the SLM process. Consideration of physical phenomena on all of these three length scales is mandatory to establish the understanding needed to realize high part quality in many applications, and to fully exploit the potential of SLM and related metal AM processes

Polysulfide Mitigation at the Electrode-Electrolyte Interface: Experiments in Rechargeable Lithium Sulfur Batteries

In the field of energy storage technology, the lithium sulfur battery is intensely studied in interest of its great theoretical gravimetric capacity (1672 Ah kg–1) and gravimetric density (2600 Wh kg–1). The theoretical performance values satisfy viability thresholds for petroleum–free electric vehicles and other emerging technologies. However, this elusive technology remains in the research sector due to a wealth of challenges resulting from its complex chalcogenide electrochemistry. The most infamous challenge remains the polysulfide redox shuttle, a phenomenon in which lithium polysulfide intermediates are produced as elemental sulfur S8 is reduced to lithium sulfide Li2S during the discharge cycle. Because the higher order polysulfides are soluble in organic electrolyte, battery cycling can result in dissolution of the cathode, dendrite formation upon the lithium metal anode, and passivation of electrode surfaces. These problems can ultimately cause rapid capacity fade and unstable Coulombic efficiency. As lithium sulfur battery research enters its 3rd decade, it is becoming increasingly clear that solutions will be holistic or synergistic; that is, addressing the aforementioned issues by suppressing their source in the polysulfide redox shuttle rather than isolated symptoms of the underlying mechanism. This thesis serves as a summary of research performed to study polysulfide suppression and mitigation through electrode material synthesis, electrolyte design, and in situ characterization. Synthesis techniques include solid state pyrolysis, autogenic synthesis, and ultrasound sonochemistry. Material characterization techniques include isothermal nitrogen sorption; scanning, transmission, and scanning transmission electron microscopy; thermogravimetric analysis; energy dispersive X–ray spectroscopy; organic elemental analysis; X– ray diffraction; and Raman spectroscopy. Electrochemical characterization includes galvanostatic battery cycling, differential potentiometric analysis, and electrochemical impedance spectroscopy. Altogether, this research demonstrates the challenges of polysulfide degradation are not sufficiently addressed by symptomatic approaches. Synthesis pathways for carbon sulfur cathodes that encourage homogeneous sulfur distribution (i.e., autogenic or sonochemical synthesis) improve specific capacity across extended cycling, but show excessive polysulfide production at slow cycling rates. In combination with fluorinated electrolyte, carbon sulfur cathode morphology improves Coulombic efficiency at cycling rates between C/20 — 2 C but at the cost of gravimetric capacity. Synchrotron tomography characterization, developed for Advanced Photon Source Beamline 6–BM–A, evidences that fluorinated electrolytes may also effectively suppress dendrite formation on lithium metal anodes. This suggests more holistic and optimized techniques, or their combinations, may lead to effective polysulfide suppression and successful commercialization of the lithium sulfur battery. Supplementary research explores broader impact of synthesized carbon applications in lithium sulfur batteries. Pyrolysis synthesis processes are evaluated for health and environment impacts using optical by–product sizing and life cycle analysis, respectively. In the context of pyrolytic synthesis of carbon microsheets, micro and nano-sized carbonaceous particulate by–products released during synthesis must be collected to minimize health exposure risks. The environmental impact of this synthesis process is a function of mode of oxygen deficiency, that is, whether pyrolytic atmosphere is facilitated by vacuum or inert gas stream. Finally, submicron carbon spheres, a carbon morphology produced by pyrolysis of sonochemically-synthesized polymer spheres, demonstrate gravimetric capacity which is a strong function of microstructure (i.e., pore distribution, crystallite size, structural disorder). In turn, microstructural properties are determined by synthesis temperature, a dimension of synthesis pathway

Nanoporous SiO2/vycor membranes for air separation

Porous Vycor tubes with 40Å initial pore diameter were modified using low pressure chemical vapor deposition (LPCVD) of silicon dioxide N02). Diethylsilane (DES) in conjunction with nitrous oxide (N2O) was used as a precursor to synthesize these SiO2 films. The aim of this study was to obtain a considerable selectivity between species of comparable size and hence N2O was used. The use of N2O was believed to make the process self-limiting. DES was allowed to flow through the tube and N2O on the outside in the chamber at 550°C in a counter-flow mechanism. This deposition geometry provided an optimum pore narrowing rate and eliminated the possibility of film cracking. The pore size of the Vycor tube was reduced with successive depositions and the stage at which maximum selectivity between oxygen and nitrogen was obtained was recorded. The value of selectivity was confirmed using mass spectroscopy and reproducing the results using another Vycor tube. The temperature dependence on selectivity was also studied. Characterization of the Vycor membranes was carried out to observe the SiO2 coating. Calculation of permeability was done using ASTM standards

Experimental and numerical assessment of weld pool behavior and final microstructure in wire feed laser beam welding with electromagnetic stirring

Advantages such as element homogenization and grain refinement can be realized by introducing electromagnetic stirring into laser beam welding. However, the involved weld pool behavior and its direct role on determining the final microstructure have not been revealed quantitatively. In this paper, a 3D transient heat transfer and fluid flow model coupled with element transport and magnetic induction is developed for wire feed laser beam welding with electromagnetic stirring. The magnetohydrodynamics, temperature profile, velocity field, keyhole evolution and element distribution are calculated and analyzed. The model is well tested against the experimental results. It is suggested that a significant electromagnetic stirring can be produced in the weld pool by the induced Lorentz force under suitable electromagnetic parameters, and it shows important influences on the thermal fluid flow and the solidification parameter. The forward and downward flow along the longitudinal plane of the weld pool is enhanced, which can bring the additional filler wire material to the root of the weld pool. The integrated thermal and mechanical impacts of electromagnetic stirring on grain refinement which is confirmed experimentally by electron backscatter diffraction analysis are decoupled using the calculated solidification parameters and a criterion of dendrite fragmentation.DFG, 416014189, Simulation des Einflusses der elektromagnetisch unterstützten Durchmischung beim Laserstrahlschweißen dickwandiger Stahlbauteile mit Zusatzmateria

Development of a Thermoelectric Characterization Platform for Electrochemically Deposited Materials

Die erfolgreiche Optimierung der Leistung von thermoelektrischen Materialien, die durch zT beschrieben wird, ist entscheidend für ihre Anwendung für das Wärmemanagement und die Kühlung von Leistungselektronik. Im Gegensatz zu Bulk-Proben bleibt die vollständige zT-Charakterisierung von Dünn- und Dickfilmmaterialien eine große Herausforderung. Dies ist insbesondere relevant für Filme, die durch elektrochemische Abscheidung synthetisiert werden, wo das Material auf eine elektrisch leitende Schicht abgeschieden wird. In dieser Dissertation habe ich ein Transport-Device für eine vollständige zTCharakterisierung von elektrochemisch abgeschiedenen Materialien entwickelt, während der Einfluss der elektrisch leitenden Schicht, sowie des Substrats beseitigt wird. Die zT-Charakterisierung erfolgt unter Verwendung eines auf einer freistehenden Membran suspendierten thermoelektrischen Materials innerhalb des entwickelten Transport-Devices, die durch eine Kombination von Fotolithografie und Mikrostrukturierungstechnik zusammen mit Ätzprozessen hergestellt wurde. Für die Messung der Wärmeleitfähigkeit habe ich eine eindimensionale, analytische, stationäre Methode eingesetzt, welche mit Hilfe von dreidimensionalen Finite-Elemente-Simulationen bestätigt wurde. Darüber hinaus habe ich die temperaturabhängigen thermoelektrischen Eigenschaften von zwei Dickschichten mit Hilfe des entwickelten Devices untersucht und mit Bulk-Proben und Dünnfilmen verglichen. Auf diese Weise konnte die Validität des Transport-Devices nachgewiesen werden. Neben der Optimierung von mikro-thermoelektrischen Materialien, die mit dem Transport- Device charakterisiert werden, ist die Leistung von thermoelektrischen Devices von den Faktoren Design, Geometrie und Konstruktion beeinflusst. Daher habe ich den Einfluss der Geometrie auf die Leistung eines elektrochemisch hergestellten mikrothermoelektrischen Generators mit Hilfe einer Finite-Elemente-Simulation untersucht

DPD Guided Insight on the Formation Process of Polyethersulfone Membranes by Nonsolvent Induced Phase Separation and the Effects of Additives

Dissipative particle dynamics (DPD), a coarse grain simulation method, was applied to the membrane formation process of non-solvent induced phase separation (NIPS) to gain further insight on the mechanism of certain variables and how they affect the final morphology. NIPS involves two solutions, an organic polymer dissolved in an organic solvent colloquially called the dope and an aqueous coagulation bath, brought into contact with one another. The solvents then mix, causing the polymer to fall out of solution as an asymmetric membrane with a dense surface layer and a more open subsurface layer in response to the decreasing solubility. Polyethersulfone (PES), a common industrial choice we have previously studied, was utilized as the polymer with N-methyl-2-pyrrolidone (NMP) as the organic solvent and water as the coagulation bath. In this study, our previous model construction was altered in several ways. Firstly, the simulation area was enlarged, allowing for a better sampling of subsurface behavior. Secondly, polymer chain length was increased to bring it more in line with the high molecular weight of industrially common polymers, with our experimental systems ranging from 100 to 200 monomers. Lastly, polyvinylpyrrolidone (PVP) was introduced as an additive to the polymer solution in concentrations varying from 1 to 10% by volume of the dope. PVP is a common polymer additive utilized in industry to produce larger pores, a result that was successfully replicated. We also investigated the effects of adding solvent to the coagulation bath as well as the effects of adding water to the polymer solution

DPD Guided Insight on the Formation Process of Polyethersulfone Membranes by Nonsolvent Induced Phase Separation and the Effects of Additives

Dissipative particle dynamics (DPD), a coarse grain simulation method, was applied to the membrane formation process of non-solvent induced phase separation (NIPS) to gain further insight on the mechanism of certain variables and how they affect the final morphology. NIPS involves two solutions, an organic polymer dissolved in an organic solvent colloquially called the dope and an aqueous coagulation bath, brought into contact with one another. The solvents then mix, causing the polymer to fall out of solution as an asymmetric membrane with a dense surface layer and a more open subsurface layer in response to the decreasing solubility. Polyethersulfone (PES), a common industrial choice we have previously studied, was utilized as the polymer with N-methyl-2-pyrrolidone (NMP) as the organic solvent and water as the coagulation bath. In this study, our previous model construction was altered in several ways. Firstly, the simulation area was enlarged, allowing for a better sampling of subsurface behavior. Secondly, polymer chain length was increased to bring it more in line with the high molecular weight of industrially common polymers, with our experimental systems ranging from 100 to 200 monomers. Lastly, polyvinylpyrrolidone (PVP) was introduced as an additive to the polymer solution in concentrations varying from 1 to 10% by volume of the dope. PVP is a common polymer additive utilized in industry to produce larger pores, a result that was successfully replicated. We also investigated the effects of adding solvent to the coagulation bath as well as the effects of adding water to the polymer solution

Fluid transport through heterogeneous pore matrices: Multiscale simulation approaches

Fluids confined in nanopores exhibit several unique structural and dynamical characteristics that affect a number of applications in industry as well as natural phenomena. Understanding and predicting the complex fluid behavior under nano-confinement is therefore of key importance, and both experimental and computational approaches have been employed toward this goal. It is now feasible to employ both simulations and theoretical methods, the results of which can be validated by cutting-edge experimental quantification. Nevertheless, predicting fluid transport through heterogeneous pore networks at a scale large enough to be relevant for practical applications remains elusive because one should account for a variety of fluid–rock interactions, a wide range of confined fluid states, as well as pore-edge effects and the existence of preferential pathways, which, together with many other phenomena, affect the results. The aim of this Review is to overview the significance of molecular phenomena on fluid transport in nanoporous media, the capability and shortcomings of both molecular and continuum fluid modeling approaches, and recent progress in multiscale modeling of fluid transport. In our interpretation, a multiscale approach couples a molecular picture for fluid interactions with solid surfaces at the single nanopore level with hierarchical transport analysis through realistic heterogeneous pore networks to balance physical accuracy with computational expense. When possible, comparison against experiments is provided as a guiding roadmap for selecting the appropriate computational methods. The appropriateness of an approach is certainly related to the final application of interest, as different sectors will require different levels of precision in the predictions