Theses and Dissertations - Department of Metallurgical and Materials Engineering

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    Rapid Solidification of Austenitic Stainless Steels by Splat Quenching
    (University of Alabama Libraries, 2020) Morales, Sydney Mackenzie; Brewer, Luke; University of Alabama Tuscaloosa
    This thesis explores the phase transformations and microstructural evolution that are observed in rapid solidification of austenitic stainless steels by splat quenching. Splat quenching is an experimental method that produces rapidly solidified structures under cooling rates that are comparable to that of additive manufacturing. In additive manufacturing, specifically selective laser melting (SLM), metal is subjected to a very rapid heating and cooling that produces cooling rates in the range from 105-106 C/s. This rapid solidification produces microstructures that deviate from equilibrium. Five compositions of austenitic stainless steel with varying Cr/Nieq were studied to assess the effect of composition and cooling rate on the solidification microstructures that take place during splat quenching. The five compositions studied in this thesis were produced via arc melting to provide feedstock for splat quenching experiments. Splat quenched samples were analyzed for the presence of microsegregation, phase content, and solidification mode and morphology. Analysis techniques included electron dispersive spectroscopy, backscatter imaging, secondary imaging, and electron backscatter diffraction. Cooling rates achieved during splat quenching were evaluated utilizing electrolytic etching to estimate cell size of the solidification microstructures. Based on these analyses, the cooling rates were estimated to be in excess of 106 C/sec with a solidification velocity range of 0.1-1 m/s. The phase content of the splat quench microstructures as a function of alloy composition agreed well with the current rapid solidification literature.
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    Experimental Study on Electrical Conductivity of Imidazolium- Based Ionic Liquids and Their Application to Current Density Simulation
    (University of Alabama Libraries, 2021) Nahian, Md Khalid; Reddy, Ramana G.; University of Alabama Tuscaloosa
    Over the last few decades, ionic liquids (ILs) have been the focus of considerable research in the field of electrochemistry for their exceptional physio-chemical characteristics. In this study, the electrical conductivity of imidazolium-based ionic liquid was investigated by electrochemical impedance spectroscopy (EIS). Aluminum chloride (AlCl3) was mixed with three different kinds of imidazolium ionic liquids, 1-butyl-3-methylimidazolium chloride (BMIC), 1-ethyl-3-methylimidazolium chloride (EMIC), and 1-hexyl-3-methylimidazolium chloride (HMIC), individually. The electrical conductivity of these chloroaluminates was determined as a function of temperature and molar ratio. For AlCl3:BMIC and AlCl3:EMIC, electrical conductivity decreases with an increase in AlCl3 content, whereas electrical conductivity increases for AlCl3: HMIC with an increase in AlCl3. Electrical conductivity increases with temperature for all three ILs systems. Following this, the effects of titanium tetrachloride (TiCl4) and temperature on the electrical conductivity of ILs systems were studied. In this case, electrical conductivity for all ionic liquid systems increases rises to a certain TiCl4 ratio and then decreases with the additional TiCl4, and electrical conductivity also increases with temperature. Activation energy was also calculated from electrical conductivity data. The obtained AlCl3:BMIC electrical conductivity data was used to simulate the current density and contact resistance for aluminum refining electrochemical cell by ANSYS Fluent. When Al metal matrix composite anode was used, a consistent electrode-electrolyte contact resistance (0.041 m2) was found regardless of applied voltage. But the contact resistance was not consistent with applied potential when Al2020 was chosen as anode.
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    Microstructure and Mechanical Properties of a Dual Phase Transformation Induced Plasticity Fe-Mn-Co-Cr High Entropy Alloy
    (University of Alabama Libraries, 2021) Hossain, AFM Monowar; Kumar, Nilesh; University of Alabama Tuscaloosa
    The multicomponent alloys, also known as High Entropy Alloys (HEAs), have been the subject of intense exploration for over a decade now for a wide range of potential applications. To investigate microstructure – mechanical property correlation in a newly developed transformation-induced plasticity dual-phase high entropy alloy (DP-HEA) Fe50Mn30Co10Cr10, microstructural analysis, mechanical testing, and fractography were performed on the DP-HEA. The microstructural state of the alloy was analyzed by optical microscope, scanning electron microscope (SEM), energy dispersive spectroscopy, and electron backscatter diffraction (EBSD). The plastic deformation behavior of the alloy was assessed using uniaxial tensile testing and micro indentation hardness tests. The digital image correlation (DIC) was used during the tensile test to evaluate the Young’s modulus and localized strain values at different points in the gage section of the tensile specimens as a function of time. An insight in to the micromechanism of plastic deformation was gleaned from stress relaxation test (SRT). A variation in the hardness values was observed probably due to the presence of two different phases (one with face-centered cubic and the other with hexagonal closed pack crystal structures) in the microstructure. The yield strength, ultimate tensile strength, and % elongation were estimated from the tensile test results. The fractography of the fractured surface of the tensile specimens revealed ductile fracture of the alloy. The strain values, in the gage section, were found to differ from each other at different points during the tensile test due to differences in the deformation of crystals with different crystallographic orientations, phases, or both. An in-depth discussion of deformation mechanism(s) is presented based on the analysis of DIC and SRT data.
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    Modeling of Solidification Microstructure During Ultrasonic Processing of Cast Aluminum Alloys
    (University of Alabama Libraries, 2021) Dong, Aqi; Nastac, Laurentiu; University of Alabama Tuscaloosa
    In order to study the formation of the solidification structure including the columnar-toequiaxedtransition (e.g., CET) under the influence of ultrasound, a 2-zone furnace system and an ultrasonic equipment were built and utilized. The 2-zone furnace system consists of an induction furnace with a top coil and a bottom coil, a graphite crucible, and. a water-cooled chill block located at the bottom of the crucible. By controlling both the top and bottom coil output power independently, the furnace can create various temperature gradients and cooling rates in different regions of the graphite crucible. The ultrasound Nb probe was inserted at the top of the crucible. The top of the crucible was also thermally insulated. Temperature measurements were performed at different locations in the crucible. The effects of ultrasound on the microstructure formation during solidification of A356 alloy was studied. A numerical model was developed to simulate the solidification process in the crucible and to assist in developing of solidification maps. In addition, with the help of machine learning techniques, the mutual interaction between Al-Si based alloys content, physical properties and processing parameters have been studied. Quantitative relationship equations and the significance of each key factors were developed and analyzed via machine learning, which helps to better understand the complex nonlinear relationship of “alloy composition-physical properties-solidification processing parameters-mechanical performance” in cast Al-Si based alloys. In the first chapter, the background of A356 alloy, ultrasonic stirring technology and the application of machine learning technique on metallurgical and materials engineering are introduced. The second chapter describes the methodology of the ultrasonic refining and modification mechanisms as well as several relevant machine learning algorithms that can be applied to study metallic materials. In the third chapter, the ultrasound effects on the formation of the solidification structure of A356 ingots processed via a 2-Zone induction melting furnace are studied. The fourth chapter focuses on modeling of segregation and microstructure evolution during the solidification of A356. Finally, in the fifth chapter a prediction of a quantitative relationship of “alloys content-physical properties-processing parameters-mechanical properties” using machine learning is performed.
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    Cerium Oxide Based Interlayer and Cathode Materials for High Performance Lithium Sulfur Battery
    (University of Alabama Libraries, 2021) Azam, Sakibul; Wang, Ruigang; University of Alabama Tuscaloosa
    Investigation of sluggish redox kinetics and polysulfide shuttling is crucial to design advanced lithium sulfur battery. Cerium oxide (CeO2) has remarkable polysulfide adsorption capability and has been recently investigated in lithium sulfur battery application and novel catalyst design. With the goal of bridging towards commercialization of lithium sulfur battery, several interlayer and cathode materials based on cerium oxide have been developed in this thesis. This literature involves understanding of the mechanism of CeO2 based materials in lithium sulfur battery. Chapter 3 focuses on cellulose paper derived carbon fiber decorated with CeO2 nanorods to be used as interlayer material for lithium sulfur battery. The carbon fiber provides physical confinement and the CeO2 adsorbs lithium polysulfides chemically to reduce shuttle effect to achieve long lifetime and high capacity for lithium sulfur battery. With a sulfur content of 2 mg, a high capacity of 1177 mAh/g was achieved. The improved performance is attributed to the binding of lithium polysulfides by the CeO2 and the blocking of polysulfide physically by the compact conducting carbon fiber. Chapter 4 is focused on Prussian blue derived carbon cubes and CeO2 nanorods co-decorated on carbon fiber as lithium sulfur battery interlayer. The carbon cubes provide room for sulfur to expand during battery cycling, further leading to excellent rate capability. The battery could last 350 cycles at high current rate of 1C. The superior performance was compared with other existing literatures as well and it could be shown that the performance improved a few folds. Chapter 5 describes the use of copper oxide (CuO) impregnated CeO2 as a cathode host material for lithium sulfur battery. The redox potential of CuO lies in the optimal range to convert lithium polysulfides to polythionate and thiosulfate species which helps to improve the battery kinetics. As a result, 10wt% of CuO impregnated in CeO2 nanorods maintain excellent discharge capacity of over 1100 mAh/g for at least 60 cycles. This catalytic effect of the material is exciting prospect for further research in Li-S battery.
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    Scalable and Highly Efficient Antimony Selenide Thin Film Solar Cells by Close Spaced Sublimation
    (University of Alabama Libraries, 2021) Guo, Liping; Yan, Feng; University of Alabama Tuscaloosa
    Sb2Se3 has emerged as a promising absorber material for solar cells owing to its superior optical and electronic properties, such as high absorption coefficient and suitable bandgap. It is a simple binary compound comprised of environmentally friendly and earth-abundant constituents, with one stable phase under normal conditions. More importantly, the crystal structure of Sb2Se3 is the unique one-dimensional ribbons that are stacked together by weak van der Waals force. The grain boundary is inert without dangling bonds, leading to significantly reduced combination centers. Furthermore, based on the S-Q limit, the theoretical maximum power conversion efficiency of Sb2Se3 can achieve as high as ~32%, implying the great potential to be employed for highly efficient solar cells. The goal of this dissertation work is to fabricate Sb2Se3 thin-film solar cells using close-spaced sublimation (CSS), which falls into three parts: (1) growth of high-quality Sb2Se3 thin films with CSS for thin-film solar cells, (2) investigation of the various buffer layer, and (3) hole-transport layer exploration for Sb2Se3 thin-film solar cells. In the first study, we systematically studied the growth behavior of Sb2Se3 with the CSS, particularly the dependence of the grain orientation on growth conditions, e.g., substrate temperature, thickness, etc. After optimization, we obtained Sb2Se3 films with desired crystal texture, i.e., (211)- and (221)-preferred orientation, thereby realizing efficiency of 4.27%. This work lays the foundation for our future work further optimizing the Sb2Se3 solar cells. The second study focuses on buffer layer engineering for the Sb2Se3 solar cells. Here we tried three different buffer layers: the oxygenated CdS via chemical bath deposition, oxygenated CdS via sputtering and sputtered CdSe. We carefully studied the optical response, band alignment with Sb2Se3, element diffusion, and the impact on grain orientation for each buffer layer. After careful engineering, we achieved 6.3%, 7.01%, and 4.5% for each buffer layer, respectively. Lastly, we applied NiOx as the hole-transport layer (HTL) on Sb2Se3 devices to construct a p-in configuration. The addition of NiOx HTL benefits charge extraction and helps reduce recombination at the backside via surface passivation. The thickness of the NiOx is a critical parameter for device performance. After the systematic screening, a decent efficiency enhancement from 6.6% to 7.3% was achieved with optimized NiOx HTL.
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    Investigation of high-performance lithium-ion batteries based on highly conductive Li7La3Zr2O12 solid-state electrolyte and stable electrode-electrolyte interface
    (University of Alabama Libraries, 2021) Li, Junhao; Wang, Ruigang; University of Alabama Tuscaloosa
    With the merits of high Li+ conductivity, wide potential window, and electrochemical stability against metallic lithium anode (the highest theoretical capacity: 3,860 mAh g−1), cubic phase garnet-type Li7La3Zr2O12 (LLZO) solid-state electrolyte has attracted much attention for developing solid-state batteries with increased safety, higher energy density, and longer lifespan. Besides, the solid-state or liquid electrolyte/electrode interface stability and low resistance are important to their optimized electrochemical performance of lithium-ion batteries. The goal of this dissertation is to develop high-performance lithium-ion batteries based on LLZO solid-state electrolyte and stable/low resistance electrode-electrolyte interface via (1) low-temperature synthesis/densification of Al/Bi-doped cubic LLZO electrolytes, (2) surface modification of LiNi1/3Co1/3Mn1/3O2 cathode particles, (3) composite polymer electrolytes, and (4) application of plastic-crystal interfacial modification.Chapter 3 explores a low-temperature synthesis strategy to obtain cubic LLZO powders via a combination of sol-gel method and ball milling induced tetragonal to cubic phase transition, which is ~200 °C lower than the thermally induced phase transition temperature. Chapter 4 investigates the role of a facial B2O3 surface modification of LiNi1/3Co1/3Mn1/3O2 cathode particles to achieve a stable cathode-electrolyte interface, which enables improved high-rate discharge performance and enhanced cycling stability of the batteries. Chapter 5 reveals the effects of LLZO ceramic filler distribution and doping elements (Al and Bi) on the ambient-temperature ionic conductivity, Li+ transference number, electrochemical stability window, and ability to suppress lithium dendrite growth of poly(vinylidene fluoride) based composite polymer electrolytes, as well as solid-state battery performance based on these composite polymer electrolytes. In Chapter 6, cubic Bi-doped LLZO ceramic pellets with a high relative density (>90%) and ionic conductivity (~1.32×10^(-4) S cm-1 at 20 °C) were achieved with a sintering temperature as low as 900 °C. A succinonitrile-based plastic-crystal interlayer at the Li/LLZO interface was demonstrated to be very effective to reduce interfacial resistance and enable stable cycling of a Li/LLZO/Li symmetric cell. With the help of the plastic-crystal interlayer and a composite cathode, a Li/LLZO/LiCoO2 all-solid-state battery was fabricated, which displayed a stable cycling at 0.1C for 40 times at 20 °C with a discharge capacity of ~115 mAh g-1 and a Coulombic efficiency of ~99%.
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    Investigation of biodegradable zn-li-cu alloys for cardiovascular stent applications
    (University of Alabama Libraries, 2020-12) Young, Jacob Steele; Reddy, Ramana G.; University of Alabama Tuscaloosa
    Zinc has been recently proposed as a suitable biodegradable material for temporary medical device applications due to an intermediate corrosion rate, high biocompatibility, and ease of processing. While pure zinc fails to meet the minimum mechanical property requirements for cardiovascular stents, Zn can be easily customized through alloying with non-toxic elements to improve both its strength and ductility while maintaining an ideal degradation behavior and biocompatibility.In this research project, the novel Zn-Li-Cu alloy system has been investigated and developed to optimize the mechanical properties and corrosion behavior for cardiovascular stent applications through simulation and in vitro examination. Through this research work, the primary accomplishments were: • Synthesized Zn-Li-Cu alloy ingots and sheets through an inert atmosphere casting, annealing, and hot rolling procedure. Individual samples are sectioned, ground, and polished for characterization and corrosion testing. • Characterized the morphology and chemistry of rolled alloys through optical microscopy, SEM, XRD, EDS, ICP-OES, FTIR, and XPS. Experimental and nominal chemistries are compared through thermodynamic modeling of the Zn-Li-Cu phase diagram with PANDAT software. • Simulated the biodegradation behavior in the human body through in vitro immersion testing and EIS using HBSS to examine weight loss-based corrosion rate, corrosion products and their effect on corrosion resistance, and long-term structural stability. • Determined the ultimate tensile strength, yield strength, elongation to failure, modulus of elongation, and Vickers hardness of rolled alloys. Tensile fracture surfaces were analyzed to find relationships between mechanical properties, fracture behavior, and alloying element concentration. • Measured the cell viability and relative metabolic behavior of NIH3T3 fibroblast cells on Zn alloy substrates through indirect MTS cytotoxicity testing to ascertain potential toxicity effects. • Established the optimal alloy chemistry to best meet the design criteria for cardiovascular stent devices and ascertain avenues for future property improvements and optimization.
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    Development of aluminum electrorefining in ionic liquids: the effects of experimental conditions on the deposition behavior and microstructure
    (University of Alabama Libraries, 2020) Wang, Yifan; Wang, Ruigang; University of Alabama Tuscaloosa
    The electrochemical refining of Al from aluminum alloy scrap (Al2020) on copper substrate cathode at low temperature ionic liquid electrolytes was studied. The main components of ionic liquid electrolytes were the mixture of AlCl3 and 1-butyl-3-methyl imidazolium chloride ([BMIM]Cl), 1-ethyl-3-methyl imidazolium chloride ([EMIM]Cl) or 1-hexyl-3-methyl imidazolium chloride ([HMIM]Cl). The electrodeposition experiments were conducted in a 50-mL glass beaker fitted with Teflon cap. Al2020 aluminum alloy scrap was used as anode material and Cu sheet was used as cathode material. The aluminum alloy scrap (anode material) was cut into thin plate shape and polished mechanically before deposition. To study the effects of various experimental parameters, Al electrodeposition was conducted with temperature ranging from 80 oC to 140 oC, cell voltage ranging from 1.0 V to 1.75 V, stirring rate ranging from 0 to 180 rpm, type of ionic liquids changing from [BMIM]Cl to [EMIM]Cl and [HMIM]Cl, and electrolyte composition ranging from IR=1.0 to 2.4 (IR = AlCl3 : [BMIM]Cl). In addition, the surface roughness of Cu cathode was controlled by polishing through 320, 600, 800, 1200 grits SiC sandpapers and mirror polishing procedure (with 3 μm SiO2 fine colloidal suspension). For all anode materials (Al sheet), they were polished by 320 grits SiC sandpaper. The experiments were performed for 2 h throughout the research and the samples were cleaned by acetone and DI water before characterization. The phase characteristics and crystallinity of Al deposits on Cu cathode sheet were analyzed using X-ray diffraction (XRD). The morphology and chemical composition characterization of Al deposits were carried out using Apreo field-emission scanning electron microscope (Apreo FE-SEM, ThermoFisher Scientific) equipped with an energy dispersive X-ray spectrometer (Bruker XFlash EDS). Al deposits with purity higher than 99.7% were obtained. Among many other merits, this study demonstrates that electrochemical refining of Al from Al2020 alloy scraps using ionic liquid electrolytes is an energy-efficient (current efficiency > 90% and energy consumption < 5 kWh/kg Al) and environmentally friendly method. Furthermore, the microstructure of Al deposits was controlled by the design of experiments. Especially the formation of dendritic structure (a typical structure formed during Al electrodeposition), which can add additional processing cost and has a profound adverse effect on refining of Al, was prevented on smoother cathode surface. The mechanisms of Al electrodeposition and formation of crystal dendrite structure were investigated. The electrodeposition process is primarily controlled by the diffusion of Al2Cl7- and the microstructure formation is concerning the surface energy, local electrolyte concentration, and apparent contact angle of ionic liquids on the substrates.
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    Precursor gas comparison for the growth of silicon carbide fibers via laser chemical vapor deposition
    (University of Alabama Libraries, 2020) Matt, Mia Catherine; Thompson, Gregory B.; University of Alabama Tuscaloosa
    Laser chemical vapor deposition (LCVD) is a processing technique that can be used to grow fibers. In this study, the relationships between deposited SiC fiber composition, fiber microstructure, and fiber mechanical properties (ambient temperature failure stress in tension) were compared using two different precursor gases - Tetramethylsilane (TMS) and Dimethylsilane (DMS) – using laser chemical vapor deposition (LCVD). Each set of fibers was grown at 2, 4 and 6 bar at a growth rate of 50 um/s. Furthermore, each set of fibers contained a nanocrystalline core. In some cases, the presence of nodular structures were noted, but these features were comprised of nanocrystalline grains. The crystalline structure for both fibers were indexed by X-ray diffraction as the 3C-SiC cubic phase. The fibers were carbon-rich. In general, the TMS fibers had generally higher average stress at failures that were 2280 MPa as compared to the average DMS fibers being 1150 MPa. Considerable spread in the tensile strength at failure was noted for the DMS fibers and is contributed to residual stresses, as they fibers were qualitatively more delicate to handle. A Weibull analysis revealed that the fibers had a low Weibull modulus, which is an indication of a large and unpredictable variation of flaws within the fibers.
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    Coating yttria stabilized zirconia powders by magnetron sputtering
    (University of Alabama Libraries, 2019) Togaru, Maanas; Thompson, Gregory B.; University of Alabama Tuscaloosa
    This thesis describes the application of Physical Vapor Deposition (PVD) for coating powders, with the work motivated by the need to provide conformal coatings for nuclear fuel for use in Nuclear Thermal Propulsion (NTP). The coated material was tungsten, because of its high melting point and low neutron cross-section, yttria-stabilized zirconia (YSZ) was used for the nuclear powder surrogate. The coating was done in a rotating drum that held and moved the powders under a cylindrical cathode. The sphericity of the powders, to improve their flow in the drum, was achieved using a gravity-based plasma Powder Alloying Spheroidization (PAS) process. The particles were coated between 5.5 kWh to 40 kWh resulting in a coating thickness between approximately 70 nm to 540 nm. The coatings were found to have powdery morphology spheres resulting from the particle-to-particle collisions. To further understand the stress state of the deposited film, a series of 100 nm tungsten films were deposited at two different rates (0.05 and 0.2 nm/s) and three pressures (2, 5 and 10 mTorr). At the lowest pressure, regardless of rate, the films had a compressive stress state. Upon increasing the pressure for both rates, the residual stress was near zero. X-ray diffraction revealed that the nominally body centered cubic tungsten film adopted the A15 phase referred to as beta-tungsten.
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    High-temperature corrosion study of alloys in molten MgCl₂-KCl eutectic salt
    (University of Alabama Libraries, 2019) Peng, Yuxiang; Reddy, Ramana G.; University of Alabama Tuscaloosa
    The MgCl₂-KCl molten salt has been proposed as one of the heat transfer fluids (HTFs) in the solar energy system because of its potential thermal properties. This research investigates the corrosion behavior and protection strategies of Haynes 230 (H230), Incoloy 800H (800H), and Stainless Steel 316 (SS316) in MgCl₂-KCl molten salt at high temperature. First of all, the corrosion rates of different alloys are determined by performing the long-term static corrosion experiments. The morphologies of the surface and corrosion depths are examined by scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). Secondly, the effect of Ni on the corrosion behavior of the alloys is studied in this research. When the alloy is connected with Ni, the corrosion rate is higher than that without connection in the same condition. Thirdly, the electrochemical tests of three alloys are also investigated in present research work. Tafel curves are plotted to calculate the corrosion potentials and corrosion rates of three alloys at different temperatures. Fourthly, two methods are proposed and studied to protect the alloys from severe corrosion in the MgCl₂-KCl molten salt: 1) adding a sacrificial anode into the salt, and 2) coating a high-corrosion-resistance alloy. Mn, Zn, and Zr are chosen as the sacrificial anode materials to protect the alloys from further corrosion. The result showed that additions of inhibitor into the molten salts decrease the corrosion rates of the tested alloy. Pure Ni, Al₂O₃, Ni-Al alloy, and Cr-Fe-Al alloy are chosen to coat the tested alloy. Long-term corrosion experiments, as well as electrochemical tests of alloys with and without different coatings, are studied in present work. The results demonstrated that all these coatings decreased the corrosion rates of the tested alloys. Finally, the diffusion models are constructed to predict the distribution of the Cr in the different alloys after corrosion. And the results are comparable to the experimental data. In conclusion, H230 showed the highest corrosion resistance in the MgCl₂-KCl salt proved by long-term corrosion test and electrochemical tests. Different inhibitors and coatings all improved the corrosion resistance of alloys in the molten salt.
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    Oxidation behavior of refractory complex concentrated alloys: computational and experimental studies
    (University of Alabama Libraries, 2019) Hunter, Brett Matthew; Weaver, Mark L.; University of Alabama Tuscaloosa
    In recent years, high entropy alloys (HEAs) and, more specifically, refractory complex concentrated alloys (RCCAs) have been of increased interest due to their potential as replacements in high temperature environments. This dissertation work has systematically investigated the phase equilibria and oxidation resistance of an alloy system with the basis AlHfNbTiZr. The three alloys investigated in the five-component system were all found to contain a single phase microstructure composed of B2 while the seven component alloy was comprised of a B2 and C14 Laves phase. While the oxidation behavior was parabolic for all alloys studied, the five component alloys exhibited 2-stage parabolic behavior compared to a single stage in the seven-component alloy. The oxidation behavior was governed by a combination of thermodynamics and kinetics with regards to diffusion of oxygen into the system. The five component alloys were found to form an external scale comprised of a ZrTiO4-based structure containing all five elements in their respective stoichiometry. Internal oxidation also occurred in these alloys and exhibited a relationship between diffusion of cations out of the alloy and diffusion of anions into the alloy. However, the seven-component alloy did not form an external scale and was governed completely by oxygen diffusion into the alloy and thermodynamic factors as to the composition of the scale. This work has furthered the fundamental understanding of the oxidation behavior of RCCAs and the accuracy of modelling on the phase equilibria of these alloys.
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    Prediction of heat transfer and microstructure in high-pressure die-cast A383 aluminum alloy
    (University of Alabama Libraries, 2019) Karkkainen, Mikko; Nastac, Laurentiu; University of Alabama Tuscaloosa
    Predicting the microstructure of the as-cast HPDC (high-pressure die cast) product is valuable, because micro-scale features often determine its mechanical properties. To predict the microstructure, the effect of processing parameters such as pressure and cooling rates must be known. The object of this study is to create state-of-the-art models for predicting heat transfer in the HPDC process, and apply those models to predict the evolution of one feature of the microstructure: the size of polyhedral α-Fe intermetallic phase. In the study, we develop a new empirical correlation for the Nusselt number in water cooling channels. This can be used to validate heat transfer coefficients for water cooling channels in commercial software to assist in modelling heat transfer in the HPDC process. Additionally, we develop a model for impact pressure in HPDC, which augments the state-of-the-art Hamasaiid model for peak IHTC (interfacial heat transfer coefficient) in HPDC, and relaxes some of their empirical assumptions. We integrate the IHTC model as a custom boundary condition in FLUENT 18.1 using SCM and UDF-files. Finally, we predict the size of polyhedral Fe-rich intermetallics using commercial casting simulation NOVAFLOW&SOLID for cooling rates and classical solidification theory for intermetallic size, and validate the results using optical micrograph size measurements.
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    Investigation of influencing factors in liquid metal embrittlement of advanced high strength steel
    (University of Alabama Libraries, 2019) Massie, Daniel Joseph Woodson; Brewer, Luke N.; University of Alabama Tuscaloosa
    This thesis explored the influence of temperature, steel type, galvanization method, and macro-strain level on the sensitivity of advanced high strength steels (AHSS) to zinc-based liquid metal embrittlement (LME). It is critical to understand the influencing factors of LME because zinc coatings are commonly used to protect steel parts from corrosion, and the use of advanced high strength steel in the automotive industry is increasing. Electro-galvanized and zinc free samples of a transformation induced plasticity steel, TBF1180, and a complex phase steel, CP1200, were studied to examine the sensitivity of each to LME. Hot-dip galvanized samples of CP1200 were examined alongside the electro-galvanized samples to investigate the effect of coating method on the LME effect. Hot tension tests were performed and ductility trough graphs were created for all samples to examine the effect of these factors on LME during fracture. Additionally, small-strain tensile tests were designed and performed on the steels to examine LME crack nucleation. From the results it was determined that LME response is temperature and steel dependent. It was shown that TBF 1180 nucleated LME cracks at 600 °C while CP1200 did not. It was also determined that hot-dip galvanized coatings more readily nucleate LME cracks than electro-galvanized coatings. Finally, these results suggest that macro-plastic deformation may not be required to initiate an LME response.
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    Hyperbaric growth of carbon fibers by laser chemical vapor deposition
    (University of Alabama Libraries, 2019) Rife, Justin Lee; Thompson, Gregory B.; University of Alabama Tuscaloosa
    Laser Chemical Vapor Deposition (LCVD) is a promising new processing technique by which freestanding structures, such as fibers, can be deposited. The deposition of carbon fibers by use of ethylene as a precursor gas can be easily achieved and has been investigated as a way to complement or even replace current carbon fiber production techniques. The properties of carbon fibers deposited from ethylene via LCVD have been investigated for low precursor pressures thus far. However, deposition rates for low precursor pressures are limited and rates that are orders of magnitude faster can be achieved by use of higher precursor pressures. No detailed studies on properties of fibers processed at these higher pressures have been conducted. This thesis fills this knowledge gap by exploring the relationships between processing conditions, growth behavior, microstructure and mechanical properties of carbon fibers deposited from ethylene at hyperbaric pressures. It is found that the fiber growth rates are limited by surface reaction kinetics at low temperatures, while they are mass transport limited or gas phase nucleation limited at high temperatures. When grown under mass transport limited conditions, fibers exhibit drastic changes in morphology and microstructure. The tensile strengths of the carbon fibers grown by LCVD are generally found to be poor due to the nature of graphitic carbon deposits. However, the Weibull modulus among the LCVD grown carbon fibers is found to be high. Trends in mechanical properties with processing conditions and microstructure are observed.
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    Experimental and numerical study of the reduction of silica in a thermal plasma reactor
    (University of Alabama Libraries, 2019) Li, Yudong; Reddy, R. G.; University of Alabama Tuscaloosa
    High purity silicon production is impeding the expansion of solar energy industry due to high cost. Using high purity SiO2 and carbon, it is possible to economically produce solar grade silicon through two-step process with SiC as an intermediate product. This work investigates the reduction behavior of SiO2 by natural gas in a thermal plasma reactor from both experimental and numerical approaches. Effects of CH4/SiO2 and plasma power input were studied by conducting experiments. Products were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The composition of each phase, including both crystalline and amorphous phases, were quantified using partial or no known crystal structure (PONKCS) and internal standard methods based on whole pattern Rietveld refinement. SiC is the major product in this study. Higher power input and higher CH4/SiO2 ratio gives higher SiC yield. Maximum SiC yield of 69% was achieved at 20kW with CH4/SiO2 = 7.5. Reaction kinetics model was developed based on the reaction mechanism. Activation energy is 184.81 kJ/mol. With X represent the reduction degree and a_C represent the activity of carbon, the overall kinetic rate expression is: dX/dt=(8.43×〖10〗^5)∙exp⁡((-184812)/RT) ∙(a_C )^2∙(3×(1-X)^(2/3)) A 3D comprehensive computational fluid dynamics (CFD) model was developed based on experimental set-up. A new plasma nozzle boundary conditions determination method was developed based on empirical expressions and experimental conditions. The model was validated with experimental temperature data. Temperature and velocity profiles in the reactor was developed. Based on CFD simulation results, the SiO2 particle interaction with plasma gas stream and the reduction behavior was studied using Lagrangian method. An algorithm was developed to optimize the kinetics parameters based on CFD results. The optimized activation energy is 217 kJ/mol. The optimized kinetic rate expression is: dX/dt=(2.70×〖10〗^6)∙exp⁡((-217000)/RT) ∙(a_C )^2∙(3×(1-X)^(2/3)) To summarize, the reduction of SiO2 by methane in our thermal plasma reactor is successful in producing SiC. Yield of SiC in the thermal plasma reactor is comparable to literature data. A 3D comprehensive CFD model was developed and verified. The optimized kinetic rate expression obtained in this study can be used to predict the SiC production process using CFD simulations.
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    Advanced characterization of the oxidation behavior of grain refined NiAl
    (University of Alabama Libraries, 2019) White, Rachel Ellen; Weaver, Mark Lovell; University of Alabama Tuscaloosa
    Reactive element doped β-NiAl is one of the most oxidation resistant materials available for high temperature use. It has been extensively studied to create the most adherent, slow growing, and passive layer possible. One recent area of interest is grain refinement, whereby the reduced metal grain size improves mechanical properties, transports reacting elements rapidly to the oxidizing surface, and facilitates the growth of a more adherent scale. This research focused on the effect of substrate grain refinement on the microstructure of its thermally grown oxide, in comparison to the oxide grown on extruded and single crystal NiAl alloys. The oxidation behavior of grain refined materials produced by via sputter deposition, ball milling, and cryomilling was found to vary significantly. Sputter deposition was shown to significantly increase the parabolic steady state oxidation rate constant, while decreasing the length of transient oxidation. Ball milling did not result in an increase in oxidation rate, but did show increased interfacial void formation as a result of the Al2O3 dispersions incorporated during the milling process. Last, cryomilling resulted in an increase in steady state oxidation rate and increased interfacial void formation that was correlated to AlN dispersions incorporated during milling. All three grain refinement methods were found to decrease the oxide grain size approximately three-fold in comparison with the oxide grown on extruded NiAl, though a consistent relationship between oxide grain size and steady state oxidation rate was not observed. This suggests that microstructural features other than substrate and oxide grain size dominate the oxidation behavior.
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    Processing-microstructure-property relations in high pressure cold spray of AA2024 and AA7075
    (University of Alabama Libraries, 2018) Story, William Andrew; Brewer, Luke N.; University of Alabama Tuscaloosa
    In this dissertation work, the processing-microstructure-property relations are examined for cold spray deposited AA2024 and AA7075. Unlike traditional thermal spray technologies, powders are never melted during the cold spray process. This approach allows for heat-sensitive materials such as 2000 and 7000 series aluminum alloys to be used for repair of components damaged by corrosion or fatigue. While a small body of literature for cold spray of AA7075 and AA2024 exists, further understanding on processing-microstructure relationships is needed and improvements to deposition characteristics, microstructure and mechanical properties are necessary for successful application. This dissertation examines four aspects of the cold spray process applied to AA2024 and AA7075. First the effects of processing parameters on process efficiency, deposit microstructure, and deposit properties of AA2024 and AA7075 are examined. Spraying with helium is found to be more cost effective than nitrogen or any mixture of nitrogen and helium and generally results in higher quality deposits. A novel heat treatment method is developed to solution heat treat gas-atomized AA2024 and AA7075 powders prior to deposition. This approach is found to significantly improve deposition efficiency of powders and homogenize the deposit microstructure by solutionization of segregated solute. The influence of the deposit geometry on the development of residual stresses are investigated for AA2024 and AA7075 using neutron diffraction. Generally compressive residual stresses are found in deposits and deposit geometry is found to significantly effect to magnitude of residual stresses evolved. Finally, the effects of heating the substrate/deposit using an in situ laser during deposition are assessed. Laser assisted cold spray is found to improve deposition efficiency and improving ductility of AA7075 deposits but can result in significant heat damage to the substrate material.
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    Phase equilibria and reaction kinetics of borides based high-temperature thermoelectric materials
    (University of Alabama Libraries, 2018) Imam, Muhammad Ali; Reddy, R. G.; University of Alabama Tuscaloosa
    In this study, the phase equilibria of the binary borides (Mg-B and Si-B) were determined by solid-state electrochemical measurements. This study provides the equilibrium thermodynamic properties of boride based thermoelectric materials. Experiments were performed to measure the electromotive force (EMF) as a function of temperature. The activities of Mg/Si in the boride alloys were determined to obtain the partial and integral thermodynamic properties (∆GM, ∆HM) of boride based TE materials. The tangent rule was used to estimate the Gibbs energies of formation (∆G0f) of SiB3, SiB6, and SiB14-50 from 823 K to 923 K in the Si-B system. The ∆G0f of SiB6 evaluated to be -12.78 ± 0.64 kJ/mole-atoms at 923 K, was found to be the most stable within this temperature range. In the Mg-B system, the integral Gibbs energy of formation (∆G0f ) of MgB2, MgB4, and MgB7 was also estimated using tangent rule and reported for 773 k to 873 k. The Gibbs energy of formation (∆G0f ) of MgB2, MgB4, and MgB7 are -15.48, -22.03, and - 15.89 kJ/mol-atoms at 873 K. Additionally, Thermogravimetric Analysis (TGA) were also done to study the oxidation stability and reaction kinetics of the boride base TE materials. The activation energy for the oxidation process was also calculated from the parabolic rate constant, obtained from the mathematical fitting of the specific weight gain with time. The oxidation activation energy for the SiB6 is 250.72, 235.64, and 232.65 kJ/mol for PO2 = 0.1, 0.23 and 0.33 atm respectively. Again, The thermal decomposition of MgB2 to MgB4 was studied to determine the kinetic barriers associated with the decomposition process. The activation energy of decomposition is 205.81 ± 1.5 kJ/mol and formation is 241.5 ±2.6 kJ/mol, which is in close agreement with the published literature, 238.1±2.6 kJ/-mol.Three different rates (10K/min, 15K/min and 20K/min) were used for an iso-conversional method as model-free kinetics to compare with the model based kinetics. The activation energy is 239.98 kJ/mol for Kissinger-Akahira-Sunrose (KAS) method and 247.8 kJ/mol for Ozawa-Flynn-Wall (OFW) method, which is in close agreement with the published literature. The CALPHAD approach was also exploited using the equilibrium emf data to get a better understanding of the binary boride base TE materials.