Theses and Dissertations - Department of Metallurgical and Materials Engineering
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Item Advanced characterization of the oxidation behavior of grain refined NiAl(University of Alabama Libraries, 2019) White, Rachel Ellen; Weaver, Mark Lovell; University of Alabama TuscaloosaReactive 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.Item An analysis of the grain refinement of magnesium by zirconium(University of Alabama Libraries, 2010) Saha, Partha; Viswanathan, S.; University of Alabama TuscaloosaA Design of Experiments (DOE) approach was used to conduct a systematic study of the grain refinement of magnesium by zirconium; variables included the amount of zirconium, the pouring temperature, and the settling time prior to casting. Samples were poured into a special "hockey puck" mold designed to reproduce the conditions in permanent mold casting. Optical and scanning electron microscopy (SEM) was utilized to measure the grain size in the final microstructure. Sample dissolution followed by SEM was used to characterize zirconium particle size and morphology both in the master alloy and grain refined samples, while an AccuSizer 770 Photozone/Light Obscuration instrument was used to measure total particle size distributions in the master alloy and grain refined samples. Transmission Electron Microscopy (TEM) was used to identify particles that likely act as suitable heterogeneous nucleation sites for grain refinement. The TEM results show that a range of particle sizes are likely substrates and that only zirconium particles which are faceted are likely nucleation sites. It is apparent that only 1 to 3% of the total particles serve as nucleation sites, but a comparison of the grain density vs. faceted particle density shows close agreement. Equal Channel Angular Extrusion (ECAE) processing of the magnesium-15wt% zirconium master alloy to increase the number of faceted particles resulted in improved grain refinement efficacy. This work suggests that there is a tremendous potential to engineer a more efficient grain refiner.Item 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 TuscaloosaInvestigation 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.Item Coating yttria stabilized zirconia powders by magnetron sputtering(University of Alabama Libraries, 2019) Togaru, Maanas; Thompson, Gregory B.; University of Alabama TuscaloosaThis 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.Item Containerless melting and characterization of cast magnesium AZ31-B alloy at low superheat in the magnetic suspension melting process(University of Alabama Libraries, 2011) Rimkus, Nathan Wayne; El-Kaddah, N.; University of Alabama TuscaloosaThis study deals with containerless induction melting and characterization of cast magnesium AZ31-B alloy at low superheat via the Magnetic Suspension Melting (MSM) process. The operating conditions for melting and confinement of a 63.5mm molten column of the alloy with respect to current and frequency were found to range from 770-830 Ampere and 3552-3600 Hz, respectively. The macro/micro-structure, oxide entrapment, segregation of alloying elements, and the formation of intermetallic precipitates in MSM cast AZ31-B alloy in a unidirectional bottom chilled ceramic mold were investigated. For a baseline comparison, the alloy was also cast using conventional methods at a superheat of 60°C. Analysis of cooling curves showed that the cooling rate during solidification is essentially constant at about 1°C/s. The growth velocity, V, for MSM casting produced at low superheat is almost constant during solidification, around 1 mm/s, while the thermal gradient, G, decreases with increasing solid fraction from 2.07°C/mm at f=0, to 1.10°C/mm at f=0.5. In contrast, V for castings produced at high superheat is four times smaller than that for low superheat castings--the conditions that favor equiaxed dendritic solidification morphology. Metallographic examination of the MSM alloy cast at low superheat shows no evidence of oxide formation. It was also found that casting at this low superheat produced a fine globular grain structure compared to the equiaxed dendritic structure in conventionally cast alloys at high superheat. The average grain sizes for 5 and 8°C MSM produced castings were 83.96 and 94.81μm, respectively. The 60°C superheat castings were found to have a much larger average grain size of 334.42μm. Elemental segregation analysis was performed and showed the presence of primary-α Mg and secondary-α Al rich Mg phases, along with γMg_17 Al_12 and Al_8 Mn_5 intermetallic phases. For the globular structures, the intermetallic phases were found to form in the secondary-α phase along the grain boundaries. Comparatively, the dendritic entrapment of the secondary-α phase was found to lead to intermetallic phase formation in the matrix of the grains.Item Correlation between heat input and residual stresses from friction stir welding of AA5052 plate(University of Alabama Libraries, 2018) Zhu, Ning; Brewer, Luke N.; University of Alabama TuscaloosaThis thesis investigates the connections between friction stir welding (FSW) parameters, simple energy/heat input metrics, and the resultant residual stresses on AA5052-H32 plates. A range of weldments were produced with different tool rotational and traverse speeds to produce the same values of the pseudo heat index (PHI). Average residual stresses inside the stir zone and peak residual stress in the thermo-mechanically affected zone were systematically recorded using laboratory x-ray diffraction. In addition, thermal cycles on the advancing side of the welds were collected and analyzed for comparison with the predictions of heat input based upon FSW parameters. Based upon these results, the PHI is not a good predictor of the peak residual stress for welding conditions which produced sound welds. Increasing traverse speed, V, with fixed rotational speed does increase the residual stresses inside the stir zone. The data in this thesis suggests that there is a complex relationship between frictional heating and mechanical stirring of the material. As a result, there is a rotational speed, which requires minimum torque during welding.Item Corrosion behavior of haynes 230, ns-163 and incoloy 800h alloys in lif-naf-kf, mgcl2-kcl and mgcl2 molten salt(University of Alabama Libraries, 2015) Peng, Yuxiang; Reddy, R. G.; University of Alabama TuscaloosaThe behavior of Haynes 230 and NS-163 alloys in fluoride (FLiNaK) and chloride (MgCl2-KCl) salts as well as NS-163 and Incoloy 800H alloys in MgCl2 salt were evaluated based on thermodynamic analysis. Also, corrosion behavior of alloys with the addition of corrosion inhibitors (Zr and Mg) to the molten salts were investigated in this paper. Corrosion studies were performed using thermodynamic modeling software to understand the corrosion mechanisms and to investigate the compatibility of Haynes and Incoloy alloys for thermal storage applications in the molten salts. Equilibrium conditions were considered for predicting the corrosion products, corrosion potentials and decomposition of molten salt with and without inhibitor for Haynes 230 and NS-163 alloys in FLiNaK and MgCl2-KCl at 700, 750, 800, 850, 900 and 1000oC. The same procedure was applied for NS-163 and Incoloy 800H alloys in MgCl2 at 750, 800 and 850oC. Results illustrate these alloys are all stable in the molten salt. From calculation, K3AlF6 and MnCl2 are the major products observed in FLiNaK and chloride salts respectively. In addition, corrosion inhibitors (Zr and Mg) protect these alloys from further corrosion acting as sacrificial anode. Furthermore, with known amount of impurities added into molten salt, calculations show that Cr and Mn metals are transferred to molten salt readily. Experiment of Haynes 230 and Stainless Steel corrosion in FLiNaK were performed at 1000oC under 1 bar Ar atmosphere for 1000 hours and NS-163 for 720 hours to detect the corrosion rate. SEM was performed to evaluate the corrosion mechanism for these alloys.Item Deformation and phase stability behavior in transition metal carbides(University of Alabama Libraries, 2018) Smith, Chase; Thompson, Gregory B.; University of Alabama TuscaloosaExtreme environment applications require materials that have melting temperatures in excess of 3000 °C and that can retain good mechanical properties at high temperatures. Transition metal carbides (TMCs) are excellent candidates due to their high melting temperatures, high hardness, and low chemical reactivity. These materials display a duality of mechanical responses dependent on structure and temperature. This research aims at understanding the phase stability and deformation behavior of TMCs at compositions and temperatures that have received little investigation. A series of HfxTa1-xC compositions were computationally predicted, fabricated, and verified by experimentally identifying their phase formation, hardness, and dislocation behavior. Hardness values obtained via nanoindentation verified computational trends which predicted a modest rise in the Hf-rich ternary compositions. The presence of small amounts of Ta in Hf-rich ternary compositions yielded a change in slip system from the reported <110>{110} in HfC to <110>{111} commonly observed in TaC. To gain insight into the deformation and slip behavior of TaC and HfC at ultra-high temperatures, a thermo-mechanical testing apparatus was built for deforming specimens between 2100 °C to 2900 °C. Samples were resistively heated in the presence of a magnetic field to produce a non-contact Lorentz force. Greater deflection was observed for HfC up to 2300 °C attributed to differences in grain size. TaC deflection increased with rising temperature whereas HfC deflection decreased. This unexpected observation was discovered to be an artifact of plastic deformation that occurred during the preload. Mass transport and diffusional creep were found dominant with a preference for <110>{110} slip behavior observed for both carbides. To understand phase stability in the Nb-C system, a series of NbCx compositions were fabricated to span between the single-phase NbC and single-phase Nb2C with several compositions residing in multi-phase regions of the phase diagrams. Equiaxed grains formed for all compositions with those between ~ 0.56 to ~ 0.63 C/Nb exhibiting a lath–like microstructure as well. Additionally, a diffusion couple was processed near the same conditions to establish the phase transformations that lead to the observed microstructures. Carbon was observed to deplete from NbC and react with the Nb metal to form β-Nb2C.Item Designing, manufacturing, testing, and optimizing of micro-fuel cells(University of Alabama Libraries, 2009) Lu, Yuhao; Reddy, R. G.; University of Alabama TuscaloosaMicro-fuel cells are considered as promising electrochemical power sources in portable electronic devices. Performance of micro-fuel cells are closely related to many factors, such as processes of fabrication, designs of flow fields, and operating conditions. In the present research, micro-proton exchange membrane fuel cells (PEMFCs) and micro-direct methanol fuel cells (DMFCs) were systematically investigated from the aspects of structure design, bipolar/end plates (BPs) fabrication, and fuel cells evaluation. In chapter 3, compared with conventional machining and rapid prototyping (RP) technology, microelectromechanical system (MEMS) technology was the practicable method to fabricate the BPs with channels of a few microns width. Experimental and modeling methods were employed to analyze performance of the micro-PEMFC in chapter 4. Contact resistance changed significantly the distribution of overpotential in the micro-PEMFC and decreased the current output. Small dimensions of the micro-channel drastically affected the species transport and resulted in a non-uniform current distribution along channel direction at low cell potential (high current). In chapters 5, four kinds of flow fields, mixed multichannel serpentine with wide channels, single channel serpentine, double channel serpentine, and mixed multichannel serpentine with narrow channels, were applied to micro-PEMFCs. Results suggested that the micro-PEMFC with good performance should use the flow field with a mixed multichannel design and long micro-channels. In chapter 6, the same flow fields were studied in the micro-DMFCs. Concentration and flow rates of methanol solution affected performance of micro-DMFCs. A micro-DMFC with the long and narrow channels needed to take long time to reach the stable stage when an electric load on it was changed. In chapter 7, a passive air-breathing micro-DMFC with low loading of catalysts was developed. Performance of the passive micro-DMFC became poor with the increase in concentration of methanol solution. Power densities of the passive micro-DMFC drastically depended on the current scanning rate. Finally, cobalt phthalocyanine was introduced to platinum catalyst system to improve and optimize the micro-DMFCs. After heat-treatment at 635 oC, CoPc-Pt/C demonstrated good electrocatalytic activity for oxygen reduction reaction (ORR) and high methanol tolerance. However, CoPc-Pt/C heated at 980 oC showed a good electrocatalytic activity for MOR in DMFCs.Item 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 TuscaloosaThe 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.Item The effect of copper on the eutectoid transformation in ductile iron(University of Alabama Libraries, 2010) Samuel, Chris; Viswanathan, S.; University of Alabama TuscaloosaAs a result of the shortage in the availability of suitable steel scrap, trace elements are unintentionally added to ductile iron from the scrap available for melting. The effect of some of these trace elements on graphite shape, the resulting microstructure, and the dimensional behavior of the cast component are not well understood. The lack of control of these trace elements leads to excessive scrap as well as additional heat treatment costs, especially when ferritic or fully pearlitic microstructures are required. This work focuses on the effect of one element, copper, that occurs as a trace element or is often deliberately added when pearlitic microstructures are desired. Ductile iron samples with copper levels ranging from 0 to 0.8 wt. % were investigated. Gleeble dilatometry was used to characterize phase transformations and microstructure development. The diffusion coefficient of carbon in ferrite in the presence of copper and silicon was measured using multicomponent solid-solid diffusion experiments. Copper appears to have little or no effect on the diffusion coefficient of carbon in ferrite. Interrupted solidification experiments are used to explain solidification and segregation in ductile iron, and a revised model of ductile iron solidification is presented. It is shown that the segregation of copper during solidification is key to the pearlite promoting effect of copper and is related to the decrease in the driving force for the diffusion of carbon through the ferrite shell.Item Effect of electromagnetic stirring on grain refinement of Al-4.5%Cu alloy(University of Alabama Libraries, 2013) Heyen, Matthew Josef; El-Kaddah, N.; University of Alabama TuscaloosaThere is considerable interest in electromagnetic stirring (EMS) of molten metal during solidification as a means to refine the grain structure and to modify the microstructure of the cast alloy by introducing a convective flow across the solid-liquid interface (SLI). . This convective flow induces changes in the thermal and solutal profiles in front of the SLI that affect both the morphology of the solidified metal as well as the composition variation throughout the metal when solidified. Morphology changes due to EMS consist of increased solutal fragmentation due to liquid with high solutal concentrations being forced into the SLI and deep into the mushy zone and causing the remelting of secondary dendrite arms (SDA's) which can be carried off in the convective flow created by the EMS. If the thermal undercooling conditions allow, these fragments serve as nuclei for new grains thereby increasing the nucleation rate during solidification. Another fragmentation mechanism caused by linear EMS, where recirculating flow loops of liquid are generated, is mechanical shearing. Albeit this mechanism appears to be minuscule compared to solutal fragmentation, but some evidence has been discovered supporting its existence.Item Effect of nickel content on the crystallization behavior in nanocrystallin (Co_(1-x)Ni_x)_88Zr_7B_4Cu1 soft magnetic alloys(University of Alabama Libraries, 2012) Hornbuckle, Billy Chad; Thompson, Gregory B.; University of Alabama TuscaloosaA series of (Co_(1-x)Ni_x)_88Zr_7B_4Cu1 soft magnetic alloys, where X was varied from 0 to 1, were fabricated by a melt spinning process into thin ribbons of the material. This process was followed by an isothermal anneal to produce a nanocomposite alloy, i.e. nanocrystalline grains in a residual amorphous matrix. The alloy series was designed to investigate crystallization kinetics and limits to the compositional regime where a nanocomposite could be formed. The primary and secondary crystallization temperatures of each alloy were determined using Differential Scanning Calorimetry (DSC) from which the crystallization activation energies were calculated using the Kissinger Method. When X exceeded 0.75, the as-spun ribbons exhibited partial crystallization, resulting in reduced exothermic crystallization peaks. For lower Ni contents, the ribbons were amorphous in the as-spun state. The activation energy for crystallization decreased with increasing Ni content. Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT) revealed fine nanocrystallite and boron segregation to the grain boundaries with increasing Ni content. The previously suspected use of Cu clustering, which can act as heterogeneous nucleation sites, showed no clear correlation with observed spatial location of the crystallites. Chemical partitioning between species in the as-spun and primary crystallization heat treatments were correlated to the resulting changes in magnetic properties. As Ni content increased, the saturation magnetization and normalized magnetization for these samples decreased accordingly.Item Electro-deposition of zinc and lead from their oxide compound using choline chloride and urea based ionic liquid(University of Alabama Libraries, 2014) Yang, Haoxing; Reddy, R. G.; University of Alabama TuscaloosaA novel method for electrodepositing metal such as zinc and lead was investigated by using a urea and choline chloride based ionic liquid as electrolyte at low temperature. The solubility analysis of both metal oxides in the resulting ionic liquid was investigated by comparing the absorption peak from FTIR. The solubility was illustrated as a function of temperature and amount of added metal oxide. Electrochemical behavior was measured from a three electrode cell which consisted of tungsten wire as cathode, platinum wire as anode and silver wire as quasireversible reference electrode using EG&G electrochemistry workstation setup. The reduction potential was found at -1.1V for the zinc system and -0.37V for the lead system, both used Ag as reference electrode. This study has been focused on measuring values of diffusion coefficient and charge transfer coefficient in different experimental conditions. In the Zn system, the charge transfer coefficient has been calculated in different temperature and found to be ranging from 0.180 to 0.258 while in the Pb system ranges from 0.528 to 0.627, and the diffusion coefficient from 1.14×10-7to 2.84×10-6 in the Zn system while 1.09×10-8to 1.42×10-7in the Pb system. The activation energy of diffusion of electro-active species in both systems are calculated and compared to a value of 107.23 kJ/mol in Zn to 124.70 kJ/mol in Pb. The nucleation mechanism was investigated for both systems. A three-dimensional nucleation model has been compared to experimental data. Instantaneous nucleation has been found for both systems. The electrodeposition for both Zn and Pb has been carried out on the Cu substrate in different voltages. Deposits are characterized by scanning electron microscopy (SEM) for morphology and X-ray diffraction (XRD) for structural analysis. Compositional analysis of both metal deposits on the cathode has been performed using Energy Dispersive Spectrometer (EDS). Metals in high purity were obtained. To improve the quality of deposited metal, effect of [BMIM]HSO4 an additive agent has been investigated. [BMIM]HSO4 has been shown to effectively improve surface layer quality by promoting a dense and uniform deposited film with finer particle size.Item Electrodeposition of Al-Ni-Cr alloys from chloroaluminate ionic liquid for bond coat on gamma-TiAl(University of Alabama Libraries, 2014) Tan, Kai; Acoff, Viola L.; University of Alabama TuscaloosaIn order to expand the utilization of γ-TiAl for high temperature structural applications, constant efforts have been dedicated to the development of coating systems to provide adequate oxidation resistance above 800oC. However, most coating methods currently employed, such as magnetron sputtering, plasma spray, and EBPVD are regarded as line-of-sight techniques, which limit the possible geometries to be coated. In this study, the possibility of electrodeposition, which emerges as a cost-effective and non-line-of-sight method, of metallic bond coats on TiAl was explored. Based on the practical feasibility, this research was targeted on Al-Ni-Cr alloy coatings. Lewis acidic chloroaluminate ionic liquid, consisting of 66.7 at.% AlCl3 and 33.3 at.% 1-ethyl-3-methylimidazolium chloride, was adopted as the baseline plating bath. The alloying elements, such as Cr and Ni were introduced by anodic dissolution of pure metals. The concentrations of Ni(II) and Cr(II) ions as a function of applied charge were measured by ICP-AES. With the conditions adopted for the experiments, the highest concentration of Ni(II) ions was 141 mM, while the concentration of Cr(II) ions was around 30 mM due to its limited solubility. The electronic absorption spectrum indicated that Cr(II) ions were tetrahedrally-coordinated, while Ni(II) ions were octahedrally-coordinated. The diffusion coefficient of Cr(II) ions was almost 10 times that of the Al(III) ions. With the large difference between the work functions of Al and Ni, Al(III) ions could be reduced by under-potential deposition in Ni solution, where the highest reduction potential was at least 0.55 V above Al(III)/Al reversible potential. In Cr solution, Al(III) ions could also be reduced under-potentially, but the under-potential was less than 0.1 V. However, in the Ni-Cr solution, no under-potential reduction of Cr(II) was observed at the presence of Ni(II). The morphologies and microstructures of the electrodeposits differ significantly as potential goes from under-potential through equilibrium potential to over-potential, and accordingly, similar microstructures can be obtained at corresponding current densities. The in-situ phase composition of Al-Cr deposits was measured by anodic stripping voltammetry as a function of deposition potential and convection. The addition of toluene as a co-solvent for Al-Cr electrodeposition was also studied, which led to the effect of decrystallization. The potentiostatic current transient for Al-Ni-Cr electrodeposition demonstrated three stages of the deposition process including nucleation, diffusion controlled growth and steady state growth, and illustrated the effect of convection on the last two stages. The pitting corrosion behaviors of the coatings were tested in both Na2SO4 and NaCl solutions by cyclic polarization. The oxidation tests were conducted on electrodeposits obtained in the over-potential region, which contains a duplex structure of Al matrix and embedded AlNiCr amorphous particles. An under coat of Ni was introduced between the TiAl substrate and Al-Ni-Cr top coat by nickel strike and nickel plating to provide adequate adhesion. Severe inter-diffusion occurs between Ni and TiAl. A continuous Al2O3 was formed in the top coat which provided good oxidation resistance.Item Electrodeposition of titanium aluminides from aluminum chloride: 1-butyl-3-methyl imidazolium chloride ionic liquid(University of Alabama Libraries, 2015) Bogala, Mallikharjuna Reddy; Reddy, R. G.; University of Alabama TuscaloosaThe key for achieving sustainable future depends not only on the discovery of new advanced structural materials, but also on the development of novel energy-efficient technologies for their production and processing in industries. Titanium aluminides are new emerging class of high-temperature structural materials that show promising application in aerospace and automotive industries. Because of their unique properties like low weight, high strength, good oxidative and creep resistance, titanium aluminides are increasingly finding their way to replace steel as structural materials. Compared to the existing industrial techniques, electrodeposition of titanium aluminides in ionic liquid electrolytes is an attractive process because of its low cost, low temperature, good current efficiency, and low energy consumption. In the present study, electrodeposition of titanium aluminides from aluminum chloride-1-butyl-3-methylimidazolium chloride (2:1 mole ratio) electrolyte was investigated at 100 °C. Chronopotentiometric experiments were conducted at different current densities (135 to 891 A/m2) for 4 h. Dissolution of titanium anode into the melt and electrodeposition of TiAl alloys onto titanium cathode took place at different current densities. Surface morphology and compositional analysis of TiAl alloys were examined using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) methods. The morphology of TiAl alloy deposits changed from dendritic to granular shape at higher current densities. XRD and EDS analysis have confirmed the presence of disordered face-centered cubic (FCC) lattice in high pure TiAl alloys. The TiAl electrodeposits containing 14.56 to 20.75 atom% Ti were produced with current efficiencies of 60.96 - 81.35% and energy consumptions of 6.71 - 12.08 kWh/kg.Item 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 TuscaloosaHigh 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.Item Experimental and theoretical analyses on the ultrasonic cavitation processing of al-based alloys and nanocomposites(University of Alabama Libraries, 2015) Jia, Shian; Nastac, Laurentiu; University of Alabama TuscaloosaStrong evidence is showing that microstructure and mechanical properties of a casting component can be significantly improved if nanoparticles are used as reinforcement to form metal-matrix-nano-composite (MMNC). In this paper, 6061/A356 nanocomposite castings are fabricated using the ultrasonic stirring technology (UST). The 6061/A356 alloy and Al2O3/SiC nanoparticles are used as the matrix alloy and the reinforcement, respectively. Nanoparticles are injected into the molten metal and dispersed by ultrasonic cavitation and acoustic streaming. The applied UST parameters in the current experiments are used to validate a recently developed multiphase Computational Fluid Dynamics (CFD) model, which is used to model the nanoparticle dispersion during UST processing. The CFD model accounts for turbulent fluid flow, heat transfer and the complex interaction between the molten alloy and nanoparticles using the ANSYS Fluent Dense Discrete Phase Model (DDPM). The modeling study includes the effects of ultrasonic probe location and the initial location where the nanoparticles are injected into the molten alloy. The microstructure, mechanical behavior and mechanical properties of the nanocomposite castings have been also investigated in detail. The current experimental results show that the tensile strength and elongation of the as-cast nanocomposite samples (6061/A356 alloy reinforced by Al2O3 or SiC nanoparticles) are improved. The addition of the Al2O3 or SiC nanoparticles in 6061/A356 alloy matrix changes the fracture mechanism from brittle dominated to ductile dominated.Item Experimental and theoretical investigation of ultrasonic cavitation processing of Al-based alloys and nanocomposites(University of Alabama Libraries, 2018) Xuan, Yang; Nastac, Laurentiu; University of Alabama TuscaloosaUltrasonic Treatment (UST) is one of the most promising manufacturing methods to refine the microstructure of casting alloys by transforming the morphology of the grains from dendritic to globular, decreasing the grain size, and modifying the precipitates. The applied temperature and/or temperature range during the ultrasonic and solidification processing are the key parameters that will influence the grain refinement. In this study, the effects of the temperature and/or temperature range applied during the ultrasonic and solidification processing on the microstructure and nano-particles distribution of the metal-matrix-nano-composites (MMNCs) have been investigated in detail. Aluminum alloy A356 and Al2O3/SiC nano-particles are used as the matrix alloy and the reinforcement, respectively. UST is applied during the solidification of the molten alloy. Experimental results indicated that the application of UST during solidification has positive effects on the microstructure of the as-cast ingots. Different UST application temperature/temperature range causes different refinement results. Moreover, the added nanoparticles refined the microstructure of the ingot section that is located adjacent to the immersed cylindrical face of the probe. Al-Si-Cu alloys have been widely used in the automotive industry. Fe-rich intermetallics are regarded as the most detrimental impurities that diminish the mechanical properties of alloys. In this study, the effect of ultrasound application temperature/temperature range on the pre-dendritic Fe-rich intermetallics (i.e, sludge) has been also investigated. Aluminum alloy A383 is used as the base alloy. Experimental results indicated that by applying UST on the melt highly influences the morphology and distribution of the precipitated Fe-rich intermetallics. Different UST application temperature/temperature range causes different modification and distribution results of the Fe-rich intermetallics. To create various temperature gradients in the laboratory scale ingot, an innovative two-zone furnace -ultrasound system has been set up in this study. A numerical model for simulation of the temperature-output power correlation that was validated by using experimental measurements has been built as well. The specific ultrasonic zone that will strongly affect the ingot microstructure has been identified and the ultrasonic attenuation coefficient of aluminum A356 melt has been determined.Item 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 TuscaloosaOver 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.