Department of Metallurgical and Materials Engineering

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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.
An analysis of the grain refinement of magnesium by zirconium
(University of Alabama Libraries, 2010) Saha, Partha; Viswanathan, S.; University of Alabama Tuscaloosa
A 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.
Carbide Nanoparticle Dispersion Techniques for Metal Powder Metallurgy
(MDPI, 2021) Rocky, Bahrum Prang; Weinberger, Christopher R.; Daniewicz, Steven R.; Thompson, Gregory B.; University of Alabama Tuscaloosa; Colorado State University
Nanoparticles (NP) embedded into a matrix material have been shown to improve mechanical properties such as strength, hardness, and wear-resistance. However, the tendency of NPs to agglomerate in the powder mixing process is a major concern. This study investigates five different mechanochemical processing (MCP) routes to mitigate agglomeration to achieve a uniform dispersion of ZrC NPs in an Fe-based metal matrix composite. Our results suggest that MCP with only process controlling agents is ineffective in avoiding aggregation of these NPs. Instead, the uniformity of the carbide NP dispersion is achieved by pre-dispersing the NPs under ultrasonication using suitable surfactants followed by mechanically mixing of the NPs with iron powders in an alcohol solvent which is then dried. High-energy MCP is then used to embed the NPs within the powders. These collective steps resulted in a uniform dispersion of ZrC in the sintered (consolidated) Fe sample.
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.
Chemical variation induced nanoscale spatial heterogeneity in metallic glasses
(Taylor & Francis, 2018) Wang, Neng; Ding, Jun; Luo, Peng; Liu, Yanhui; Li, Lin; Yan, Feng; University of Alabama Tuscaloosa; United States Department of Energy (DOE); Lawrence Berkeley National Laboratory; Chinese Academy of Sciences; Institute of Physics, CAS
Metallic glasses possess amorphous structures with inherent heterogeneity at the nanoscale. A combined experimental and modeling investigation to elucidate the chemical effect on such nanoscale heterogeneity in a Cu-Zr-Al metallic glass system is conducted. By using the dynamic atomic force microscopy, we reveal a reduction of the nanoscale spatial heterogeneity in the local viscoelastic response after introducing Al into the Cu50Zr50 metallic glass. The change of such nanoscale heterogeneity can be contributed to the variation of local atomic structures. The addition of Al increases the population of the icosahedral short-range ordered clusters, thus reducing the structural heterogeneity at the nanoscale. IMPACT STATEMENTThis paper provides a combination between the nanoscale experimental and theoretical understanding of the chemical variation induced spatial heterogeneity in CuZrAl metallic glass and their impacts on the mechanical properties.
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.
Composition-dependent apparent activation-energy and sluggish grain-growth in high entropy alloys
(Taylor & Francis, 2019) Gwalani, B.; Salloom, R.; Alam, T.; Valentin, S. G.; Zhou, X.; Thompson, G.; Srinivasan, S. G.; Banerjee, R.; University of North Texas System; University of North Texas Denton; University of Alabama Tuscaloosa; United States Department of Energy (DOE); Pacific Northwest National Laboratory
Experimental results reveal that the apparent activation-energy for grain-growth in an fcc-based AlxCoCrFeNi high entropy alloy (HEA) system increases from 179 to 486kJ/mol when the Al content increases from x=0.1 to 0.3. These unexpectedly high apparent activation-energy values can be potentially attributed to solute clustering within the fcc solid-solution phase that develops with increasing Al content in this HEA. Detailed microstructural analysis using atom-probe tomography and density functional theory (DFT) calculations strongly indicate the presence of such nanoscale clusters. This phenomenon can change grain-growth from a classical solute-drag regime to a much more sluggish cluster-drag based mechanism in these HEAs. [GRAPHICS] IMPACT STATEMENTFirst report on a composition dependent change in apparent activation-energy for grain-growth in high entropy alloys. A novel cluster drag effect inhibiting grain-growth kinetics is suggested.
Compositional Dependent Thin Film Stress States
(2010-08-23) Thompson, G. B.; Fu, B.; University of Alabama Tuscaloosa
This paper addresses in situ stress evolution of two-component FexPt1−x, where x spanned 0 to 1, alloy thin films. The stresses of the high-temperature, quenched-in, solid solution phase was determined by in situ wafer curvature measurements during ambient temperature growth. The measured stresses were shown to be compositional dependent and spanned both compressive and tensile stress states. Under specific growth conditions, a “zero-stress” state could be achieved. The alloy stress states did not show any significant stress recovery upon ceasing the deposition, i.e. the stress state during growth was retained in the film. X-ray diffraction, transmission electron microscopy, and atom probe tomography were used to characterize the microstructures of each thin film. The evolution of the stress state with composition is described in terms of a chemical potential term for preferential segregation of one species in the alloy to the grain boundaries.
Compositional Evolution During the Synthesis of FePt Nanoparticles
(2008-09-25) Thompson, G. B.; Srivastava, Chandan; Nikles, David E.; University of Alabama Tuscaloosa
A series of FePt nanoparticles was synthesized by the thermal decomposition of iron pentacarbonyl and reduction in platinum acetylacetonate in phenyl ether solvent. A range of precursor molar ratios of 2, 1.5, and 1 between iron pentacarbonyl and platinum acetylacetonate was studied. After 30 min of reflux, the synthesis method produced a wide distribution in composition and size for the nanoparticles. Given 200 min of reflux, it was observed that the particle-to-particle composition and size narrowed, and the atomic ratio of Fe to Pt, for the majority of nanoparticles, approached the initial precursor molar ratios except for the molar ratio of 1. It is speculated that the compositional variability may be a result of the slow kinetics of iron pentacarbonyl’s decomposition in the reaction.
A computational investigation into the microstructures and stability of the zeta phase in transition metal carbides and nitrides
(Taylor & Francis, 2018) Weinberger, Christopher R.; Yu, Hang; Wang, Billie; Thompson, Gregory B.; Colorado State University; Drexel University; University of Alabama Tuscaloosa
A high-volume fraction of the zeta phase in multiphase group VB transition metal tantalum carbides has been shown to dramatically increase fracture toughness. This has been attributed to its unique nanoscale lath-based microstructure. However, what governs the microstructure and how it forms is still not well understood. In this paper, we propose a precipitation model for the formation of these phases and demonstrate that the anisotropic surface energies govern the observed zeta-phase morphology. The energetics and zeta-phase microstructure for other group VB carbides were found to be similar. In contrast, multiphase hafnium nitrides can form both thin-lath-based microstructure as well as large, single zeta-phase grains. The difference between hafnium nitride and the group VB carbides is attributed to the relative bulk free energies and low-temperature stability between the phases.
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 Tuscaloosa
This 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.
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 Tuscaloosa
This 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.
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 Tuscaloosa
The 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.
Deformation and phase stability behavior in transition metal carbides
(University of Alabama Libraries, 2018) Smith, Chase; Thompson, Gregory B.; University of Alabama Tuscaloosa
Extreme 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.
Design and discovery of a novel half-Heusler transparent hole conductor made of all-metallic heavy elements
(Nature Portfolio, 2015-06-24) Yan, Feng; Zhang, Xiuwen; Yu, Yonggang G.; Yu, Liping; Nagaraja, Arpun; Mason, Thomas O.; Zunger, Alex; Northwestern University; University of Colorado System; University of Colorado Boulder; University of Alabama Tuscaloosa
Transparent conductors combine two generally contradictory physical properties, but there are numerous applications where both functionalities are crucial. Previous searches focused on doping wide-gap metal oxides. Focusing instead on the family of 18 valence electron ternary ABX compounds that consist of elements A, B and X in 1:1:1 stoichiometry, we search theoretically for electronic structures that simultaneously lead to optical transparency while accommodating intrinsic defect structures that produce uncompensated free holes. This leads to the prediction of a stable, never before synthesized TaIrGe compound made of all-metal heavy atom compound. Laboratory synthesis then found it to be stable in the predicted crystal structure and p-type transparent conductor with a strong optical absorption peak at 3.36 eV and remarkably high hole mobility of 2,730 cm(2)V(-1)s(-1) at room temperature. This methodology opens the way to future searches of transparent conductors in unexpected chemical groups.
Designing, manufacturing, testing, and optimizing of micro-fuel cells
(University of Alabama Libraries, 2009) Lu, Yuhao; Reddy, R. G.; University of Alabama Tuscaloosa
Micro-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.
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.
Effect of bottom electrodes on nanoscale switching characteristics and piezoelectric response in polycrystalline BiFeO3 thin films
(American Institute of Physics, 2011-10-17) Yan, F.; Zhu, T. J.; Lai, M. O.; Lu, L.; National University of Singapore; Zhejiang University; University of Alabama Tuscaloosa
We have investigated the nanoscale switching characteristics and piezoelectric response based on polycrystalline BiFeO3 (BFO) thin films with different orientations deposited on different oxide bottom electrodes. The BFO film deposited on the LaNiO3 (LNO)-coated Si substrate shows a (001) preferred orientation and higher ferroelectric properties, while the BFO film grown on the SrRuO3 (SRO) buffered Si substrate shows a random orientation. The domain structures have been determined via piezoresponse force microscopy (PFM) for both films, predicting that the BFO film with the LNO bottom electrode has a larger piezoelectricity property corresponding to the ferroelastic domain. Through local switching spectroscopy measurements, the evidence of ferroelectric switching and the origin of the enhanced piezoresponse properties have been provided. A greatly improved piezoelectric response has been demonstrated using PFM that is 66.8 pm V-1 for the BFO with a SRO bottom electrode, while we obtain a value of 348.2 pm V-1 for the BFO with a LNO bottom electrode due to the increased density of the polarization vectors along the external electrical field. (C) 2011 American Institute of Physics. [doi:10.1063/1.3651383]
The effect of copper on the eutectoid transformation in ductile iron
(University of Alabama Libraries, 2010) Samuel, Chris; Viswanathan, S.; University of Alabama Tuscaloosa
As 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.
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 Tuscaloosa
There 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.

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