Theses and Dissertations - Department of Metallurgical and Materials Engineering
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Browsing Theses and Dissertations - Department of Metallurgical and Materials Engineering by Author "Acoff, Viola L."
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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 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 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 Influence of grain misorientation on grain growth in nanocrystalline metals(University of Alabama Libraries, 2013) Brons, Justin Glen; Thompson, Gregory B.; University of Alabama TuscaloosaIt is well known that the grain size of a material controls its properties, including mechanical strength, electrical conduction, and corrosion resistance. Typically, a fine grain size is desirable, since it allows for these properties to be increased. Nanocrystalline materials have been engineered in order to maximize the benefits associated with this fine grain size. Unfortunately, the high density of grain boundaries for a given volume of the material leads to an increase in the excess energy that is associated with grain boundaries. This excess energy can act as a driving force for grain growth, which causes these nanocrystalline structures to be unstable, with this grain growth often times being detrimental to the material properties. This research investigated the influence of grain boundary mobility and the applied driving force on grain growth in nanocrystalline metal films by focusing on the role grain boundary misorientation plays in regulating grain growth. The was be accomplished by completing two types of studies: (i) Annealing sputter-deposited thin films to study mobility in a case where the driving force is assumed to be dominated by grain boundary curvature and (ii) Mechanically indenting thin films with different microstructural features while submerged in liquid nitrogen. In terms of the latter study, the mobility was expected to be extremely low due to the cryogenic temperatures. Both sets of films were then characterized using precession-enhanced diffraction-based orientation analysis in the transmission electron microscope to quantify the evolution in grain size, grain morphology, and in the grain-to-grain misorientation.Item The investigation of accumulative roll bonding for processing TI/Al multilayered composite targets for perforation testing(University of Alabama Libraries, 2015) Stokes, Derrick D.; Acoff, Viola L.; University of Alabama TuscaloosaMultilayered composites (MLCs) processed using accumulative roll bonding (ARB) have great potential as candidates for perforation testing. In the current study, multilayered composites comprised of alternating layers of titanium and aluminum have been investigated. Since the ARB process has been shown to induce anisotropy, the Ti/Al MLCs were first subjected to quasi-static loading to determine the effects of anisotropy. The MLCs were then subjected to perforation testing using projectiles with various apex angles. The effects of perforation testing were studied in terms of varying ballistic parameters and characterization of the fracture surfaces of the MLCs. The results of this study show that ARB-processed Ti/Al MLCs are promising for use in ballistic and impact applications.Item Investigation of the effect of hafnium and chromium additions on the microstructures and short-term oxidation properties of DC magnetron sputtered β-nial bond coats deposited on Ni-based superalloys(University of Alabama Libraries, 2010) Bestor, Michael Alan; Weaver, Mark Lovell; University of Alabama TuscaloosaThermal barrier coatings play a major role in protecting turbine blades from extreme operating environments and extending service lifetimes. Reactive elements (e.g. Zr, Hf, Y, Si, etc.) have been shown to enhance the oxidation performance of such coating systems when added in appropriate amounts to overlay bond coats. This study investigated the influences of processing parameters along with Hf and Cr additions on the short-term oxidation performance of β-NiAl based coatings deposited via direct current magnetron sputtering onto CMSX-4 and René N5 substrates. Sputtering parameters were optimized to yield a zone T microstructure. The results indicate that the coatings are deposited as a solid solution and precipitation with the RE-doped coatings occur following annealing at 1000°C for times up to four hours. Precipitates form heterogeneously within the coatings with larger precipitates forming at grain boundaries and smaller ones forming within the grains at prior dislocation lines. Small incorporations of Cr into the NiAl-1Hf coating increased the average grain size and precipitate size. Transmission electron microscopy and atom probe tomography confirmed that the chemistry of the precipitates is mostly β'-Ni2AlHf accompanied by HfC and α-Cr. The results from the isothermal oxidation studies at 1050°C indicated that increasing the preoxidation annealing time from two to four hours decreased the mass gains observed with the specimens up to 100 hours. However, significant oxidation at the coating/substrate interface was discovered and the amount of oxidation increased with Hf content and preoxidation annealing time. This oxidation is thought to be caused by the large number of pinhole defects with the iii zone T microstructure and large grain boundary volume. Increased Hf concentrations were also found at the coating/substrate interface and this has been shown to lead to dramatic internal oxidation. The NiAlCrHf samples contain larger grain sizes and precipitates and a thinner TGO than the NiAl-Hf coatings. This combined with the lower mass gains during isothermal oxidation indicate that the NiAlCrHf coatings rapidly form a thin, protective α-Al2O3 layer that limits additional transport of oxygen to the bond coat. The results have been analyzed and discussed relative to previous research on sputter deposited NiAl-Hf coatings.Item Microstructural analysis of TI-6Al-4V components made by electron beam additive manufacturing(University of Alabama Libraries, 2017) Coleman, Rashadd; Acoff, Viola L.; University of Alabama TuscaloosaElectron Beam Additive Manufacturing (EBAM) is a relatively new additive manufacturing (AM) technology that uses a high-energy electron beam to melt and fuse powders to build full-density parts in a layer by layer fashion. EBAM can fabricate metallic components, particularly, of complex shapes, in an efficient and cost-effective manner compared to conventional manufacturing means. EBAM is an enabling technology for rapid manufacturing (RM) of metallic components, and thus, can efficiently integrate the design and manufacturing of aerospace components. However, EBAM for aerospace-related applications remain limited because the effect of the EBAM process on part characteristics is not fully understood. In this study, various techniques including microhardness, optical microscopy (OM), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and electron backscatter diffraction (EBSD) were used to characterize Ti-6Al-4V components processed using EBAM. The results were compared to Ti-6Al-4V components processed using conventional techniques. In this study it is shown that EBAM built Ti-64 components have increased hardness, elastic modulus, and yield strength compared to wrought Ti-6Al-4V. Further, it is also shown in this study that the horizontal build EBAM Ti-6Al-4V has increased hardness, elastic modulus, and yield strength compared to vertical build EBAM due to a preferential growth of the β phase.Item Microstructural formations and phase transformation pathways in tantalum carbides(University of Alabama Libraries, 2010) Morris, Robert Allen; Thompson, Gregory B.; University of Alabama TuscaloosaTransition metal carbides have a large assortment of applications because of their high hardness, chemical resistance, and high melting temperatures. Tantalum carbide (TaC) and its sub-stoichiometric Ta2C and Ta4C3 phases have emerged as candidate materials for ultra-high temperature structural applications. A consequence of the high melting temperature is the limiting methods to fabricate near-net shape, near full density tantalum carbides. In general, hot-isostatic pressing (HIP) and/or arc melting/vacuum plasma spraying (VPS) of powders are the viable means of manufacturing. In HIP'ing, the phase formation is through solid-state reactions whereas arc melting/VPS involves rapid solidification. Additionally, the precipitation of multiple phases generates various orientation relationships that influence the grain morphology. Depending on carbon content, the grains were equiaxed, equiaxed with a cross-hatch pattern of thin laths of secondary phases, to acicular grains. The microstructures were quantified through a series of different 2D and 3D analytical techniques. To understand how these microstructures developed, a series of XTa:(1-X)C (0.5Item Nanostructure characterization, fabrication and devices of 1D & 2D ZNO and 2D MOS2(University of Alabama Libraries, 2018) Waters, Joseph Lionel; Kung, Patrick; University of Alabama TuscaloosaOne of the main complications in the synthesis of Zinc oxide (ZnO) nanowires (NWs), is the ability to reproduce well aligned wires. ZnO was studied due to its optoelectronic applications. Its simple crystal growth abilities, lead to potentially lower cost for ZnO based devices. Other semiconductors such as MoS2 in bulk form contain an indirect bandgap of 1.2 eV. As layers are removed, the materials band gap undergoes a shift and switches from indirect to direct bandgap for a single monolayer. This monolayer of MoS2 contains a bandgap of 1.8 eV, therefore the goal was to synthesis single layer MoS2 on various substrates. In this doctoral research wide bandgap ZnO and emerging MoS2 were studied individually. These novel semiconductors were then fabricated together to form heterostructures to enhance the functionality of ZnO and MoS2 by covering the UV (380 nm) to the visible region (650 nm). ZnO powders were reduced by carbo-thermal reduction and grown onto sapphire substrates to act as a ZnO NW scaffold. The material properties such as the crystalline phase of the hexagonal wurtzite ZnO were examined by SEM, TEM and complemented by optical characterizations. X-ray photoelectron spectroscopy determined the chemical species and lack of impurities present in the NW. Local electrode atom probe analysis of the crystal stoichiometry and concentration gradient of oxygen content from the center of the NW to the edge wall. A single ZnO NW was removed from the surface and a Schottky diode device was fabricated to determine the effects of UV illumination. Defects at the edge of the wire can lead to external growth of MoS2 and good bonding at the interface of the heterostructure. The triangular formation, thickness and edge effects at the grain boundaries were studied by SEM and AFM. The two main phonon modes in MoS2 are used to determine the number of layers present by Raman spectroscopy. TEM led to determination of the 2H phase of MoS2. When the 1D-2D hybrid structure is fabricated regions of molybdenum and sulfur on the ZnO NW were mapped by EDS on a SEM and TEM.Item Quantitative microanalysis techniques for magnetic nanostructures(University of Alabama Libraries, 2010) Henry, Karen Torres; Thompson, Gregory B.; University of Alabama TuscaloosaMicroanalysis techniques are used to characterize magnetic nanostructures. To advance these materials in many applications, it is necessary to understand the microstructure. For example, subtle compositional fluctuations within a nanostructure can significantly influence the material properties. In this work, microanalysis techniques have been used to quantify composition, volume fraction, and long-range order parameter in magnetic nanostructures. A methodology for determining an optimal voxel dimension range was developed using a model system for the experimentally collected atom probe tomography data. The influence of voxel dimension on volume fraction and composition of chemically partitioned phases is examined. Atom probe tomography is used to understand the influence of Pt enrichment at grain boundaries in the A1 to L10 polymorphic phase transformation. The Pt enrichment at grain boundaries in atom probe tomography analysis provides experimental verification of modeling predictions of Pt surface segregation. It is also observed that upon phase transformation to L10, the Pt grain boundary enrichment decreased. Field ion microscopy and atom probe tomography is used to evaluate the field evaporation behavior of (001) planes in ordered FePt. Both experimental and simulation results have shown that the difference in evaporation field between the two components of the alloy contributed to the trajectory aberrations near the (002) pole and zone axes. The model system shows that chemical order within a structure introduces aberrations in the reconstruction of the atomic planes limiting the spatial and chemical fidelity of characterizing such structures using current reconstruction methodologies. Finally, a comparison of the experimental results to simulations is used to assess the viability of electron diffraction in the quantification of S in FePt thin films and nanoparticles. A multislice approach was used to simulate CBED patterns of FePt films with various thicknesses, compositions, orientations, and S values. In general, electron diffraction provides a technique to determine order parameter of small volumes with the implementation of multislice simulations.Item Relating processing-microstructure-mechanical properties of inconel 718 fabricated by selective laser melting(University of Alabama Libraries, 2018) Holland, Sharniece; Li, Lin; University of Alabama TuscaloosaSelective laser melting (SLM), uses a computer controlled scanning beam to selectively melt pre-spread powders in a layer by layer fashion. SLM has become a powerful tool to fabricate Inconel 718 due to the ability to obtain a desired microstructure and mechanical properties such as hardness and tensile strength. The combination of rapid melting and solidification in the SLM process, produces a very unique microstructures, which results in different material properties in comparison to conventional routes. Although Inconel 718 has been thoroughly studied, an in-depth analysis of grain boundary (GB) network structures and multiscale correlation of microstructure and mechanical behaviors is lacking. This work aims to relate the processing-microstructure-property relationship of Inconel 718 fabricated by SLM using experimental characterization, computational simulation, and theoretical analysis. These investigations reveal: 1. The as-SLM Inconel 718 exhibits spatial heterogeneity with a typical fast solidification structure with melting pool, columnar and equiaxial grains. In the melt pools, finer dendritic structure and a cellular structure, reflecting micro-segregation due to the laser induced localized thermal history. Upon heat treatment, the unique SLM structures are completely removed. The HT sample displays a fully recrystallized grain structure with appearance of twin boundaries and precipitates along the GBs and within the grain interiors. 2. Effective properties are influenced by the character and connectivity. The GBs diffusivity for both random and crystallographically consistent networks exhibits two types of behaviors, based on the contrast of local diffusivity of individual GBs. The as- SLM samples are occupied by one large connected general cluster reflecting the dominance of J0 triple junctions. While the HT sample multiple special clusters reflecting the interweaving of general and special boundaries. 3. The Vickers hardness of the as-SLM sample is ~ 3.14 GPa and the measured wear rate is ~ 2.16 ×10-6 mm2 while the HT sample had a significant increase in the Vickers hardness to ~ 4.49 GPa, and a reduction of wear rate to 1.75×10-6 mm2. Such enhancement of the mechanical performance can be contributed to the improvement of part quality via lowering the porosity, the removal of residual thermal stress, and the precipitation of nanoscale strengthening phases.Item Texture evolution of niobium, aluminum, and titanium in Ti-Al-Nb composites processed by accumulative roll bonding(University of Alabama Libraries, 2012) Qu, Peng; Acoff, Viola L.; University of Alabama TuscaloosaMultilayered Ti/Al/Nb composites were produced by the accumulative roll bonding (ARB) process utilizing pure Ti, Al and Nb element sheets. Up to four cycles of ARB were applied to the composites. The microstructure and texture evolution of the elemental Nb, Al and Ti phase was studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). Nanoindentation was performed as well. Nb and Ti layers necked and fractured as the number of ARB passes increased. After four ARB cycles, a nearly homogeneous distribution of Nb and Ti layers in an Al matrix was achieved. The as-received Nb sheet exhibited a fully lamellar structure and had a strong cold rolling texture. After subjecting to ARB, slight grain refining was observed and the high angle grain boundary (HAGB) fraction was increased. The intensity of the α-fiber texture was weakened while that of the γ-fiber texture was strengthened during ARB. The texture evolution was attributed to partial recrystallization during the ARB process as a result of adiabatic heating. In the Al phase, grain refinement occurred with increased ARB cycles as a result of the increased fraction of HAGBs. Strong recrystallization texture occurred for samples subjected to increased ARB cycles as a result of the adiabatic heating produced. The shear bands at the Ti/Al interface reduced the intensity of the cold rolling fiber textures of Al. There was no evidence of shear component from the orientation distribution function (ODF) results. Twinning was observed in the Ti phase for all stages of deformation but had little influence on microstructure. Grain refinement was achieved as a result of the accommodating of shear bands to the limited slip system of Ti. Recrystallization occurred at higher ARB cycles as a result of adiabatic heating. The Schmid factor shows that Basal and prismatic slip systems dominate at low ARB cycles, while at higher deformation, the first-order and second-order pyramidal slips are active.Item The texture-structure relationship in ti-al-nb multilayered composites processed by accumulative roll bonding(University of Alabama Libraries, 2014) Zhou, Liming; Acoff, Viola L.; University of Alabama TuscaloosaMultilayered Ti/Al/Nb composites were processed by the accumulative roll bonding (ARB) process using elemental foils of titanium, aluminum, and niobium. The rolled multilayered composites (MLCs) were prepared by ARB process up to two ARB cycles. The microstructure and texture evolution of the Ti, Al, and Nb in the MLCs were studied utilizing X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with electron backscattered diffraction (EBSD). The characterizations of crystallographic texture and microstructure were conducted using a creative approach; a layer by layer method on the rolling plane Texture evolution in the MLCs produced by symmetric rolling and asymmetric rolling was also studied in a layer by layer manner. In addition to studying the texture evolution of the Nb in the MLCs produced by the ARB process, the Bingham distribution was used to model the orientation distribution function (ODF) by employing MTEX, a quantitative texture analysis toolbox for Matlab®. This provided a bridge for the gap between experiments and Bingham modeling in terms of the crystallographic texture. As the numbers of ARB cycles increased, the microstructures tended to be heterogeneous through the thickness. Also, the texture development of the mating layers in the MLCs exhibited multiple texture domination rather than random. Furthermore, the developed textures of the layers in the MLCs during the ARB process were significantly different from that produced by conventional rolling. The characteristic textures formed in the MLCs subjected to the ARB process implied that the partial recrystallization and recovery occurred as a result of the adiabatic heat. The shear and compressive strain distributions were inhomogeneous through the thickness. Thus, the texture developments of the layers in the MLCs suggested a strong locational dependence. Where, the surface and the middle layers tended to form textures attributed to the shear, while, the transitory layers tended to form texture components induced by the compression.