Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item Microstructural Evolution in Friction Stir Welding of Ti-5111(2010) Wolk, Jennifer Nguyen; Salamanca-Riba, Lourdes; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Titanium and titanium alloys have shown excellent mechanical, physical, and corrosion properties. To address the needs of future naval combatants, this research examines an alternative joining technology, friction stir welding (FSW). Friction stir welding uses a non-consumable tool to generate frictional heat to plastically deform and mix metal to form a consolidated joint. This work focuses on FSW of Ti-5111 (Ti-5Al-1Sn-1Zr-1V-0.8Mo), a near alpha alloy. This study aims to gain a fundamental understanding of the relationship between processing parameters, microstructure, and mechanical properties of experimental 12.7mm and 6.35mm Ti-5111 friction stir welds. The resulting weld microstructure shows significant grain refinement within the weld compared to the base metal. The weld microstructures show a fully lamellar colony structure with peak welding temperatures exceeding beta transformation temperature. The friction stir weld shows material texture strengthening of the BCC F fiber component before transformation to D2 shear texture in the stir zone. Transmission electron microscopy results of the base metal and the stir zone show a lath colony-type structure with low dislocation density and no lath grain substructure. In situ TEM heating experiments of Ti-5111 friction stir welded material show transformation to the high temperature beta phase at significantly lower temperatures compared to the base metal. Thermal and deformation mechanisms within Ti-5111 were examined through the use of thermomechanical simulation. Isothermal constant strain rate tests show evidence of dynamic recrystallization and deformation above beta transus when compared with the FSW thermal profile without deformation. Subtransus deformation shows kinking and bending of the existing colony structure without recrystallization. Applying the friction stir thermal profile to constant strain rate deformation successfully reproduced the friction stir microstructure at a peak temperature of 1000ºC and a strain rate of 10/s. These results provide unique insight into the strain, strain rates, and temperatures regime within the process. Finally, the experimental thermal and deformation fields were compared using ISAIAH, a Eulerian based three-dimensional model of friction stir welding. These results are preliminary but show promise for the ability of the model to compute thermal fields for material flow, model damage prediction, and decouple texture evolution for specific thermomechanical histories in the friction stir process.Item RELIABILITY OF LEAD-FREE HIGH TEMPERATURE SURFACE MOUNT COMPONENT ATTACHES(2008) Oberc, Timothy James; McCluskey, F. P.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work investigates the relative reliabilities of SAC305, eutectic Au-Sn, and Ag-In transient liquid phase sintered (TLPS) solder joints subjected to high temperature passive thermal cycling. These solder materials were monitored for electrical resistance, mechanical pull strength, and microstructural changes during cycling. In fabricating the test assemblies, SAC305 and eutectic Au-Sn manufacturing parameters were gathered from the paste distributors while fabrication with Ag-In TLPS required in-house development. Work with the Ag-In TLPS paste revealed that reducing the additive (In) particle size led to statistically significant improvements in the solid volume fraction of the joints. Successful attachment of ceramic quadflat packs (CQFPs) to polyimide circuit boards using Ag-In TLPS demonstrated that surface mount joints with mechanical and electrical integrity could be manufactured from the material. Au-Sn was found to be the strongest of the materials while cracking in the SAC305 and Ag-In samples during cycling resulted in weaker joints.Item The Practical Engineers' Rebellion: Evans Patent Safety Guard and the Failure of Scientific Technology in the Steam Boat Inspection Service, 1830-1862(2008-07-31) Bernhardt III, John A.; Friedel, Robert; History; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The U.S. Congress's initiative to solve the problem of steamboat boiler explosions in the mid-nineteenth century resulted in the Steamboat Act of 1852. The Act brought radical changes to the western rivers, including reform of the engineering cadre, introduction of new safety devices and procedures, and the creation of a new bureaucracy (the Steam Boat Inspection Service). One of the new safety devices introduced by the Treasury Department was the controversial Evans Patent Safety Guard. This is the story of the safety guard as a central actor in framing the expertise of scientists, inventors, and practical engineers in attempting to make technology safe. The safety guard helps us to understand where expertise came from, how it was defined and justified by government officials, and why the notion of technological expertise depends on a complex mix of technical, institutional, and socioeconomic factors.Item An Experimental and Theoretical Investigation of the Low Temperature Creep Deformation Behavior of Single Phase Titanium Alloys(2006-10-26) Oberson, Paul Gregory; Ankem, Sreeramamurthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Titanium alloys are used for many applications due to their desirable properties, including high strength-to-weight ratio, corrosion resistance, and biocompatibility. They are used for aerospace, chemical, nuclear, industrial, biomedical, and consumer applications. Often, titanium components are subject to stresses for an extended time. It is known that single-phase hexagonally close-packed (HCP) alpha and body-centered cubic (BCC) beta-titanium alloys deform over time, or creep, at low temperatures (<0.25*Tm). However, factors that affect creep behavior including microstructure and alloy chemistry are not well understood. The aim of this investigation is to experimentally and theoretically study the creep deformation behavior of single-phase alpha and beta-titanium alloys. The first part of the investigation concerns alpha-Ti alloys. The low temperature creep behavior was studied experimentally, using alpha-Ti-1.6wt.%V as the model alloy. Creep testing was performed at a range of temperatures and slip and twinning were identified as creep deformation mechanisms. The activation energy for creep was measured for the first time for an alpha-Ti than deforms by twinning. A change in activation energy during creep is explained by a model for twin nucleation caused by dislocation pileups. The theoretical aspect of the investigation concerns the phenomenon of slow twin growth (time-dependent twinning) during low temperature creep of alpha and beta-Ti alloys. This phenomenon is unusual and poorly understood as twins in bulk metals are expected to grow very fast. It was suggested that interstitial atoms, particularly oxygen could be responsible for time-dependent twinning but there were no models to explain this. As such, crystallographic models were developed for the HCP lattice of alpha-Ti and the BCC lattice of beta-Ti to show how the octahedral interstitial sites where atoms such as oxygen can reside are eliminated by the atomic movements associated with twinning. As such, the rate of twin growth, and in turn the creep strain rate is controlled by the diffusion of oxygen away from these eliminated sites. The results of these findings are valuable when designing Ti alloys for improved creep resistance and mechanical reliability. This work was supported by the National Science Foundation under Grant Number DMR-0513751.Item Dynamics of Near-Alpha Titanium Welding(2004-10-12) Neuberger, Brett William; Ankem, Sreeramamurthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Typically, when gas tungsten arc welding (GTAW) is employed to join near-alpha titanium alloys, the resulting weld fusion zone (FZ) is much harder than that of the base metal (BM), thereby leading to lost ductility. The aim of this investigation was to improve FZ ductility of Ti-5Al-1Sn-1V-1Zr-0.8Mo by modifying filler metal chemistry. In this regard, metallic yttrium was added to the filler metal and aluminum concentration reduced. It was believed that additions of yttrium would lead to formation of yttria in the weld melt, thereby promoting heterogeneous nucleation. Since oxygen and aluminum both act as alpha-stabilizers, expected pickup of oxygen during the welding process will be offset by the aluminum reduction. Tensile testing indicated that modified filler metal welds showed a dramatic increase in ductility of the FZ. Fracture toughness testing showed that while JIC values decreased in all welds, the tearing modulus, T, in modified filler metal welds was significantly higher than that of matching filler metal welds. Microhardness mapping of the weld zones illustrated that modified filler metal welds were significantly softer than matching filler metal welds. Microstructural examinations were completed through the use of optical, SEM and TEM studies, indicating that there was a presence of nano-particles in the weld FZ. XPS analysis identified these particles as yttrium oxysulfate. WDS analysis across the welds' heat affected zones demonstrated that there is an internal diffusion of oxygen from the BM into the FZ. Research results indicate yttrium oxysulfide particles form in the weld pool, act as a drag force on the solidification front and limit growth of prior-beta grain boundaries. The reduced prior-beta grain size and removal of interstitial oxygen from the matrix in modified filler metal welds, further enhanced by oxidation of yttrium oxysulfide to yttrium oxysulfate, leads to increased ductility in the weld's FZ. Addition of yttrium to the weld also acts to modify the surface tension of the melt, leading to an increased weld depth penetration. Results of this work indicate that the goals of this project and a significant advancement in the understanding of yttrium effects on titanium grain refinement have been achieved.Item Investigation of Low Temperature Creep Deformation Behavior of a Metastable Beta Titanium-14.8Wt%Vanadium Alloy(2004-07-13) Hudson, Candi Monica; Ankem, Sreeramamurthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents the results of investigated low temperature creep behavior of a metastable beta phase Ti-14.8Weight%V alloy (Ti-14.8V). It is the first such study which relates the activation energy and microstructure with low temperature creep deformation mechanisms in the temperature range of 298K to 800K. A Ti-14.8V alloy with a grain size of 350 m was tensile and creep tested in the temperature range of 298 - 458 K; creep tests were conducted at 95% of the 0.2% yield stress. Activation energies were determined by utilizing strain rate models and resulting least squared Arrhenius plots, which were found to be in the range of 36.6-112.42 kJ/mole for the measured temperature range of 298 - 458K. The resulting activation energies plotted as a function of strain was found to be linear dependent. The determined activation energy values of 36.6 57.55 kJ/mole at the low end of the strain are within the range of activation energy values for dislocation motion. The higher activation energy value of 112.42 kJ/mole is within range of for activation energy value for diffusion of oxygen in beta titanium alloy. These activation energy values are consistent with SEM and TEM observations of deformation mechanisms as dislocations, slip, and stress induced plates (SIP) in the form of twinning were the dominant creep deformation mechanisms for this alloy. The deformation mechanisms changed from predominantly slip to SIP in the form of twins at the higher test temperatures. Further, these findings are consistent with observations, characterization by TEM analysis identified slip dislocations of the 1/2<111> type and twins of the {332}<113> type, which are consistent with time dependent twinning deformation. The results strongly support the mechanism of oxygen controlled time dependent twinning deformation as proposed earlier.