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
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item Finite Element Modeling and Cellular Studies on Controlled Pores with Sub-Surface Continuity for Biomedical Applications(2012) Lambert, Paul Keslar; Ankem, Sreeramamurthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work investigated a novel process for improving the reliability of load-bearing joint prosthetics, in which electrical discharge machining (EDM) is used to create pores with sub-surface continuity on a conventionally-fabricated prosthetic material. The first part of this investigation utilized in vitro studies to verify the biocompatibility of deep, high-aspect-ratio EDM-produced pores. Mesenchymal stem cells were seeded onto Grade 4 titanium samples with EDM-created pores, and osteodifferentiation and mineralization were induced and assessed. It was found that such pores allowed for cell proliferation and mineralization indicating good biocompatibility. The second part of this work utilized three dimensional finite element modeling (FEM) to characterize simulated porous implant interfaces under stress. Interlocking strengths of selected structures were verified, interface separation under applied stress was measured for these structures with implications for wear particle intrusion in the interfaces, and stress shielding analysis was performed on simulated implants containing intersecting and non-intersecting pores. This work was supported in part by the National Science Foundation under Grant Number CMMI-0733522.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 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 EFFECT OF MICROSTRUCTURE ON THE ROOM TEMPERATURE TENSILE AND CREEP DEFORMATION MECHANISMS OF ALPHA-BETA TITANIUM ALLOYS(2005-04-13) Jaworski, Allan Wayne; Ankem, Sreeramamurthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Two-phase alpha-beta titanium alloys are used in many applications because of their high specific strength, corrosion resistance, processability, and biocompatibility. The room temperature tensile and creep deformation mechanisms of alpha-beta alloys must be understood in order to design alloys with desired properties and improved creep resistance. There is a lack of understanding in this regard. The aim of this investigation is to systematically study the effects of microstructure, stability of the beta phase, and alloying elements on the deformation mechanisms of alpha-beta titanium alloys using Ti-6.0wt%Mn and Ti-8.1wt%V as the model systems. The tensile and creep deformation mechanisms and microstructure were studied using SEM, TEM, HREM, and optical microscopy. In addition, theoretical modeling was performed in terms of crystallographic principles and stress analysis. It was found for the first time in an alpha-beta titanium alloy (Ti- 8.1wt%V) that the alpha phase deforms by twinning and the beta phase deforms by stress induced martensite, different mechanisms than the single-phase alpha and beta alloys with similar grain size. Single-phase alpha deforms predominantly by slip, and single-phase beta deforms predominately by twinning. This is also the first time that stress induced martensite has been observed in a creep deformed alpha-beta titanium alloy. However in the case of Ti-6.0wt%Mn, where the beta phase stability is higher, stress induced martensite was not observed. The deformation mechanisms are modeled in terms of the beta phase stability and interactions between phases, including elastic interaction stresses, alpha phase templating, interactions of deformation products, and alpha-omega interactions. A model is also proposed which explains anisotropic interface sliding based on locking of growth ledges. These results are extremely valuable when designing new alloys with improved resistance to creep and other failure modes. The observed deformation mechanisms can directly affect the mechanical reliability of systems. For instance, increased creep strain can alter the dimensional tolerances of components and the observed stress induced products can act as nucleation sites for fracture initiation and stress corrosion cracking. This work was supported by the National Science Foundation under grant number DMR-0102320.