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 Modeling the Influence of Phase Boundaries and Oxygen Interstitials on the Nucleation and Growth of Deformation Twins in the Alpha-Phase of Titanium Alloys(2015) Joost, William Joseph; Ankem, Sreeramamurthy; Kuklja, Maija M; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Twinning is an important deformation mechanism in many hexagonal close packed metals, including alpha-titanium alloys. However, the processes of twin nucleation, growth, and interaction with other defects are not well understood. Further, many aspects of deformation twinning are difficult to interrogate experimentally owing to the small time and length scales of the governing mechanisms. In this study we apply a combination of theoretical and computational materials science techniques, leveraged with experimental data, to quantify the effects of alpha-beta phase boundaries and oxygen interstitials on twin nucleation, twin growth, and ultimately mechanical behavior in titanium alloys. Combined results from finite element method and analytical dislocation modeling demonstrate that elastic and plastic interaction stresses across the interface between the alpha- and beta-phases are responsible for the experimentally observed anisotropy in the deformation behavior of dual-phase alloys. Interaction stresses also promote slip and twinning at up to 30% lower applied stress than predicted from Schmid's Law, significantly affecting performance in many applications. The complex interactions of phase boundaries, dislocations, and deformation twins modify the preferred deformation mechanism and promote twinning for some loading orientations. In order to quantify the interaction between oxygen interstitials and (10-12) twin boundaries, we employ atomistic simulations using a newly developed modified embedded atom method potential and density functional theory. Our investigation reveals that a twin boundary alters interstitial formation energy by as much as 0.5 eV while also stabilizing a tetrahedral interstitial, which is unstable in the bulk. Further, the activation barriers for diffusion in the region near a twin are uniformly lower than in the bulk; an atom diffusing across the twin boundary moves through several paths with peak activation barriers more than 0.3 eV lower than for comparable diffusion far from the twin. Despite accelerated kinetics, oxygen diffusion still occurs much more slowly than twin growth, suggesting that oxygen interstitials contribute to experimentally observed time-dependent twinning. Together, these results provide new insight while enabling predictive modeling and purposeful development of improved titanium alloys across a wide range of applications.Item The Effect of Phase Constitution and Morphology on Room Temperature Deformation Behavior of Binary Titanium Alloys(2011) Wyatt, Zane W.; Ankem, Sreeramamurthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Currently, titanium alloys are used in a variety of applications, including defense, aerospace, biomedicine, and even common consumer products such as bicycles and golf clubs. In many applications such as the landing gear of aircraft and geothermal energy production, titanium components may be subjected to stresses for extended periods of time. It has long been known that single-phase α (HCP), single-phase β (BCC), and two-phase α + β Ti alloys can creep at low temperatures (<0.25Tm). For this reason, creep is an important factor to consider when designing titanium alloys for various applications. The first part of this investigation is concerned with single-phase α-Ti alloys. It was found that the twin size (lamellar thickness) decreases with an increase in strain rate. This behavior is unexpected based on the classical understanding of instantaneous twinning. This investigation was able to for the first time demonstrate a time-dependent twinning phenomenon during high strain rate tensile deformation. The second part of this investigation is concerned with experimentally and theoretically studying low-temperature creep deformation behavior of two-phase α + β Ti alloys. Deformation mechanisms were seen in two-phase α + β Ti alloys that are not present during creep of the respective single-phase alloys with compositions equivalent to the individual phases. To investigate the possible interphase interaction stresses, 3D anisotropic Finite element modeling (FEM) was used. These simulations revealed that due to the Burgers orientation relationship between the two phases, deformation such as slip or twinning in the α phase can create very high additional shear stresses on different slip systems in the β phase. This work also revealed that the interfacial stresses that develop between the two phases during elastic deformation will often be much greater than the applied stress. These results were used to help explain the additional deformation mechanisms seen in two-phase alloys that are not seen in the respective single-phase alloys during creep. This work was supported by the National Science Foundation under Grant Number DMR-0906994.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.