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 MANUFACTURING TECHNIQUES FOR TITANIUM ALUMINIDE BASED ALLOYS AND METAL MATRIX COMPOSITES(2010) Kothari, Kunal B; Wereley, Norman M; Radhakrishnan, Ramachandran; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Dual phase titanium aluminides composed vastly of gamma phase (TiAl) with moderate amounts of alpha2 phase (Ti3Al) have been considered for several high temperature aerospace and automobile applications. High specific strength coupled with exceptional high temperature performance in the areas of creep and oxidation resistance makes titanium aluminides "materials of choice" for next generation propulsion systems. Titanium aluminides are primarily being considered as potential replacements for Ni-based superalloys in gas turbine engine components with the aim of developing more efficient and leaner engines with high thrust-to-weight ratio. As titanium aluminides lack room temperature ductility, traditional manufacturing techniques such as casting, forging and rolling are more expensive to perform. To overcome this, research over the past decade has examined powder metallurgy techniques such as hot-isostatic pressing, sintering and hot-pressing to produce titanium aluminides parts. Enhancements in these powder metallurgy techniques has produced near-net shape parts of titanium aluminides possessing a homogeneous and refined microstructure and thereby exhibiting better mechanical performance. This study presents a novel powder metallurgy approach to consolidate titanium aluminide powders. Traditional powder consolidation processes require exposure to high temperatures over a lengthy duration. This exposure leads to grain growth in the consolidated part which adversely affects its mechanical properties. A rapid consolidation process called Plasma Pressure Compaction (P2C) has been introduced and utilized to consolidate titanium aluminide powders to produce titanium aluminide parts with minimal grain growth. The research also explores the role of small alloying additions of Nb and Cr to enhance ductility of the consolidated parts. The grain size of the consolidated parts is further reduced in the sub-micrometer range by milling the as-received powders. Finally, a metal matrix composite with TiAl matrix reinforced with TiB was developed by first blending the matrix and the reinforcement powders and then consolidating the powder blend.Item Effect of Dynamic Flexural Loading on the Durability and Failure Site of Solder Interconnects in Printed Wiring Assemblies(2007-12-04) Varghese, Joseph; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation investigates the durability of solder interconnects of area array packages mounted on Printed Wiring Assemblies (PWAs) subjected to dynamic flexural loads, using a combination of testing, empirical curve fitting and mechanistic modeling. Dynamic 4-point bend tests are conducted on a drop tower and with an impact pendulum. Failure data is collected and an empirical rate-dependent durability model, based on mechanistic considerations, is developed to estimate the fatigue failure envelopes of the solder, as a function of solder strain and strain-rate. The solder plastic strain histories are obtained from the PWA flexural strain and strain rate, using transfer functions developed from 3D transient Finite Element Analysis (FEA) with rate-dependent solder material properties. The test data also shows the existence of multiple competing failure sites: solder, copper trace, PWB under solder pads, and layers of intermetallic compound (IMC) between the solder and solder pads. The failures in the IMC layers are found to be either in the bulk of the IMC layers or at the interface between different species of IMC layers. The dominant failure site is found to be strongly dependent on the loading conditions. The empirical model is demonstrated for solder failures as well as Cu trace failures, and the transition between their competing failure envelopes is identified. This dissertation then focuses in detail on two of these competing failure sites: (i) the solder and (ii) the interface between two IMC layers. A strain-range fatigue damage model, based on strain-rate hardening and exhaustion of ductility, is used to quantify the durability and estimate the fatigue constants of the solder for high strain rates of loading. Interfacial fracture mechanics is used to estimate the damage accumulation rates at the IMC interface. The IMC failure model and the solder failure model provide a mechanistic perspective on the failure site transitions. Durability metrics, based on the mechanics of these two failure mechanisms, are used to quantify the competing damage accumulation rates at the two failure sites for a given loading condition. The results not only identify which failure site dominates but also provide estimate of the durability of the solder interconnect. The test data shows good correlation with the model predictions. The test vehicles used in this study consist of PWAs with Sn37Pb solder interconnects. But the proposed test methodologies and mechanistic models are generic enough to be easily extended to other emerging lead free solder materials. Wherever possible, suggestions are provided for the development of test techniques or phenomenological models which can be used for engineering applications. A methodology is proposed in the appendix to implement the findings of this thesis in real-world applications. Damage in the solder interconnect is quantified in terms of generic empirical metrics, PWA flexural strain and strain rate. It is shown that the proposed metrics (PWA strain and strain rate) can quantify the durability of the solder interconnect, irrespective of the loading orientation or the PWA boundary conditions.