Browsing by Subject "Microstructure"
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Item Biopharmaceutical classification and development of limonene-based self-nanoemulsified capsule dosage form of coenzyme Q10(Texas Tech University, 2004-05) Palamakula, AnithaCoenzyme QIO (CoQ) is a challenging micronutrient for oral formulation due to its low aqueous solubility and bioavailability. The present dissertation deals with a systematic approach to classify CoQ biopharmaceutically according to FDA biopharmaceutical classification system (BCS), and to develop a self-nanoemulsified capsule dosage form (SNCDF) with chiral limonenes. We have hypothesized that the oral bioavailability of CoQ may be enhanced by limonene based SNCDF. In vitro transport studies using Caco-2 cells and solubility studies indicated that CoQ is moderately permeable and has low solubility. However, CoQ exhibits substantial solubility in limonenes. The permeability of CoQ across isolated rat GI segments revealed regional differences with maximum absorption through duodenum. Based on these results, a limonene based self-nanoemulsified formulation of CoQ was prepared and evaluated by in vitro and in vivo methods. Dissolution studies in water have shown CoQ release of > 90% within 5 minutes. Thermal analysis showed no significant change in CoQ endotherm. FT-IR and X-ray diffraction studies revealed the preservation of CoQ structure, indicating no interactions. Particle size, turbidity and zeta potential measurements have indicated that R-(+)-limonene provided superior self-nanoemulsified formulation of CoQ when compared with S-(-)-limonene. A three-factor, three-level optimization design was used to evaluate the effect of critical process variables on the drug release characteristics. Mathematical relationships, contour plots and response surface methodology were employed with constrained optimization to predict levels of factors that provide maximum drug release. The predicted and observed responses were in good agreement. The long term stability of the formulation was ascertained by subjecting to various temperature and humidity conditions for 6 months. The results indicated no significant effect on turbidity, particle size, zeta potential, DSC, FT-IR and total drug release at room temperature. The in vivo performance of CoQ limonene based SNCDF and eutectic based self-nanoemulsified drug delivery systems (SNEDDS) was evaluated by assessing the pharmacokinetic parameters, Tmax, Cmax, and AUC in rats. The oral bioavailability of SNCDF and SNEDDS was found to increase by 650% and 730% respectively when compared with CoQ powder (control). Preliminary assessment in human volunteers indicated increased tendency of rate and extent and metabolism of nanoemulsified preparations as compared to control.Item Characterization of carbon fibers: coefficient of thermal expansion and microstructure(Texas A&M University, 2006-04-12) Kulkarni, Raghav ShrikantThe focus of the research is to develop a consistent and repeatable method to evaluate the coefficient of thermal expansion (CTE) of carbon fibers at high temperatures. Accurate measurement of the CTE of carbon fibers is essential to understand and develop optimal processing procedures as well as computational simulations to predict properties and allowables for fiber-reinforced composites. The mismatch between the coefficient of thermal expansion of the fiber and the matrix has a profound impact on the development of residual stresses and the subsequent damage initiation and progression, potentially diminishing the performance of composite structures. In situ transmission electron microscopy (TEM) is selected to perform the experimental work on account of the high resolution and the capability of evaluating both the longitudinal and transverse CTE. The orthotropy in the CTE is tested by rotating the fibers through 45?? about their axis. The method is validated by testing standard tungsten filaments of known CTE. Additionally, the microstructure of the fibers is studied in a field emission scanning electron microscope as well as through selected area diffraction patterns in a TEM to observe presence of any potential orthotropy. The pitch based P55 fiber revealed a cylindrically orthotropic microstructure, but the PAN based IM7 and T1000 fibers did not reveal any orthotropy. Finite element models of hexagonally arranged IM7 fibers in a 977 epoxy matrix are developed using PATRAN and analyzed using the commercial FEA code ABAQUS 6.4. The fiber properties were considered temperature independent where as the matrix properties were varied linearly with temperature. The lamina properties evaluated from the finite element modeling are in agreement with the experimental results in literature within 10% in the temperature range of room temperature to the stress free temperature of the epoxy, however at cryogenic temperatures the difference is greater. The residual stresses developed during processing of the composite indicated a potential location for fiber matrix debonding to be in the matrix dominant regions.Item Effects of scaling on microstructure evolution of Cu nanolines and impact on electromigration reliability(2014-12) Cao, Linjun; Ho, P. S.; Im, Jang-Hi; Huang, Rui; Ferreira, Paulo J; Rabenberg, Llewellyn K; Gall, MartinScaling can significantly degrade the electromigration (EM) lifetime for Cu interconnects, raising serious reliability concerns. Different methods have emerged to enhance the EM resistance of Cu by suppressing the interface diffusion (the historically fastest diffusion path), notably using CoWP metal cap and Mn alloying. With further scaling of Cu interconnects, EM reliability becomes increasingly complex due to changes in Cu microstructure. In ultra-fine Cu lines a large population of small grains mix with bamboo-type grains, resulting in an additional contribution of grain boundary diffusion to EM degradation. With the interface diffusion suppressed by CoWP or Mn alloying, the grain structure effect becomes even more important. The objective of this study is to investigate the EM reliability of ultra-fine Cu interconnects, focusing on the scaling effect on grain structure and mass transport. First, the detailed microstructure information of Cu interconnects down to the 22 nm node was analyzed using a transmission electron microscope (TEM)-based high resolution diffraction technique. A dominant sidewall growth of {111} grains was observed for 70 nm Cu lines (45 nm node), reflecting the importance of interfacial energy in controlling grain growth. The strength of the {111} texture was found to significantly increase as line width was reduced to 40 nm (22 nm node), while the length fraction of coherent twin boundaries was reduced to ~1%. Secondly, the results from microstructure together with the deduced interfacial and grain boundary diffusivities were used to identify flux divergent sites for void formation and to analyze the grain structure effect on EM statistics using a microstructure-based kinetic model. Finally, based on the analysis of Cu grain structure evolution with downscaling, the scaling behavior of EM drift velocity was investigated for Cu interconnects with CoWP capping and Mn alloying. This enables us to project the EM lifetime and statistics for future technology nodes. The Mn alloying effect on mass transport in combination of grain structure control was found to provide an effective means to improve EM reliability especially with further scaling. In summary, this study establishes a correlation between the microstructure of Cu nanolines, void formation kinetics, and EM statistics.Item Effects of scaling on microstructure evolution of Cu nanolines and impact on electromigration reliability(2014-12) Cao, Linjun; Ho, P. S.; Im, Jang-Hi; Huang, Rui; Ferreira, Paulo J; Rabenberg, Llewellyn K; Gall, MartinScaling can significantly degrade the electromigration (EM) lifetime for Cu interconnects, raising serious reliability concerns. Different methods have emerged to enhance the EM resistance of Cu by suppressing the interface diffusion (the historically fastest diffusion path), notably using CoWP metal cap and Mn alloying. With further scaling of Cu interconnects, EM reliability becomes increasingly complex due to changes in Cu microstructure. In ultra-fine Cu lines a large population of small grains mix with bamboo-type grains, resulting in an additional contribution of grain boundary diffusion to EM degradation. With the interface diffusion suppressed by CoWP or Mn alloying, the grain structure effect becomes even more important. The objective of this study is to investigate the EM reliability of ultra-fine Cu interconnects, focusing on the scaling effect on grain structure and mass transport. First, the detailed microstructure information of Cu interconnects down to the 22 nm node was analyzed using a transmission electron microscope (TEM)-based high resolution diffraction technique. A dominant sidewall growth of {111} grains was observed for 70 nm Cu lines (45 nm node), reflecting the importance of interfacial energy in controlling grain growth. The strength of the {111} texture was found to significantly increase as line width was reduced to 40 nm (22 nm node), while the length fraction of coherent twin boundaries was reduced to ~1%. Secondly, the results from microstructure together with the deduced interfacial and grain boundary diffusivities were used to identify flux divergent sites for void formation and to analyze the grain structure effect on EM statistics using a microstructure-based kinetic model. Finally, based on the analysis of Cu grain structure evolution with downscaling, the scaling behavior of EM drift velocity was investigated for Cu interconnects with CoWP capping and Mn alloying. This enables us to project the EM lifetime and statistics for future technology nodes. The Mn alloying effect on mass transport in combination of grain structure control was found to provide an effective means to improve EM reliability especially with further scaling. In summary, this study establishes a correlation between the microstructure of Cu nanolines, void formation kinetics, and EM statistics.Item Enhanced Radiation Tolerance in Sputtered Cu/V Multilayers(2010-10-12) Fu, EngangHigh energy particle (neutron, proton and He ions) irradiation to materials typically leads to deteriorating properties, including void swelling, blistering, embrittlement, fracture and exfoliation of surfaces. This dissertation examines size dependent radiation damage in nanostructured metallic multilayers synthesized by the magnetron sputtering technique at room temperature. It reveals the roles of interface in achieving enhanced radiation tolerance in metallic materials. The microstructure and mechanical properties of as-deposited Cu/V multilayer films are systemically investigated, providing the basis for studying radiation damage mechanisms. Sputter-deposited Cu/V multilayers are subjected to helium (He) ion irradiation at room temperature with a peak dose of 6 displacements per atom (dpa). The average helium bubble density and lattice expansion induced by radiation decrease significantly with decreasing h, where h is individual layer thickness. The magnitude of radiation hardening decreases with decreasing h, and becomes negligible when h is 2.5 nm or less. The interactions between interfaces and radiation induced point defects and the evolution of microstructurs and mechanical behavior are discussed. This study indicates that nearly immiscible Cu/V interfaces spaced a few nm apart can effectively reduce the concentration of radiation induced point defects. Dose dependent radiation damage at room temperature in these Cu/V multilayers is systematically investigated with a peak dose in the range of 1-12 dpa. Peak bubble density increases with increasing dose, but it is much lower in Cu/V 2.5 nm multilayers than that in Cu/V 50 nm specimens. A similar radiation hardening trend is observed in multilayers irradiated at different fluences. Radiation hardening increases with dose and seems to reach saturation at a peak dose of 6 dpa. Negligible hardening for fine ( h less than/equal to 2.5 nm) multilayers is observed at all dose levels. Thermal stability of Cu/V multilayers is revealed by in situ annealing inside a transmission electron microscope. During isothermal annealing at 600 degrees C grain boundary grooving occurs across layer interfaces in Cu/V 50 nm specimens, whereas Cu/V 5 nm multilayers appear rather stable. Annealing of Cu/V multilayers at 400 degrees C leads to hardening of multilayers, whereas softening occurs in Cu/V multilayers annealed at 600 degrees C. The evolution of mechanical properties during annealing is correlated to the degradation of the layer interface and the consequent reduction of interface resistance to the transmission of single dislocation.Item Experimental study on transport phenomenon during deep fat frying of chicken nuggets(2011-05) Lalam, Sravan; Takhar, Pawan S.; Thompson, Leslie D.; Alvarado, Christine Z.Two important factors affecting oil uptake of food products during deep fat frying (DFF) are water content and pressure development. In the past frying studies, pressure has not been measured physically but was calculated using computer models, which has resulted in some disagreements in the literature about its magnitude. The present study tries to explain the complex mass transfer mechanisms (fat uptake and moisture loss) taking place during DFF with respect to real time pressure variations inside chicken nuggets. The first objective of this study was to measure the pressure changes inside nuggets during frying by using a fiber optics sensor (FISO Technologies Inc, Québec, Canada). The second objective was to perform fat and moisture analysis for methylcellulose (MC) and control nuggets at different frying times and temperatures. The third objective was to visualize the microstructure in the core, crust and cross-section of chicken nuggets using scanning electron microscopy. The fourth objective was to visualize the oil uptake by the nuggets by frying them for various times in oil containing a fat-soluble dye (Sudan Red) and then observing them under light microscope. Breaded chicken nuggets were made with and without 5% MC added to predust. All the frying experiments were performed at two temperatures (175ºC and 190ºF) for 0, 30, 60, 120 and 240 sec. The gauge pressure increased rapidly above the atmospheric pressure immediately after the nuggets were introduced into hot oil. This was expected due to sudden moisture flash-off. As the temperature of the nugget increased, the pressure inside the nugget decreased to negative values (suction). As the nugget was removed from the fryer after 240 sec (post-frying cooling phase), the pressure decreased further for another 2 to 3 min. The negative pressure values caused rapid absorption of surface oil. During the post-frying cooling phase, the pressure tends to reach an equilibrium negative value and then starts rising back to 0 bars (ambient pressure) in 2 to 3 hour. The highest value of pressure was 0.0018 bars and the lowest was -0.19 bars. The MC-coated nuggets had lower fat uptake and higher moisture retention when compared to control nuggets in the core and crust regions for both frying temperatures. From the scanning microscopic analysis, control nuggets had higher levels of randomness in the crust, core and meat layers in terms of microstructure development, surface texture, rigidity and pore sizes when compared to MC-coated nuggets. With an increase in frying temperature, the nuggets had more surface damage and increased complexity of microstructure for both treatment and control nuggets. The nuggets fried in dyed oil showed oil penetration only from 1 mm to 4 mm into the meat layer from the crust. This implied that the oil uptake in the frying process was a surface phenomenon when observed under the light microscope. The present results provided scarce of real time pressure variation data during DFF with respect to the simultaneous mass transfer processes taking place. This will aid in understanding and elucidating of the oil uptake mechanisms, oil distribution, microstructure development and other parameters needed for optimizing the frying process to obtain healthier, low fat fried foods.Item Flow behavior and microstructure of cement-based materials(2014-05) Han, Dongyeop; Ferron, Raissa D.; Fowler, David W.; Juenger, Maria G.; Zhu, Jinying; Ferraris, Chiara F.The flow behavior of concrete is highly impacted by the inherent structure of the paste matrix, which in turn is governed by the aggregation mechanisms within the paste matrix. Further, the mixing process is an essential process of cement-based materials preparation that influences the rheology of cement paste via the microstructure formation of cement paste. Due to the difficulty of measuring the rheology of concrete with aggregates, cement paste is used to represent the rheology of concrete. Based on literature in this area of research [1], it is known that a faster mixing speed is appropriate to simulate the condition of the cement paste inside of the concrete mixture during mixing. In 2011, the American Society for Testing and Materials (ASTM) published a new standard for high-shear mixing of hydraulic cement paste (ASTM C1738, “Standard Practice for High-shear Mixing of Hydraulic Cement Pastes,” 2011) to provide guidance for preparation of cement paste samples for rheological studies in hydraulic cement systems. Despite the improvements gained in the implementation of several hydraulic cement paste standards for mixing throughout the years, the relationship between the rheology and fresh state microstructure of cement paste with different mixing intensities—especially those with a very high mixing intensity range is not known yet. Overall, there is a lack of fundamental knowledge about the influence of the applied mixing forces on the internal structure of cement paste, the role of the microstructure on the rheological behavior, and microstructure formation mechanisms on rheological behavior. The objectives of this research are to (1) evaluate the influences of sample preparation on the rheology of cement paste, (2) analyze the influence of the mixing intensity on rheological behaviors, (3) characterize the microstructure of fresh state cement paste (4) understand the mechanisms of determining cement agglomerate size and identify the relationship between the microstructure and rheology of fresh state cement paste. In order to accomplish these objectives, rheology studies on cement pastes mixed with different mixing intensities were conducted. Based on the rheology studies with a high mixing intensity, it was found that increasing the mixing intensity does not always result in a reduction in the rheological properties. Rather interesting results can occur when a high-range water reducer is incorporated, and possible explanations for this unexpected behavior are presented. To prove the reasons for this unexpected result, two hypotheses were proposed: (1) If cement paste has increased ionic concentration under the high mixing intensity conditions, then the cement flocs are aggregated (flocculated), and thus these aggregated cement flocs likely contribute to the increased apparent viscosity. (2) If the polycrboxylate-based high-range water reducer (HRWR) produces unexpected air bubbles under the high mixing intensity conditions, then those air bubbles will have an influence on increasing apparent viscosity. To prove these hypotheses, a series of experiments were conducted, and based on the results of these experiments microstructural formation mechanisms were suggested to explain the unexpected flocculation under the high mixing intensity, and the numerical relationship was investigated between the microstructure and rheology properties of fresh state cement paste. In this research, the relationship between microstructural change and rheological behaviors of fresh state cement paste were investigated and a better understanding of the mechanisms of microstructural formation with various mixing intensity conditions for cement-based materials was obtained.Item Laser/microstructure interaction and ultrafast heat transfer(2006) Heltzel, Alexander John; Howell, John R.The industrial demand for smaller structures required for the manufacture of quantum devices, high-density recording media, etc., have resulted in the need for fabrication technology at the nanometer scale. However, most lithography and milling techniques are limited either by their inability for large-area fabrication or by the diffraction limit and in most cases the high manufacturing costs. To overcome the diffraction limit and to spatially-control matter on a nanometer scale, near-field optics techniques have been employed. It has been shown that laser/microstructure interaction can create surface modifications below the diffraction limit in both localized and parallel fashions. This dissertation investigates laser/microstructure interaction using both numerical and analytical tools for computation. Two fundamental problems required for predictive optical nanolithography are addressed: the electrodynamic response of the laser energy in the vicinity of micro/nanostructures, and the resulting energy transport through the target material. The dissertation first concentrates on the interaction between lasers and dielectric microspheres. Analytical solutions provided by near-field Mie theory and numerical solutions to Maxwell’s equations are obtained. Three-dimensional electromagnetic fields are resolved in the near-field of these spheres and functional dependencies on several experimental parameters are uncovered. Energy transport through the substrate is modeled numerically in both two and three dimensions using conventional conduction formulae and ultrafast electron density evolution for the case of femtosecond pulse irradiation. The combined electrodynamic/heat transfer solutions generate final lithographic predictions which are compared to experimental characterizations. The final segment of this dissertation investigates the interaction between lasers and gold, silver, and carbon nanotube structures for the purposes of optical lithography and photonic signal propagation. Fundamentals of surface plasmons, coupled electron/photon waves at conductor/dielectric interfaces, are explored computationally. Practical lithographic and photonic applications are optimized theoretically in an effort to advance knowledge in this area.Item Microstructural Characterization and Shape Memory Response of Ni-Rich NiTiHf and NiTiZr High Temperature Shape Memory Alloys(2014-08-14) Evirgen, AlperNiTiHf and NiTiZr high temperature shape memory alloys (HTSMAs) have drawn a great deal of attention as cheaper alternatives to Pt, Pd and Au alloyed NiTi-based HTSMAs while NiTiZr alloys also providing at least 20% weight reduction then its NiTiHf counterparts with the same stoichiometry. (Ti + Hf/Zr)-rich compositions were already reported to have high thermal hysteresis, poor dimensional and thermal stability due to their low matrix strength hampering their practical applications. However, Ni-rich compositions of NiTiHf alloys were shown to have very promising shape memory responses recently due to generation of fine Ni-rich particles after proper heat treatments not only strengthening the matrix but also leading to relatively high transformation temperatures. Comparable studies have not been performed on Ni-rich NiTiZr compositions. Furthermore, very few published work are present on these new Ni-rich NiTiHf and NiTiZr systems. Hence many critical characteristics still remains unknown and further investigation is necessary to reveal the effect of precipitation on the microstructures and its subsequent effect on the transformation characteristics and shape memory responses. The present study focuses on the extensive microstructural and thermo-mechanical property characterizations of the Ni-rich NiTiHf and NiTiZr HTSMAs in order to develop the fundamental knowledge necessary for the optimization and development of reliable, cheap, lightweight HTSMAs operating up to 300 ?C with improved thermal and dimensional stability. Several different compositions of Ni-rich NiTiHf and NiTiZr HTSMAs are systematically precipitation heat treated for the microstructural control and then subjected to multi-scale microstructural and thermo-mechanical characterizations to achieve this goal. Differential scanning calorimetry measurements are conducted on the aged samples to reveal the transformation characteristics and furthermore generate the time-temperature-transformation temperature (TTT) diagrams of the individual alloy systems. The shape memory response and characteristics of the alloys are investigated through load-biased thermal cycling and superelasticity tests. The microstructures of the aged samples are extensively characterized using transmission electron microscopy (TEM) to build up microstructure-property relationships as well as providing deeper understanding of precipitate crystal structure, composition and morphology. Such an experimental approach is crucial for the development of new ternary alloy compositions and for the careful control of the microstructure to obtain desired properties. The outcomes of the present study is expected to help to reveal the potential of these alloys to be utilized in a wide range of applications at elevated temperatures in aerospace, automotive and oil-gas industries.Item Microstructure and processing effects on stress and reliability for through-silicon vias (TSVs) in 3D integrated circuits(2015-05) Jiang, Tengfei; Ho, Paul S.; Huang, Rui; Im, Jang-Hi; Shi, Li; Zhao, Jie-HuaCopper (Cu) Through-silicon via (TSV) is a key enabling element that provides the vertical connection between stacked dies in three-dimensional (3D) integration. The thermal expansion mismatch between Cu and Si induces complex stresses in and around the TSV structures, which can degrade the performance and reliability of 3DICs and are key concerns for technology development. In this dissertation, the effects of Cu microstructure and processing conditions on the stress characteristics and reliability of the TSV structure are studied. First, the stress characteristics of Cu TSV structures are investigated using the substrate curvature method. The substrate curvature measurement was supplemented by microstructure and finite element analyses (FEA) to investigate the mechanisms for the linear and nonlinear stress-temperature behaviors observed for the TSV structure. Implications of the near surface stress on carrier mobility change and device keep-out zone (KOZ) are discussed. Second, via extrusion, an important yield and reliability issue for 3D integration, is analyzed. Synchrotron x-ray microdiffraction technique was introduced for direct measurements of local stress and material behaviors in and around the TSV. Local plasticity near the top of the via was observed which provided direct experimental evidence to support the plasticity mechanism of via extrusion. An analytical model and FEA were used to analyze via extrusion based on local plasticity. Next, the effect of Cu microstructure effect on the thermomechanical behaviors of TSVs is investigated. The contribution from grain boundary and interfacial diffusion on via extrusion and the relaxation mechanisms are discussed. Potential approaches to minimize via extrusion are proposed. Finally, the stress characteristics of 3D die stack structures are studied using synchrotron x-ray microdiffraction. High resolution stress mappings were performed and verified by finite element analysis (FEA). FEA was further developed to estimate the stress effect on device mobility changes and the warpage of the integrated structure.Item Microstructure and properties of copper thin films on silicon substrates(2009-05-15) Jain, Vibhor VinodkumarCopper has become the metal of choice for metallization, owing to its high electrical and thermal conductivity, relatively higher melting temperature and correspondingly lower rate of diffusivity. Most of the current studies can get high strength copper thin films but on an expense of conductivity. This study proposes a technique to deposit high strength and high conductivity copper thin films on different silicon substrates at room temperature. Single crystal Cu (100) and Cu (111) have been grown on Si (100) and Si (110) substrates, respectively. Single crystal Cu (111) films have a high density of growth twins, oriented parallel to the substrate surface due to low twin boundary energy and a high deposition rate. The yield strengths of these twinned Cu films are much higher than that of bulk copper, with an electrical resistivity value close to that of bulk copper. X-ray diffraction, transmission electron microscopy and nanoindentation techniques were used to show that high density twins are sole reason for the increase in hardness of these thin films. The formation of growth twins and their roles in enhancing the mechanical strength of Cu films while maintaining low resistivity are discussed.Item Microstructure and rheology of soft particle glasses(2013-12) Mohan, Lavanya; Bonnecaze, R. T. (Roger T.)Soft particle glasses like microgels and compressed emulsions are densely packed, disordered suspensions of deformable particles. Quantitative relationships among the constituent properties and the macroscopic properties of the suspension are determined for their customized design as rheological additives. The microscopic origin of their macroscopic properties is also determined. Advanced characterization techniques like Large Amplitude Oscillatory Shear (LAOS) and microrheology are studied to use them efficiently to characterize these materials. Their microstructure and rheology are investigated through theory, simulations and experiments. Soft particle glasses are used as rheological additives in many applications including coatings, solid inks and textured food and cosmetic products but their formulation is largely empirical. A quantitative connection between their formulation and rheology is critical to enable their rational design. Their microstructure will lead to the microscopic origin of some unique properties in common with other soft crowded materials like intracellular cytoplasm and clays. These are complex fluids and require novel techniques to characterize them. A study of these techniques is essential to efficiently interpret the observations in terms of their macroscopic properties and the microscopic dynamics involved. Particle scale simulations of steady and oscillatory shear flow are developed to predict the nonlinear rheology and microstructure of these glasses. The origin of yielding is determined as escape of particles from their cages giving rise to a shear induced diffusion. Microrheology is studied by developing simulations of a probe particle being pulled at a constant force and the rheological information from microrheology is quantitatively connected to that from bulk rheological measurements. Soft particle glasses develop internal stresses when quenched to a solid state by flow cessation during processing. Experiments are performed to characterize and a priori predict these stresses. Simulations are used to determine the particle scale mechanisms involved in the stress relaxation on flow cessation and the microstructural origin of internal stresses. A pairwise interaction theory is developed for quiescent glasses to quantitatively predict their microstructure and elastic properties. The theory is then extended to sheared glasses to quantitatively predict their nonlinear rheology. The implementation of the pairwise theories is computationally much faster than the full three-dimensional simulations.Item On the role of microstructure in ductile failure(2011-08) Ghahremaninezhad Mianji, Ali; Ravi-Chandar, K.; Mear, Mark E.; Liechti, Kenneth M.; Huang, Rui; Benzerga, AmineFailure in structural materials occurs initially by localization of deformation, and subsequently through a process of nucleation, growth and coalescence of voids. Predicting material failure requires a careful investigation of the different stages of damage evolution at the multiple scales. The main objective of this thesis is to explore the evolution of damage and to correlate this with the deformation of the material at the continuum and microstructural levels. This is accomplished through macroscopic measurements of strain evolution using digital image correlation and microscale measurements of strain and damage using optical and scanning electron microscopy. Three materials with different microstructure were examined. In oxygen-free, high-conductivity copper, a high-purity material without appreciable second phase particles, strain levels in the order of three were observed in the material without any trace of damage. Failure was observed to be triggered by plastic instability in the form of shear bands and the emergence of a prismatic cavity that grows in a self-similar fashion by an alternating slip mechanism. In Al 6061-T6, a material with a dispersion of second phase particles at a volume fraction of about 0.01, nucleation of damage does not appear until plastic strain levels of 0.5 to 1.0. Once damage in the form of particle fracture or decohesion at the interface initiates, subsequent failure follows by the void nucleation, growth and coalescence; but, dominated by the fluctuations in the distribution of second phase particles, final separation occurs in a highly localized layer of material on the order of the grain size, corresponding to a small increase in the overall strain. In nodular cast iron, a material with an initial porosity of about 0.10, growth of voids was observed initially, but this was terminated by a transition of the deformation into a localized region. Phenomenological models based on strain-to-failure and micromechanical models based on a mechanistic description of the microscale deformation are evaluated in light of the above examination of failure in these three classes of materials.Item Patterning and microstructure of penguin plumage(2014-08) Kulp, Felicia Briana; Clarke, Julia A.; Bell, Christopher J; Shawkey, Matthew DPenguins (Sphenisciformes) exhibit an array of derived feather features. The characters describing penguin integument that are used in the phylogenetic reconstruction have not been reassessed since they were written in 2005. I reassessed all integument characters for extant penguins and outgroup taxa. The phylogenetic tree constructed using the reassessed integument characters does not differ in topology from the original phylogenetic tree except that several outgroup relationships become less resolved. This indicates that conclusions drawn by previous authors about the relationships among extant penguins using the original tree are still valid. However, the reconstruction of the integument of the common ancestor of Spheniscidae no longer remains the same. Caution should be exercised when using museum bird skins to score integument characters because these colors can change over time, especially in the bill and legs. In addition to examining macro characteristics of penguin feathers, I also examined the microstructure of Gentoo Penguin (Pygoscelis papua) feathers in order to assess the presence of nanofibers, which had thus far only been found in the Little Penguin (Eudyptula minor). Nanofibers create the structural blue color in the dorsal feathers in the Little Penguin. I discovered nanofibers in all pigmented feathers in the Gentoo Penguin. The nanofibers in black parts of the feathers are overprinted by melanosomes. An amorphous keratin matrix exists in the white breast feathers, creating white structural color, but nanofibers are absent. My data suggest that penguin integument is even more modified relative to other birds than previously thought. Implications for penguin color patterning are presented concerning countershading and intraspecific signaling. The data presented in my study raise new questions about the origin and potential functions of penguin plumage structure and coloration.Item Phase-field Models for Solidification and Solid/Liquid Interactions(2011-02-22) Park, Min SooThe microstructure resulting from the solidification of alloys can greatly affect their properties, making the prediction of solidification phenomena under arbitrary conditions a very important tool in the field of computer-aided design of materials. Although considerable attention has been allocated to the understanding of this phenomenon in cases in which the solidification front advances freely into the liquid, the actual microstructure of solidification is strongly dependent of interfacial interactions. Over the past decade, the phase-field approach has been proved to be a quite effective tool for the simulation of solidification processes. In phase-field models, one or more phase fields ? (conserved and/or non-conserved) are introduced to describe the microstructure of a complex system. The behavior of a given microstructure over time is then simulated by solving evolution equations written in terms of the minimization of the free energy of the entire system, which is written as a functional of the field variables as well as their gradients and materials? constitutive equations. With the given free energy functional, the governing equations (phase-field equation, diffusion equation, heat equation and so on) are solved throughout the entire space domain without having to track each of the interfaces formed or abrupt changes in the topology of the microstructure. In this work I will present phase-field models for solidification processes, solid/liquid interactions as well as their applications.Item Quantitative characterization of microstructure of asphalt mixtures to evaluate fatigue crack growth(2012-05) Izadi, Anoosha; Bhasin, Amit; Smit, AndreStudies show that the microstructure of the fine aggregate matrix has a significant influence on the mechanical properties and evolution of damage in an asphalt mixture. However, very little work has been done to quantitatively characterize the microstructure of the asphalt binder within the fine aggregate matrix of asphalt mixtures. The first objective of this study was to quantitatively characterize the three dimensional microstructure of the asphalt binder within the fine aggregate matrix (FAM) of an asphalt mixture and compare the influence of binder content, coarse aggregate gradation, and fine aggregate gradation on this microstructure. Studies indicate that gradation of the fine aggregate has the most influence of the degree of anisotropy whereas gradation of the coarse aggregate has the most influence on the direction anisotropy of the asphalt mastic within the fine aggregate matrix. Addition of asphalt binder or adjustments to the fine aggregate gradation also resulted in a more uniform distribution of the asphalt mastic within the fine aggregate matrix. The second objective of this study was to compare the internal microstructure of the mortar within a full-scale asphalt mixture to the internal microstructure of the FAM specimen and also conduct a limited evaluation of the influence of mixture properties and methods of compaction on the engineering properties of the FAM specimens. Fatigue cracking is a significant form of pavement distress in flexible pavements. The properties of the sand-asphalt mortars or FAM can be used to characterize the evolution of fatigue crack growth and self-healing in full-scale asphalt mixtures. The results from this study, although limited in number, indicate that in most cases the SGC (Superpave Gyratory Compactor) compacted FAM specimen had a microstructure that most closely resembled the microstructure of the mortar within a full-scale asphalt mixture. Another finding from this study was that, at a given level of damage, the healing characteristic of the three different types of FAM mixes evaluated was not significantly different. This indicates that the healing rate is mostly dictated by the type of binder and not significantly influenced by the gradation or binder content, as long as the volumetric distribution of the mastic was the same.Item Severe plastic deformation of difficult-to-work alloys(Texas A&M University, 2004-09-30) Yapici, Guney GuvenThe present work aims to reveal the microstructural evolution and post-processing mechanical behavior of difficult-to-work alloys upon severe plastic deformation. Severe plastic deformation is applied using equal channel angular extrusion (ECAE) where billets are pressed through a 90o corner die achieving simple shear deformation. Three different materials are studied in this research, namely Ti-6Al-4V, Ti-6Al-4V reinforced with 10% TiC and AISI 316L stainless steel. Microstructure and mechanical properties of successfully extruded billets were reported using light microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), tension and compression experiments and microhardness measurements. The effects of extrusion conditions (temperature and processing route) on the microstructure and mechanical properties are investigated. The underlying mechanisms responsible for observed mechanical behaviors are explored. It is seen that ECAE shear deformation leads to refinement in ? plates and elimination of prior ? boundaries in Ti-6Al-4V. Decreasing extrusion temperature and increasing number of passes decreases ? plate size and grain size. Refined ? grain size leads to a significant increase in tensile and compressive flow stresses at room temperature. Texture produced by ECAE has a pronounced effect on mechanical properties. Specifically it leads to tension/compression asymmetry in flow strengths and strain hardening coefficients may be described by the activation of differing slip systems under tension and compression loading. ECAE of Ti-6Al-4V+10%TiC samples also improved mechanical properties due to ? plate size refinement. Nevertheless, further extrusion passes should be carried out for tailoring reinforcement size and distribution providing optimum strength and ductility. ECAE deformation of AISI 316L stainless steel at high homologous temperatures (0.55 to 0.60 Tm) results in deformation twinning as an effective deformation mechanism which is attributed to the effect of the high stress levels on the partial dislocation separation. Deformation twinning gives rise to high stress levels during post-processing room temperature tension and compression experiments by providing additional barriers to dislocation motion and decreasing the mean free path of dislocations. The highest tensile flow stress observed in the sample processed at 700 oC following one pass route A was on the order of 1200 MPa which is very high for 316L stainless steel. The ultimate goal of this study is to produce stabilized end microstructures with improved mechanical properties and demonstrate the applicability of ECAE on difficult-to-work alloys.