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 Department: Department of Materials Science and Engineering
 Collection: UNT Theses and Dissertations
Mechanics and Mechanisms of Creep and Ductile Fracture

Mechanics and Mechanisms of Creep and Ductile Fracture

Date: August 2013
Creator: Srivastava, Ankit
Description: The main aim of this dissertation is to relate measurable and hopefully controllable features of a material's microstructure to its observed failure modes to provide a basis for designing better materials. The understanding of creep in materials used at high temperatures is of prime engineering importance. Single crystal Ni-based superalloys used in turbine aerofoils of jet engines are exposed to long dwell times at very high temperatures. In contrast to current theories, creep tests on Ni-based superalloy specimens have shown size dependent creep response termed as the thickness debit effect. To investigate the mechanism of the thickness debit effect, isothermal creep tests were performed on uncoated Ni-based single crystal superalloy sheet specimens with two thicknesses and under two test conditions: a low temperature high stress condition and a high temperature low stress condition. At the high temperature, surface oxidation induced microstructural changes near the free surface forming a layered microstructure. Finite element calculations showed that this layered microstructure gave rise to local changes in the stress state. The specimens also contained nonuniform distribution of initial voids formed during the solidification and homogenization processes. The experiments showed that porosity evolution could play a significant role in the thickness debit effect. This ...
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Mechanisms of Ordered Gamma Prime Precipitation in Nickel Base Superalloys

Mechanisms of Ordered Gamma Prime Precipitation in Nickel Base Superalloys

Date: May 2011
Creator: Singh, Antariksh Rao Pratap
Description: Commercial superalloys like Rene88DT are used in high temperature applications like turbine disk in aircraft jet engines due to their excellent high temperature properties, including strength, ductility, improved fracture toughness, fatigue resistance, enhanced creep and oxidation resistance. Typically this alloy's microstructure has L12-ordered precipitates dispersed in disordered face-centered cubic γ matrix. A typical industrially relevant heat-treatment often leads to the formation of multiple size ranges of γ¢ precipitates presumably arising from multiple nucleation bursts during the continuous cooling process. The morphology and distribution of these γ′ precipitates inside γ matrix influences the mechanical properties of these materials. Therefore, the study of thermodynamic and kinetic factors influencing the evolution of these precipitates and subsequent effects is both relevant for commercial applications as well as for a fundamental understanding of the underlying phase transformations. The present research is primarily focused on understanding the mechanism of formation of different generations of γ′ precipitates during continuous cooling by coupling scanning electron microscopy (SEM), energy filtered TEM and atom probe tomography (APT). In addition, the phase transformations leading to nucleation of γ′ phase has been a topic of controversy for decades. The present work, for the first time, gives a novel insight into the mechanism ...
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Micro and nano composites composed of a polymer matrix and a metal disperse phase.

Micro and nano composites composed of a polymer matrix and a metal disperse phase.

Date: December 2007
Creator: Olea Mejia, Oscar Fernando
Description: Low density polyethylene (LDPE) and Hytrel (a thermoplastic elastomer) were used as polymeric matrices in polymer + metal composites. The concentration of micrometric (Al, Ag and Ni) as well as nanometric particles (Al and Ag) was varied from 0 to 10 %. Composites were prepared by blending followed by injection molding. The resulting samples were analyzed by scanning electron microscopy (SEM) and focused ion beam (FIB) in order to determine their microstructure. Certain mechanical properties of the composites were also determined. Static and dynamic friction was measured. The scratch resistance of the specimens was determined. A study of the wear mechanisms in the samples was performed. The Al micro- and nanoparticles as well as Ni microparticles are well dispersed throughout the material while Ag micro and nanoparticles tend to form agglomerates. Generally the presence of microcomposites affects negatively the mechanical properties. For the nanoparticles, composites with a higher elastic modulus than that of the neat materials are achievable. For both micro- and nanocomposites it is feasible to lower the friction values with respective to the neat polymers. The addition of metal particles to polymers also improves the scratch resistance of the composites, particularly so for microcomposites. The inclusion of Ag ...
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Microstructure Evolution in Laser Deposited Nickel-Titanium-Carbon in situ Metal Matrix Composite

Microstructure Evolution in Laser Deposited Nickel-Titanium-Carbon in situ Metal Matrix Composite

Date: December 2010
Creator: Gopagoni, Sundeep
Description: Ni/TiC metal matrix composites have been processed using the laser engineered net shaping (LENS) process. As nickel does not form an equilibrium carbide phase, addition of a strong carbide former in the form of titanium reinforces the nickel matrix resulting in a promising hybrid material for both surface engineering as well as high temperature structural applications. Changing the relative amounts of titanium and carbon in the nickel matrix, relatively low volume fraction of refined homogeneously distributed carbide precipitates, formation of in-situ carbide precipitates and the microstructural changes are investigated. The composites have been characterized in detail using x-ray diffraction, scanning electron microscopy (including energy dispersive spectroscopy (XEDS) mapping and electron backscatter diffraction (EBSD)), Auger electron spectroscopy, and transmission (including high resolution) electron microscopy. Both primary and eutectic titanium carbides, observed in this composite, exhibited the fcc-TiC structure (NaCl-type). Details of the orientation relationship between Ni and TiC have been studied using SEM-EBSD and high resolution TEM. The results of micro-hardness and tribology tests indicate that these composites have a relatively high hardness and a steady-state friction coefficient of ~0.5, both of which are improvements in comparison to LENS deposited pure Ni.
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Mist and Microstructure Characterization in End Milling Aisi 1018 Steel Using Microlubrication

Mist and Microstructure Characterization in End Milling Aisi 1018 Steel Using Microlubrication

Date: August 2013
Creator: Shaikh, Vasim
Description: Flood cooling is primarily used to cool and lubricate the cutting tool and workpiece interface during a machining process. But the adverse health effects caused by the use of flood coolants are drawing manufacturers' attention to develop methods for controlling occupational exposure to cutting fluids. Microlubrication serves as an alternative to flood cooling by reducing the volume of cutting fluid used in the machining process. Microlubrication minimizes the exposure of metal working fluids to the machining operators leading to an economical, safer and healthy workplace environment. In this dissertation, a vegetable based lubricant is used to conduct mist, microstructure and wear analyses during end milling AISI 1018 steel using microlubrication. A two-flute solid carbide cutting tool was used with varying cutting speed and feed rate levels with a constant depth of cut. A full factorial experiment with Multivariate Analysis of Variance (MANOVA) was conducted and regression models were generated along with parameter optimization for the flank wear, aerosol mass concentration and the aerosol particle size. MANOVA indicated that the speed and feed variables main effects are significant, but the interaction of (speed*feed) was not significant at 95% confidence level. The model was able to predict 69.44%, 68.06% and 42.90% of ...
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Modifications of epoxy resins for improved mechanical and tribological performances and their effects on curing kinetics.

Modifications of epoxy resins for improved mechanical and tribological performances and their effects on curing kinetics.

Date: May 2008
Creator: Chonkaew, Wunpen
Description: A commercial epoxy, diglycidyl ether of bisphenol-A, was modified by two different routes. One was the addition of silica to produce epoxy composites. Three different silane coupling agents, glycidyloxypropyl trimethoxy silane (GPS), -methacryloxypropyl trimethoxy silane (MAMS) and 3-mercaptopropyltriethoxy silane (MPS), were used as silica-surface modifiers. The effects of silica content, together with the effects of chemical surface treatment of silica, were studied. The results indicate that epoxy composites with silica exhibit mechanical and tribological properties as well as curing kinetics different than the pure epoxy. The optimum silica content for improved mechanical and tribological properties (low friction coefficient and wear rate) was different for each type of silane coupling agent. An unequivocal correlation between good mechanical and improved tribological properties was not found. Activation energy of overall reactions was affected by the addition of silica modified with MAMS and MPS, but not with GPS. The second route was modification by fluorination. A new fluoro-epoxy oligomer was synthesized and incorporated into a commercial epoxy by a conventional blending method. The oligomer functioned as a catalyst in the curing of epoxy and polyamine. Thermal stability of the blends decreased slightly at a high oligomer content. Higher wear resistance, lower friction coefficient and ...
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Modified epoxy coatings on mild steel: A study of tribology and surface energy.

Modified epoxy coatings on mild steel: A study of tribology and surface energy.

Date: August 2009
Creator: Dutta, Madhuri
Description: A commercial epoxy was modified by adding fluorinated poly (aryl ether ketone) and in turn metal micro powders (Ni, Al, Zn, and Ag) and coated on mild steel. Two curing agents were used; triethylenetetramine (curing temperatures: 30 oC and 70 oC) and hexamethylenediamine (curing temperature: 80 oC). Variation in tribological properties (dynamic friction and wear) and surface energies with varying metal powders and curing agents was evaluated. When cured at 30 oC, friction and wear decreased significantly due to phase separation reaction being favored but increased when cured at 70 oC and 80 oC due to cross linking reaction being favored. There was a significant decrease in surface energies with the addition of modifiers.
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Molecular Dynamics Simulations of the Structures of Europium Containing Silicate and Cerium Containing Aluminophosphate Glasses

Molecular Dynamics Simulations of the Structures of Europium Containing Silicate and Cerium Containing Aluminophosphate Glasses

Date: August 2012
Creator: Kokou, Leopold Lambert Yaovi
Description: Rare earth ion doped glasses find applications in optical and photonic devices such as optical windows, laser, and optical amplifiers, and as model systems for immobilization of nuclear waste. Macroscopic properties of these materials, such as luminescence efficiency and phase stability, depend strongly on the atomic structure of these glasses. In this thesis, I have studied the atomic level structure of rare earth doped silicate and aluminophosphate glasses by using molecular dynamics simulations. Extensive comparisons with experimental diffraction and NMR data were made to validate the structure models. Insights on the local environments of rare earth ions and their clustering behaviors and their dependence on glass compositions have been obtained. In this thesis, MD simulations have been used to investigate the structure of Eu2O3-doped silica and sodium silicate glasses to understand the glass composition effect on the rare earth ions local environment and their clustering behaviors in the glass matrix, for compositions with low rare earth oxide concentration (~1mol%). It was found that Eu–O distances and coordination numbers were different in silica (2.19-2.22 Å and 4.6-4.8) from those in sodium silicate (2.32 Å and 5.8). High tendencies of Eu clustering and short Eu-Eu distances in the range 3.40-3.90 Å were ...
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Morphological properties of poly (ethylene terephthalate) (PET) nanocomposites in relation to fracture toughness.

Morphological properties of poly (ethylene terephthalate) (PET) nanocomposites in relation to fracture toughness.

Date: August 2005
Creator: Pendse, Siddhi
Description: The effect of incorporation of montmorillonite layered silicate (MLS) on poly (ethylene terephthalate) (PET) matrix was investigated. MLS was added in varying concentration of 1 to 5 weight percent in the PET matrix. DSC and polarized optical microscopy were used to determine the crystallization effects of MLS addition. Non isothermal crystallization kinetics showed that the melting temperature and crystallization temperature decrease as the MLS percent increases. This delayed crystallization along with the irregular spherulitic shape indicates hindered crystallization in the presence of MLS platelets. The influence of this morphology was related with the fracture toughness of PET nanocomposites using essential work of fracture coupled with the infra red (IR) thermography. Both the essential as well as non essential work of fracture decreased on addition of MLS with nanocomposite showing reduced toughness.
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Nano-crystallization Inhibition in 5 Nm Ru Film Diffusion Barriers for Advanced Cu-interconnect

Nano-crystallization Inhibition in 5 Nm Ru Film Diffusion Barriers for Advanced Cu-interconnect

Date: December 2013
Creator: Sharma, Bed P.
Description: As the semiconductor industries are moving beyond 22 nm node technology, the currently used stacked Ta/TaN diffusion barrier including a copper seed will be unable to fulfill the requirements for the future technologies. Due to its low resistivity and ability to perform galvanic copper fill without a seed layer, ruthenium (Ru) has emerged as a potential copper diffusion barrier. However, its crystallization and columnar nanostructure have been the main cause of barrier failures even at low processing temperatures (300 oC -350 oC). In this study, we have proposed and evaluated three different strategies to improve the performance of the ultrathin Ru film as a diffusion barrier for copper. The first study focused on shallow surface plasma irradiation/amorphization and nitridation of 5 nm Ru films. Systematic studies of amorphization and nitrogen incorporation versus sample bias were performed. XPS, XRD and RBS were used to determine the physico-chemical, crystallization and barrier efficiency of the plasma modified Ru barrier. The nitrogen plasma surface irradiation of Ru films at substrate bias voltage of -350 V showed an improved barrier performance up to 400 oC annealing temperatures. The barrier barely started failing at 450 oC due mainly to nitrogen instability. The second study involved only ...
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