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Surface Modifications to Enhance the Wear Resistance and the Osseo-integration Properties of Biomedical Ti-alloy

Description: The current study focuses on improving the wear resistance of femoral head component and enhancing the osseo-integration properties of femoral stem component of a hip implant made of a new generation low modulus alloy, Ti-35Nb-7Zr-5Ta or TNZT. Different techniques that were adopted to improve the wear resistance of low-modulus TNZT alloy included; (a) fabrication of graded TNZT-xB (x= 0, 1, 2 wt%) samples using LENS, (b) oxidation, and (c) LASER nitriding of TNZT. TNZT-1B and TNZT-O samples have shown improved wear resistance when tested against UHMWPE ball in SBF medium. A new class of bio-ceramic coatings based on calcium phosphate (CaP), was applied on the TNZT sample surface and was further laser processed with the objective of enhancing their osseo-integration properties. With optimized LASER parameters, TNZT-CaP samples have shown improved corrosion resistance, surface wettability and cellular response when compared to the base TNZT sample.
Date: August 2013
Creator: Kami, Pavani
Partner: UNT Libraries

Corrosion Protection of Aerospace Grade Magnesium Alloy Elektron 43™ for Use in Aircraft Cabin Interiors

Description: Magnesium alloys exhibit desirable properties for use in transportation technology. In particular, the low density and high specific strength of these alloys is of interest to the aerospace community. However, the concerns of flammability and susceptibility to corrosion have limited the use of magnesium alloys within the aircraft cabin. This work studies a magnesium alloy containing rare earth elements designed to increase resistance to ignition while lowering rate of corrosion. The microstructure of the alloy was documented using scanning electron microscopy. Specimens underwent salt spray testing and the corrosion products were examined using energy dispersive spectroscopy.
Date: August 2013
Creator: Baillio, Sarah S.
Partner: UNT Libraries

Dynamic Precipitation of Second Phase Under Deformed Condition in Mg-nd Based Alloy

Description: Magnesium alloys are the lightweight structural materials with high strength to weigh ratio that permits their application in fuel economy sensitive automobile industries. Among the several flavors of of Mg-alloys, precipitation hardenable Mg-rare earth (RE) based alloys have shown good potential due to their favorable creep resistance within a wide window of operating temperatures ranging from 150°C to 300°C. A key aspect of Mg-RE alloys is the presence of precipitate phases that leads to strengthening of such alloys. Several notable works, in literature, have been done to examine the formation of such precipitate phases. However, there are very few studies that evaluated the effect stress induced deformation on the precipitation in Mg-RE alloys. Therefore, the objective of this work is to examine influence of deformation on the precipitation of Mg-Nd based alloys. To address this problem, precipitation in two Mg-Nd based alloys, subjected to two different deformation conditions, and was examined via transmission electron microscopy (TEM) and atom probe tomography (APT). In first deformation condition, Md-2.6wt%Nd alloy was subjected to creep deformation (90MPa / 177ºC) to failure. Effect of stress-induced deformation was examined by comparing and contrasting with precipitation in non-creep tested specimens subjected to isothermal annealing (at 177ºC). In second condition, Mg-4.0Y-3.0Nd-0.5Zr (wt %) or WE43 alloy (with comparable Nd content as model Mg-Nd system) was subjected to hot rolling deformation at a sub-solvus temperature.
Date: December 2013
Creator: Dendge, Nilesh Bajirao
Partner: UNT Libraries

Laser Surface Modification on Az31b Mg Alloy for Bio-wettability

Description: Laser surface modification of AZ31B Magnesium alloy changes surface composition and roughness to provide improved surface bio-wettability. Laser processing resulted in phase transformation and grain refinement due to rapid quenching effect. Furthermore, instantaneous heating and vaporization resulted in removal of material, leading the textured surface generation. A study was conducted on a continuum-wave diode-pumped ytterbium laser to create multiple tracks for determining the resulting bio-wettability. Five different laser input powers were processed on Mg alloy, and then examined by XRD, SEM, optical profilometer, and contact angle measurement. A finite element based heat transfer model was developed using COMSOL multi-physics package to predict the temperature evolution during laser processing. The thermal histories predicted by the model are used to evaluate the cooling rates and solidification rate and the associated changes in the microstructure. The surface energy of laser surface modification samples can be calculated by measuring the contact angle with 3 different standard liquid (D.I water, Formamide, and 1-Bromonaphthalen). The bio-wettability of the laser surface modification samples can be conducted by simulated body fluid contact angle measurement. The results of SEM, 3D morphology, XRD, and contact angle measurement show that the grain size and roughness play role for wetting behavior of laser processing Mg samples. Surface with low roughness and large grain size performs as hydrophilicity. On the contrast, surface with high roughness and small grain size performs as hydrophobicity.
Date: December 2013
Creator: Ho, YeeHsien
Partner: UNT Libraries

Additive Manufacturing of Metastable Beta Titanium Alloys

Description: Additive manufacturing processes of many alloys are known to develop texture during the deposition process due to the rapid reheating and the directionality of the dissipation of heat. Titanium alloys and with respect to this study beta titanium alloys are especially susceptible to these effects. This work examines Ti-20wt%V and Ti-12wt%Mo deposited under normal additive manufacturing process parameters to examine the texture of these beta-stabilized alloys. Both microstructures contained columnar prior beta grains 1-2 mm in length beginning at the substrate with no visible equiaxed grains. This microstructure remained constant in the vanadium system throughout the build. The microstructure of the alloy containing molybdenum changed from a columnar to an equiaxed structure as the build height increased. Eighteen additional samples of the Ti-Mo system were created under different processing parameters to identify what role laser power and travel speed have on the microstructure. There appears to be a correlation in alpha lath size and power density. The two binary alloys were again deposited under the same conditions with the addition of 0.5wt% boron to investigate the effects an insoluble interstitial alloying element would have on the microstructure. The size of the prior beta grains in these two alloys were reduced with the addition of boron by approximately 50 (V) and 100 (Mo) times.
Date: August 2017
Creator: Yannetta, Christopher James
Partner: UNT Libraries

Laser Modified Alumina: a Computational and Experimental Analysis

Description: Laser surface modification involves rapid melting and solidification is an elegant technique used for locally tailoring the surface morphology of alumina in order to enhance its abrasive characteristics. COMSOL Multiphysics® based heat transfer modeling and experimental approaches were designed and used in this study for single and multiple laser tracks to achieve densely-packed multi-facet grains via temperature history, cooling rate, solidification, scanning electron micrographs, and wear rate. Multi-facet grains were produced at the center of laser track with primary dendrites extending toward the edge of single laser track. The multiple laser tracks study indicates the grain/dendrite size increases as the laser energy density increases resulting in multiplying the abrasive edges which in turn enhance the abrasive qualities.
Date: December 2012
Creator: Moncayo, Marco Antonio
Partner: UNT Libraries

Synchrotron Radiation X-Ray Diffraction of Nickel-Titanium Shape Memory Alloy Wires During Mechanical Deformation

Description: Shape memory alloys (SMAs) are a new generation material which exhibits unique nonlinear deformations due to a phase transformation which allows it to return to its original shape after removal of stress or a change in temperature. It shows a shape memory effect (martensitic condition) and pseudoelasticity (austenitic condition) properties depends on various heat treatment conditions. The reason for these properties depends on phase transformation through temperature changes or applied stress. Many technological applications of austenite SMAs involve cyclical mechanical loading and unloading in order to take advantage of pseudoelasticity, but are limited due to poor fatigue life. In this thesis, I investigated two important mechanical feature to fatigue behavior in pseudoelastic NiTi SMA wires using high energy synchrotron radiation X-ray diffraction (SR-XRD). The first of these involved simple bending and the second of these involved relaxation during compression loading. Differential scanning calorimetry (DSC) was performed to identify the phase transformation temperatures. Scanning electron microscopy (SEM) images were collected for the initial condition of the NiTi SMA wires and during simple bending, SEM revealed that micro-cracks in compression regions of the wire propagate with increasing bend angle, while tensile regions tend to not exhibit crack propagation. SR-XRD patterns were analyzed to study the phase transformation and investigate micromechanical properties. By observing the various diffraction peaks such as the austenite (200) and the martensite (100), (110), and (101) planes, intensities and residual strain values exhibit strong anisotropy depending upon whether the sample is in compression or tension during simple bending. This research provides insight into two specific mechanical features in pseudoelastic NiTi SMA wires.
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Date: December 2015
Creator: Zhang, Baozhuo
Partner: UNT Libraries

Mechanisms of Ordered Gamma Prime Precipitation in Nickel Base Superalloys

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 of order-disorder transformations and associated phase separation processes at atomistic length scales, by coupling high angle annular dark field (HAADF) - STEM imaging and APT. The results indicate that multiple competing mechanisms can operate during a single continuous cooling process leading to different generations of γ′ including a non-classical mechanism, operative at large undercoolings.
Date: May 2011
Creator: Singh, Antariksh Rao Pratap
Partner: UNT Libraries

First Principles Calculations of the Site Substitution Behavior in Gamma Prime Phase in Nickel Based Superalloys

Description: Nickel based superalloys have superior high temperature mechanical strength, corrosion and creep resistance in harsh environments and found applications in the hot sections as turbine blades and turbine discs in jet engines and gas generator turbines in the aerospace and energy industries. The efficiency of these turbine engines depends on the turbine inlet temperature, which is determined by the high temperature strength and behavior of these superalloys. The microstructure of nickel based superalloys usually contains coherently precipitated gamma prime (?) Ni3Al phase within the random solid solution of the gamma () matrix, with the ? phase being the strengthening phase of the superalloys. How the alloying elements partition into the and ? phases and especially in the site occupancy behaviors in the strengthening ? phases play a critical role in their high temperature mechanical behaviors. The goal of this dissertation is to study the site substitution behavior of the major alloying elements including Cr, Co and Ti through first principles based calculations. Site substitution energies have been calculated using the anti-site formation, the standard defect formation formalism, and the vacancy formation based formalism. Elements such as Cr and Ti were found to show strong preference for Al sublattice, whereas Co was found to have a compositionally dependent site preference. In addition, the interaction energies between Cr-Cr, Co-Co, Ti-Ti and Cr-Co atoms have also been determined. Along with the charge transfer, chemical bonding and alloy chemistry associated with the substitutions has been investigated by examining the charge density distributions and electronic density of states to explain the chemical nature of the site substitution. Results show that Cr and Co atoms prefer to be close by on either Al sublattice or on a Ni-Al mixed lattice, suggesting a potential tendency of Cr and Co segregation in the ? phase.
Date: August 2012
Creator: Chaudhari, Mrunalkumar
Partner: UNT Libraries

Laser Surface Modification of AZ31B Mg Alloy Bio-Implant Material

Description: Magnesium and its alloys are considered as the potential biomaterials due to their biocompatibility and biodegradable characteristics but suffer from poor corrosion performance. Various surface modification techniques are employed to improve their corrosion resistance. In present case, laser surface melting was carried out on AZ31B Mg alloy with various laser energy densities using a continuous wave ytterbium laser. Effect of laser treatment on phase and microstructure evolution was evaluated by X ray diffraction and scanning electron microscopy. Multi-physics thermal model predicted time temperature evolution along the depth of the laser treatment zone. Additionally, electrochemical method and bio-immersion test were employed to evaluate the corrosion behavior in simulated body fluid medium. Microstructure revealed grain refinement and even distribution of Mg17Al12 phase along the grain boundary for laser treated samples leading to substantial enhancement in the corrosion resistance of the laser treated samples compared to the untreated alloy. The laser processed samples also possessed a superior wettability in SBF solution than the untreated sample. This was further reflected in enhanced bio-integration behavior of laser processed samples. By changing the parameters of laser processing such as power, scanning speed, and fill spacing, a controllable corrosion resistance and bioactivity/biocompatibility of the implant material was achieved.
Date: August 2018
Creator: Wu, Tso-chang
Partner: UNT Libraries

Surface Degradation Behavior of Bulk Metallic Glasses and High Entropy Alloys

Description: In this study, the surface degradation behavior was studied for typical examples from bulk metallic glasses (BMGs), metallic glass composites (MGCs) and high entropy alloys (HEAs) alloy systems that are of scientific and commercial interest. The corrosion and wear behavior of two Zr-based bulk metallic glasses, Zr41.2Cu12.5Ni10Ti13.8Be22.5 and Zr57Cu15.4Ni12.6Al10Nb5, were evaluated in as-cast and thermally relaxed states. Significant improvement in corrosion rate, wear behavior, and friction coefficient was seen for both the alloys after thermal relaxation. Fully amorphous structure was retained with thermal relaxation below the glass transition temperature. This improvement in surface properties was explained by annihilation of free volume, the atomic scale defects in amorphous metals resulting from kinetic freezing. Recently developed MGCs, with in situ crystalline ductile phase, demonstrate a combination of mechanical properties and fracture behavior unseen in known structural metals. The composites showed higher wear rates but lower coefficient of friction compared to monolithic amorphous glasses. No tribolayer formation was seen for the composites in sharp contrast to that of the monolithic metallic glasses. Corrosion was evaluated by open circuit potential (OCP) analysis and potentiodynamic polarization. Site-specific corrosion behavior was studied by scanning vibration electrode technique (SVET) to identify formation of galvanic couples. Scanning kelvin probe microscope was used to map elecropositivity difference between the phases and linked to wear/corrosion behavior. Phases with higher elecropositivity were more susceptible to surface degradation. Wear and corrosion synergy in marine environment was evaluated for two high entropy alloys (HEAs), CoCrFeMnNi and Al0.1CoCrFeNi. Between the two alloys, Al0.1CoCrFeNi showed better wear resistance compared to CoCrFeMnNi in dry and marine conditions due to quicker passivation, a higher magnitude of polarization resistance and significantly larger pitting resistance.
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Date: December 2017
Creator: Ayyagari, Venkata A
Partner: UNT Libraries

Non-Isothermal Laser Treatment of Fe-Si-B Metallic Glass

Description: Metallic glasses possess attractive properties, such as high strength, good corrosion resistance, and superior soft magnetic performance. They also serve as precursors for synthesizing nanocrystalline materials. In addition, a new class of composites having crystalline phases embedded in amorphous matrix is evolving based on selective crystallization of metallic glasses. Therefore, crystallization of metallic glasses and its effects on properties has been a subject of interest. Previous investigations from our research group related to laser assisted crystallization of Fe-Si-B metallic glass (an excellent soft magnetic material by itself) showed a further improvement in soft magnetic performance. However, a fundamental understanding of crystallization and mechanical performance of laser treated metallic glass was essential from application point of view. In light of this, the current work employed an integrated experimental and computational approach to understand crystallization and its effects on tensile behavior of laser treated Fe-Si-B metallic glass. The time temperature cycles during laser treatments were predicted using a finite element thermal model. Structural changes in laser treated Fe-Si-B metallic glass including crystallization and phase evolution were investigated with the aid of X-ray diffraction, differential scanning calorimetry, resistivity measurements, and transmission electron microscopy. The mechanical behavior was evaluated by uniaxial tensile tests with an InstronTM universal testing machine. Fracture surfaces of the metallic glass were observed using scanning electron microscopy and site specific transmission electron microscopy. Fe-Si-B metallic glass samples treated with lower laser fluence (<0.49 J/mm2) underwent structural relaxation while higher laser flounces led to partial crystallization. The crystallization temperature experienced an upward shift due to rapid heating rates of the order of 104 K/s during laser treatments. The heating cycle was followed by termination of laser upon treatment attainment of peak temperature and rapid cooling of the similar order. Such dynamic effects resulted in premature arrest of the crystallite growth leading ...
Date: December 2017
Creator: Joshi, Sameehan Shrikant
Partner: UNT Libraries

Integrated Computational and Experimental Approach to Control Physical Texture During Laser Machining of Structural Ceramics

Description: The high energy lasers are emerging as an innovative material processing tool to effectively fabricate complex shapes on the hard and brittle structural ceramics, which previously had been near impossible to be machined effectively using various conventional machining techniques. In addition, the in-situ measurement of the thermo-physical properties in the severe laser machining conditions (high temperature, short time duration, and small interaction volume) is an extremely difficult task. As a consequence, it is extremely challenging to investigate the evolution of surface topography through experimental analyses. To address this issue, an integrated experimental and computational (multistep and multiphysics based finite-element modeling) approach was employed to understand the influence of laser processing parameters to effectively control the various thermo-physical effects (recoil pressure, Marangoni convection, and surface tension) during transient physical processes (melting, vaporization) for controlled surface topography (surface finish). The results indicated that the material lost due to evaporation causes an increase in crater depth of machined cavity, whereas liquid expulsion created by the recoil pressure increases the material pileup height around the lip of machined cavity, the major attributes of surface topography (roughness). Also, it was found that the surface roughness increased with increase in laser energy density and pulse rate (from 10 to 50Hz), and with the decrease in distance between two pulses (from 0.6 to 0.1mm) or the increase in lateral and transverse overlap (0, 17, 33, 50, 67, and 83%). The results of the computational model are also validated by experimental observations with reasonably close agreement.
Date: December 2013
Creator: Vora, Hitesh D.
Partner: UNT Libraries

An Initial Study of Binary and Ternary Ti-based Alloys Manufactured Using Laser Engineered Net Shaping (LENSTM)

Description: In this study an initial assessment of the composition – microstructure – property relationships in binary and ternary Ti – based systems are made possible using LENSTM technology. Laser Engineering Net Shaping (LENSTM), a rapid prototyping, directed laser deposition methodology of additive manufacturing (AM) was used to create bulk homogenous specimens that are compositionally graded. Compositionally graded specimens were made possible by incorporating elemental blends of powder during the LENSTM process. While there have been numerous studies assessing the influence of common elements (e.g., V, Mo, Al, and Cr) on the resulting microstructure in titanium alloys, other elements have been neglected. A systematic study of the Ti – Fe – Al ternary system based upon varying compositions of the eutectoid former, Fe with Al to stabilize the a and b phases respectively has also been neglected. This research effort focuses on exploiting the LENSTM process by rapidly assessing the composition – microstructure – property relationships in a combinatorial approach for the Ti – W, Ti – Fe, and Ti – Fe – Al systems. Compositionally graded specimens of Ti – xW (0<x<40wt.%(14.79at.%)), Ti – xFe (0<x<35wt.%(36.37at.%)), and Ti – xFe – yAl (0<x<40wt.%(36.37at.%)), y=5,10, 15wt.%) have been heat treated to also assess the influence of thermal history on microstructural features such as phase composition and volume fraction. Lastly, a Ti – xMo (0<x<40wt.%(24.96at.%)) compositionally graded specimen was deposited to re-assess the Mo-equivalency nature of W, as well as assess the role of phase separation in microstructural evolution at temperatures above and below the invariant point (~695°C) of the Ti – W binary system.
Date: December 2015
Creator: Gray, Alyn M.
Partner: UNT Libraries

Comparative Coarsening Kinetics of Gamma Prime Precipitates in Nickel and Cobalt Base Superalloys

Description: The increasing technological need to push service conditions of structural materials to higher temperatures has motivated the development of several alloy systems. Among them, superalloys are an excellent candidate for high temperature applications because of their ability to form coherent ordered precipitates, which enable the retention of high strength close to their melting temperature. The accelerated kinetics of solute diffusion, with or without an added component of mechanical stress, leads to coarsening of the precipitates, and results in microstructural degradation, limiting the durability of the materials. Hence, the coarsening of precipitates has been a classical research problem for these alloys in service. The prolonged hunt for an alternative of nickel base superalloys with superior traits has gained hope after the recent discovery of Co-Al-W based alloys, which readily form high temperature g precipitates, similar to Ni base superalloys. In the present study, coarsening behavior of g precipitates in Co-10Al-10W (at. %) has been carried out at 800°C and 900°C. This study has, for the first time, obtained critical coarsening parameters in cobalt-base alloys. Apart from this, it has incorporated atomic scale compositional information across the g/g interfaces into classical Cahn-Hilliard model for a better model of coarsening kinetics. The coarsening study of g precipitates in Ni-14Al-7 Cr (at. %) has shown the importance of temporal evolution of the compositional width of the g/g interfaces to the coarsening kinetics of g precipitates. This study has introduced a novel, reproducible characterization method of crystallographic study of ordered phase by coupling of orientation microscopy with atom probe tomography (APT). Along with the detailed analysis of field evaporation behaviors of Ni and Co superalloys in APT, the present study determines the site occupancy of various solutes within ordered g precipitates in both Ni and Co superalloys. This study has explained the role of structural ...
Date: August 2014
Creator: Meher, Subhashish
Partner: UNT Libraries

Characterization of Ti-6Al-4V Produced Via Electron Beam Additive Manufacturing

Description: In recent years, additive manufacturing (AM) has become an increasingly promising method used for the production of structural metallic components. There are a number of reasons why AM methods are attractive, including the ability to produce complex geometries into a near-net shape and the rapid transition from design to production. Ti-6Al-4V is a titanium alloy frequently used in the aerospace industry which is receiving considerable attention as a good candidate for processing via electron beam additive manufacturing (EBAM). The Sciaky EBAM method combines a high-powered electron beam, weld-wire feedstock, and a large build chamber, enabling the production of large structural components. In order to gain wide acceptance of EBAM of Ti-6Al-4V as a viable manufacturing method, it is important to understand broadly the microstructural features that are present in large-scale depositions, including specifically: the morphology, distribution and texture of the phases present. To achieve such an understanding, stereological methods were used to populate a database quantifying key microstructural features in Ti-6Al-4V including volume fraction of phases, a lath width, colony scale factor, and volume fraction of basket weave type microstructure. Microstructural features unique to AM, such as elongated grains and banded structures, were also characterized. Hardness and tensile testing were conducted and the results were related to the microstructural morphology and sample orientation. Lastly, fractured surfaces and defects were investigated. The results of these activities provide insight into the process-structure-properties relationships found in EBAM processed Ti-6Al-4V.
Date: December 2015
Creator: Hayes, Brian J.
Partner: UNT Libraries

Structure Evolution and Nano-Mechanical Behavior of Bulk Metallic Glasses and Multi-Principal Element Alloys

Description: Bulk metallic glasses and multi-principal element alloys represent relatively new classes of multi-component engineering materials designed for satisfying multiple functionalities simultaneously. Correlating the microstructure with mechanical behavior (at the microstructural length-scales) in these materials is key to understanding their performance. In this study, the structure evolution and nano-mechanical behavior of these two classes of materials was investigated with the objective of fundamental scientific understanding of their properties. The structure evolution, high temperature nano-mechanical behavior, and creep of two Zr-based alloys was studied: Zr41.2Ti13.8Cu12.5Ni10.0Be22 (Vitreloy1) and Zr52.5Ti5Cu17.9Ni14.6All0 (Vitreloy105). Devitrification was found to proceed via the formation of a metastable icosahedral phase with five-fold symmetry. The deformation mechanism changes from inhomogeneous or serrated flow to homogenous flow near 0.9Tg, where Tg is the glass transition temperature. The creep activation energy for Vitreloy1 and Vitreloy105 were 144 kJ/mol and 125 kJ/mol, respectively in the range of room temperature to 0.75Tg. The apparent activation energy increased drastically to 192 kJ/mol for Vitreloy1 and 215 kJ/mol for Vitreloy105 in the range of 0.9Tg to Tg, indicating a change in creep mechanism. Structure evolution in catalytic amorphous alloys, Pt57.5Cu14.7Ni5.3P22.5 and Pd43Cu27Ni10P20, was studied using 3D atom probe tomography and elemental segregation between different phases and the interface characteristics were identified. The structure evolution of three multi-principal element alloys were investigated namely CoCrNi, CoCrFeMnNi, and Al0.1CoCrFeNi. All three alloys formed a single-phase FCC structure in as-cast, cold worked and recrystallized state. No secondary phases precipitated after prolonged heat treatment or mechanical working. The multi-principal element alloys showed less strain gradient plasticity compared to pure metals like Ni during nano-indentation. This was attributed to the highly distorted lattice which resulted in lesser density of geometrically necessary dislocations (GNDs). Dislocation nucleation was studied by low load indentation along with the evaluation of activation volume and activation energy. This was ...
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Date: May 2017
Creator: Mridha, Sanghita
Partner: UNT Libraries

In Vitro Behavior of AZ31B Mg-Hydroxyapatite Metallic Matrix Composite Surface Fabricated via Friction Stir Processing

Description: Magnesium and its alloys have been considered for load-bearing implant materials due to their similar mechanical properties to the natural bone, excellent biocompatibility, good bioactivity, and biodegradation. Nevertheless, the uncontrollable corrosion rate in biological environment restrains their application. Hydroxyapatite (HA, Ca10(PO4)6(OH)2) is a widely used bio-ceramic which has bone-like mineral structure for bone fixation. Poor fracture toughness of HA makes it not suitable for load-bearing application as a bulk. Thus, HA is introduced into metallic surface in various forms for improving biocompatibility. Recently friction stir processing (FSP) has emerged as a surface modification tool for surface/substrate grain refinement and homogenization of microstructure in biomaterial. In the pressent efforts, Mg-nHA composite surface on with 5-20 wt% HA on Mg substrate were fabricated by FSP for biodegradation and bioactivity study. The results of electrochemical measurement indicated that lower amount (~5% wt%) of Ca in Mg matrix can enhance surface localized corrosion resistance. The effects of microstructure,the presence of HA particle and Mg-Ca intermetallic phase precipitates on in vitro behavior of Mg alloy were investigated by TEM, SEM, EDX,XRD ,and XPS. The detailed observations will be discussed during presentation.
Date: August 2016
Creator: Ho, Yee Hsien
Partner: UNT Libraries

Effects of Transition Metal Oxide and Mixed-Network Formers on Structure and Properties of Borosilicate Glasses

Description: First, the effect of transition metal oxide (e.g., V2O5, Co2O3, etc.) on the physical properties (e.g., density, glass transition temperature (Tg), optical properties and mechanical properties) and chemical durability of a simplified borosilicate nuclear waste glass was investigated. Adding V2O5 in borosilicate nuclear waste glasses decreases the Tg, while increasing the fracture toughness and chemical durability, which benefit the future formulation of nuclear waste glasses. Second, structural study of ZrO2/SiO2 substitution in silicate/borosilicate glasses was systematically conducted by molecular dynamics (MD) simulation and the quantitative structure-property relationships (QSPR) analysis to correlate structural features with measured properties. Third, for bioactive glass formulation, mixed-network former effect of B2O3 and SiO2 on the structure, as well as the physical properties and bioactivity were studied by both experiments and MD simulation. B2O3/SiO2 substitution of 45S5 and 55S5 bioactive glasses increases the glass network connectivity, correlating well with the reduction of bioactivity tested in vitro. Lastly, the effect of optical dopants on the optimum analytical performance on atom probe tomography (APT) analysis of borosilicate glasses was explored. It was found that optical doping could be an effective way to improve data quality for APT analysis with a green laser assisted system, while laser spot size is found to be critical for optimum performance. The combined experimental and simulation approach adopted in this dissertation led to a deeper understanding of complex borosilicate glass structures and structural origins of various properties.
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Date: December 2018
Creator: Lu, Xiaonan
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Effect of Retting on Surface Chemistry and Mechanical Performance Interactions in Natural Fibers for High Performance Polymer Composites

Description: Sustainability through replacement of non-renewable fibers with renewable fibers is an ecological need. Impact of transportation costs from South-east Asia on the life cycle analysis of the composite is detrimental. Kenaf is an easily grown crop in America. Farm based processing involves placing the harvested crop in rivers and ponds, where retting of the fibers from the plant (separation into fibers) can take 2 weeks or more. The objective of this thesis is to analyze industrially viable processes for generating fibers and examine their synergistic impact on mechanical performance, surface topography and chemistry for functional composites. Comparison has been made with commercial and conventional retting process, including alkali retting, enzymatic retting, retting in river and pond water (retting occurs by natural microbial population) with controlled microbial retting. The resulting kenaf fibers were characterized by dynamic mechanical analysis (DMA), Raman spectroscopy (FT-Raman), Fourier transform infrared spectroscopy (FT-IR), polarized optical microscopy (POM), X-ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM) optical fluorescence microscopy, atomic force microscopy (AFM) and carbohydrate analysis. DMA results showed that pectinase and microbe treated fibers have superior viscoelastic properties compared to alkali retting. XPS, Raman, FT-IR and biochemical analysis indicated that the controlled microbial and pectinase retting was effective in removing pectin, hemicellulose and lignin. SEM, optical microscopy and AFM analysis showed the surface morphology and cross sectional architecture were preserved in pectinase retting. Experimental results showed that enzymatic retting at 48 hours and controlled microbial retting at 72 hours yield uniform and superior quality fibers compared to alkali and natural retting process. Controlled microbial retting is an inexpensive way to produce quality fibers for polymer composite reinforcement.
Date: May 2013
Creator: Ramesh, Dinesh
Partner: UNT Libraries

Interspecimen Study of Bone to Relate Macromechanical, Nanomechanical and Compositional Changes Across the Femoral Cortex of Bone

Description: Mechanics of bone is widely studied and researched, mainly for the study of fracture. This has been done mostly on a macro scale. In this work hierarchical nature of bone has been explored to investigate bone mechanics in more detail. Flexural test were done to classify the bones according to their strength and deflection. Raman spectroscopy analysis was done to map the mineralization, collagen crosslinking changes across the thickness of the bone. Nanoindentation was done to map indentation hardness and indentation modulus across femoral cortex of the bone. The results indicate that the composition of the bone changes across the thickness of the femoral cortex. The hypothesis is confirmed as increase in mineralization, carbonate to phosphate ratio and collagen crosslinking shows the effect as increased indentation hardness and modulus and decreased deflection.
Date: May 2013
Creator: Nar, Mangesh
Partner: UNT Libraries

Anisotropic Nature of Radially Strained Metal Tubes

Description: Metal pipes are sometimes swaged by a metal cone to enlarge them, which increases the strain in the material. The amount of strain is important because it affects the burst and collapse strength. Burst strength is the amount of internal pressure that a pipe can withstand before failure, while collapse strength is the amount of external pressure that a pipe can withstand before failure. If the burst or collapse strengths are exceeded, the pipe may fracture, causing critical failure. Such an event could cost the owners and their customers millions of dollars in clean up, repair, and lost time, in addition to the potential environmental damage. Therefore, a reliable way of estimating the burst and collapse strength of strained pipe is desired and valuable. The sponsor currently rates strained pipes using the properties of raw steel, because those properties are easily measured (for example, yield strength). In the past, the engineers assumed that the metal would be work-hardened when swaged, so that yield strength would increase. However, swaging introduces anisotropic strain, which may decrease the yield strength. This study measured the yield strength of strained material in the transverse and axial direction and compared them to raw material, to determine the amount of anisotropy. This information will be used to more accurately determine burst and collapse ratings for strained pipes. More accurate ratings mean safer products, which will minimize risk for the sponsor’s customers. Since the strained metal has a higher yield strength than the raw material, using the raw yield strength to calculate burst and collapse ratings is a conservative method. The metal has even higher yield strength after strain aging, which indicates that the stresses are relieved. Even with the 12% anisotropy in the strained and 9% anisotropy in the strain aged specimens, the raw yield strengths are ...
Date: December 2015
Creator: Strickland, Julie N.
Partner: UNT Libraries