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Liu, JieCharacterization of New Rotary Endodontic Instruments Fabricated from Special Thermomechanically Processed NiTi Wire
Doctor of Philosophy, The Ohio State University, 2009, Oral Biology
Although NiTi rotary instruments are very popular for endodontic treatment, instrument separation is still a challenge in clinic. A new NiTi rotary instrument has recently been marketed that is machined from M-Wire that has been subjected to a proprietary novel thermomechanical processing procedure. The manufacturer has claimed that this new M-Wire instrument has considerably improved flexibility and resistance to cyclic fatigue, compared to conventional rotary instruments that are machined from superelastic (SE) austenitic NiTi wire. Clinical use has confirmed that these new M-Wire rotary instruments have outstanding clinical fatigue resistance. However, the mechanism for the improved clinical performance of these instruments is unknown.The objective of this study was to employ a variety of metallurgical laboratory techniques to determine the origin of these improved mechanical properties for the new rotary instruments. Specimens from as-received M-Wire instruments, clinically used M-Wire instruments, and conventional instruments were prepared for evaluation. The temperature range for phase transformation was examined by differential scanning calorimetry (DSC). Vickers hardness measurements were made since hardness variations for the same type of alloy has been found to correlate with variations in mechanical properties. The microstructures of the NiTi alloys were revealed by acid etching and examined with an optical microscope and a scanning electron microscope. Wear resistance of clinically used M-Wire instruments was investigated by examining their surfaces with an SEM. In a complementary study, bright-field images of M-Wire blanks were obtained by scanning transmission electron microscopy (STEM).DSC study showed that M-Wire instruments have much higher Af (austenite-finish) temperatures (over 40°C) than conventional superelastic rotary instruments (below room temperature), and are a mixture of martensite, R-phase and austenite at room temperature. The Vickers hardness of M-Wire instruments is significantly higher than that of conventional rotary NiTi instruments. Mean Vickers hardness number for the tip, intermediate region and shank region of size 30/.04 taper M-Wire instruments were about 374, 380 and 392 each.Better wear resistance was observed with the SEM on clinically used M-Wire instruments, which presented less microcracks and evidence of permanent deformation on the surface compared with surfaces of clinically used conventional NiTi instruments. This improved wear resistance is attributed to increased hardness for surface region of the M-Wire instrument. Acid-etched M-Wire instruments presented a classical lenticular martensite structure when observed with the optical microscope and SEM. EDS analyses of the microstructures of the M-Wire instruments revealed titanium-rich precipitates. STEM examinations of M-Wire blanks revealed much coarser grains, twinning, and a high density of dislocations, which were not observed in starting superelastic NiTi wire blanks for conventional instruments.In summary, increased hardness was found for M-Wire instruments, compared with conventional superelastic NiTi rotary instruments, which served as a control for this study. The STEM observations show that the improved mechanical properties of the starting M-Wire (and the rotary instruments manufactured from this special NiTi wire) arise from strengthening mechanisms in the martensitic structure, which were induced by extensive thermomechanical processing.

Committee:

William Brantley (Advisor); William Johnston (Committee Member); Sarandeep Huja (Committee Member); John Nusstein (Committee Member)

Subjects:

Dental Care; Materials Science

Keywords:

NiTi rotary instruments; M-Wire instrument; thermomechanical processing procedure; microstructure; phase transformation; DSC; Vickers Hardness measurement; SEM; STEM; twinning microstructure; wear resistance

Blank, Jonathan PEffect of boron additions on microstructure and mechanical properties of titanium alloys produced by the armstrong process
Doctor of Philosophy, The Ohio State University, 2008, Materials Science and Engineering
The beneficial influence of boron additions on processing, microstructure, physical and mechanical properties of various titanium alloys has been recognized since 1950’s. However, boron additions to titanium alloys to obtain specific microstructures and mechanical properties for several niche applications, including automotive and aerospace, have been actively studied during the past 25 years. The addition of boron concentrations greater than 0.05 wt.% to titanium alloys creates a dispersion of TiB. The presence of TiB enhances the tensile and fatigue strengths as well as the wear resistance as compared to the original titanium alloy. Although these improvements in mechanical properties are attractive, there are still two major obstacles in using these alloys: (1) relationship of microstructure and mechanical properties in Ti-B alloys needs further investigation to optimize the alloys for specific commercial applications; and (2) cost to benefit ratio of producing these alloys is high for a given application(s). The Armstrong process is a novel process that can produce commercially pure (CP) titanium and titanium alloy powder directly from TiCl4 (and other metal halides or as required, to obtain the desired alloy composition). The Armstrong process uses sodium as a reducing agent, with similar reactions as the Hunter process using sodium as a reducing agent and Kroll process using magnesium as a reducing agent. The Armstrong process forms CP-Ti and titanium alloyed powder, which can be directly consolidated or melted into the final product. In comparing the downstream processing steps required by the Kroll and Hunter processes with direct consolidation of Armstrong powder, several processing features or steps are eliminated: (1) restriction of batch processing of material, (2) blending of titanium sponge and master alloy material to create titanium alloys, (3) crushing of the sponge product, (4) melting, and (5) several handling steps. The main objective of this research was to characterize structure and properties of CP-Ti and Ti-B alloys produced by the Armstrong process. Particular emphasis has been placed on improved understanding of the strengthening mechanisms associated with the addition of boron to titanium alloys.

Committee:

James Williams (Advisor)

Subjects:

Textile Technology

Keywords:

Titanium; Titanium-boron; TiB; Commercially Pure Titanium; CP-Ti; Titanium microstructure; Titanium-boron microstructure; Tensile; Notched Fatigue; Fatigue Crack Growth

Telang, AbhishekA Study of the Effects of Mechanical Surface Treatments on Residual Stresses, Microstructure and Stress Corrosion Cracking Behavior of Alloy 600
PhD, University of Cincinnati, 2015, Engineering and Applied Science: Materials Science
Stress corrosion cracking (SCC) of Alloy 600 has been a major problem in commercial light water reactor (LWR) nuclear power plants. Localized corrosion and intergranular SCC (IGSCC) have been observed in Alloy 600 in the high temperature (288-340 °C) pure water environment of LWRs. Additionally, IGSCC of Alloy 600 has been reported even at room temperature under certain conditions in thiosulfate and tetrathionate solutions. In general, SCC can be attributed to the presence of tensile stress, an aggressive environment and a susceptible microstructure. Therefore, SCC mitigation techniques address these factors by modifying the environment, metallurgical processing treatments and alleviating the tensile stresses by mechanical surface treatments/stress relief. This study investigated the application of laser shock peening (LSP) as a technique to mitigate SCC in Alloy 600. LSP induced large compressive residual stresses (-550 MPa) that decreases gradually through depth. The pressure pulse generated during the LSP treatment causes plastic deformation, resulting in high dislocation density, twins and formation of misoriented sub-grains/crystallites that have sizes in the range of 50-300 nm in the near-surface region. Slow strain rate tests (SSRTs) and constant load tests performed in tetrathionate solution at room temperature were used to evaluate the effect of LSP on the SCC behavior. LSP treated samples had a significantly longer time to failure and reduced susceptibility to SCC as compared with untreated sensitized Alloy 600. These improvements were attributed to LSP induced compressive residual stresses, increased yield strength (YS) and hardening caused by near-surface microstructural changes. SSRTs in simulated PWR environment also show similar results with higher YS, tensile strength and strain to failure. Additionally, the gage section shows fewer cracks and smaller crack lengths in the LSP treated samples as compared with the untreated samples. The other approach involved using mechanical surface treatments/cold work followed by annealing to engineer the Alloy 600 microstructure for increased resistance to corrosion and IGSCC. We demonstrated a novel method of surface grain boundary engineering (SGBE) in Alloy 600 using iterative cycles of ultrasonic nanocrystalline surface modification (UNSM) treatment and strain annealing. Three cycles of UNSM and strain annealing at 900-1000 °C were used to modify the microstructure to a depth of 250 µm from the surface. This surface treatment based method increased the fraction of low coincident site lattice (CSL) grain boundaries whilst decreasing the fraction and connectivity of random high angle boundaries (HABs) in the near surface region. Similar results were achieved using thermo-mechanical processing (TMP) with iterative cycles of 10% cold work and strain annealing in Alloy 600. A disrupted random HAB network and large fraction (70%) of CSL boundaries (Σ3-Σ27) reduced the propensity to sensitization. SSRTs in tetrathionate solutions at room temperature show that SGBE and TMP lowered the susceptibility to intergranular SCC. Detailed analysis using EBSD showed cracks arrested at J1 (1-CSL) and J2 (2-CSL) type of triple junctions. The probability of crack arrest, calculated using percolative models, was higher after SGBE and TMP in Alloy 600 and explains the improved IGSCC resistance.

Committee:

Vijay Vasudevan, Ph.D. (Committee Chair); Seetha Ramaiah Mannava, Ph.D. (Committee Member); Dong Qian, Ph.D. (Committee Member); Rodney Roseman, Ph.D. (Committee Member); Vesselin Shanov, Ph.D. (Committee Member)

Subjects:

Materials Science

Keywords:

Stress Corrosion Cracking;Microstructure;Laser Shock Peening;Microstructure;Grain boundary;Residual Stress

Lee, EunhaMicrostructure evolution and microstructure/mechanical properties relationships in α+β titanium alloys
Doctor of Philosophy, The Ohio State University, 2004, Materials Science and Engineering
In this study, the microstructural evolution of Timetal 550 was investigated. Timetal 550 showed two types of phase transformations (martensitic and nucleation and growth) depending on the cooling rate from the β region. The α phase initially precipitated at the prior β grain boundaries, and it had a Burgers OR with one of the adjacent grains. It was found that colonies could grow, even in the fast-cooled Timetal 550 sample, from the grain boundary α into the prior β grain with which it exhibited the Burgers OR. Three orientation relationships were also found between α laths in the basketweave microstructure. Microhardness testing demonstrated that fast-cooled Timetal 550 samples with basketweave microstructure were harder than slowly-cooled samples with colony microstructure. Orientation-dependent deformation was found in the colony microstructure. Specically, when the surface normal is perpendicular to the [0001] of α, the material deforms easily in the direction perpendicular to the [0001] of α. Fuzzy logic and Bayesian neural network models were developed to predict the room temperature tensile properties of Timetal 550. This involved the development of a database relating microstructural features to mechanical properties. A Gleeble 3800 thermal-mechanical simulator was used to develop various microstructures. Microstructural features of tensile-tested samples were quantified using stereological procedures. The quantified microstructural features and the tensile properties were used as inputs and outputs, respectively, for modeling the relationships between them. The individual influence of five microstructural features on tensile properties was determined using the established models. The microstructural features having the greatest impact on UTS and YS were the thickness of α laths and the width of grain boundary α layer, and the microstructural features having the greatest impact on elongation were the thickness of α laths and the prior β grain size. Nanoindentation testing found that the hardness of the individual grains was related to their orientations. The hardness values were highest near the [0001] stress axis, and they decreased as the stress axis deviated from [0001] orientation. Dislocation analyses indicated that the deformation in individual grains conformed to the Schmid factor analysis where slip primarily occurs on those slip systems where RSS (SF) values are highest.

Committee:

Hamish Fraser (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

Timetal 550; Ti-6Al-4V; microstructure evolution; modeling microstructure/tensile properties relatioships; nanoindentation

Collins, Peter ChancellorA combinatorial approach to the development of composition-microstructure-property relationships in titanium alloys using directed laser deposition
Doctor of Philosophy, The Ohio State University, 2004, Materials Science and Engineering
The Laser Engineered Net Shaping (LENS™) system, a type of directed laser manufacturing, has been used to create compositionally graded materials. Using elemental blends, it is possible to quickly vary composition, thus allowing fundamental aspects of phase transformations and microstructural development for particular alloy systems to be explored. In this work, it is shown that the use of elemental blends has been refined, such that bulk homogeneous specimens can be produced. When tested, the mechanical properties are equivalent to conventionally prepared specimens. Additionally, when elemental blends are used in LENS™ process, it is possible to deposit compositionally graded materials. In addition to the increase in design flexibility that such compositionally graded, net shape, unitized structures offer, they also afford the capability to rapidly explore composition-microstructure-property relationships in a variety of alloy systems. This research effort focuses on the titanium alloy system. Several composition gradients based on different classes of alloys (designated a, a+b, and b alloys) have been produced with the LENS™. Once deposited, such composition gradients have been exploited in two ways. Firstly, binary gradients (based on the Ti-xV and Ti-xMo systems) have been heat treated, allowing the relationships between thermal histories and microstructural features (i.e. phase composition and volume fraction) to be explored. Neural networks have been used to aid in the interpretation of strengthening mechanisms in these binary titanium alloy systems. Secondly, digitized steps in composition have been achieved in the Ti-xAl-yV system. Thus, alloy compositions in the neighborhood of Ti-6Al-4V, the most widely used titanium alloy, have been explored. The results of this have allowed for the investigation of composition-microstructure-property relationships in Ti-6-4 based systems.

Committee:

Hamish Fraser (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

combinatorial method; combinatorial approach; laser deposition; directed laser deposition; LENS; titanium; molybdenum; Ti-6-4; Ti-6Al-4V; Timetal 21S; composition; microstructure; property; relationships; neural network; fuzzy logic

Wu, KaishengComputer simulation of interdiffusion microstructures in multi-component and multiphase systems
Doctor of Philosophy, The Ohio State University, 2004, Materials Science and Engineering
Understanding of the complicated interdiffusion microstructures developed between two contact alloys at elevated temperatures is critical to the design of many materials, e.g. coated turbine blades. In the present thesis, a novel computational approach based on phase field method has been developed to investigate for the first time the interdiffusion microstructures for ternary and two-phase systems. This approach possesses many advantages over the previous simulations which were limited to one-dimensional (1D) diffusion in a common matrix phase while second-phase particles were treated as point sources or sinks of solute atoms. The proposed approach can simultaneously account for the diffusion in different phases with arbitrary volume fractions and morphologies. The elastic interaction between particles as well as the stress effect on the interdiffusion can also be considered. This approach has been first applied for a fundamental research to investigate the Kirkendall effect in ternary, two-phase diffusion couples. Model α+α' alloys were designed so that changes in the microstructure could be attributed to either capillarity or the Kirkendall effect. It has been found that the Kirkendall effect, which was introduced by setting atomic mobilities to different values, changed the diffusion path slope and led to the formation of "horns" on the two-phase diffusion path. Both precipitates and the so-called Type 0 boundary migrate as a result of the Kirkendall effect. Also the initial slope of the diffusion path differs significantly from the earlier work and the path is time dependent due to temporal changes in the microstructure. In addition, the phase field method provides a detailed picture of Kirkendall marker movement in a two-phase microstructure. The marker plane bends around precipitates and individual markers move along curved paths. The practical application of the approach has then been carried out for Ni-Al-Cr ternary system, the most important system in the coating design. CALPHAD technique has been incorporated to provide chemical free energy and kinetic data. The simulated γ+β/γ and γ+β/γ+β diffusion couples present microstructure features which show reasonable agreement with experimental observations. New features such as curved diffusion paths in two-phase region and formation of single-phase layer have also been observed.

Committee:

Yunzhi Wang (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

diffusion; diffusion couple; interdiffusion microstructure; phase field method; ternary system; Ni-Al-Cr; computer simulation

Tedjaseputra, Erik NugrohoNumerical Simulations of Microstructure-based Crystal Plasticity Finite Element Model for Titanium and Nickel Alloys
Master of Science, The Ohio State University, 2012, Mechanical Engineering

With the rapid development in aviation industry, flourishing research to manufacture alloys that exhibit long service life and reliable is highly in demand. With the constraint of cost and time, modeling of alloys becomes priority to study the material response in extreme conditions of high stress and temperature, particularly in creep. This thesis will highlight the study of utilization of crystal plasticity-based finite-element method to model creep response for titanium-alloys and nickel-based superalloys. The first part of the thesis studies Ti-6242 alloy creep response that exhibit more plastic strain in a given favorable microstructure profile despite of low stress applied, compared to harder microstructure profile subjects to higher stress. The simulation result shows this phenomena based on Ti-alloys experiments of varying studied microstructure feature under the same loading. In detail, the thesis discusses heavily on the modification of existing crystal plasticity developed by Ghosh, S. et al that encompasses microstructure parameters, such as; grain or colony size, misorientation, lath-alpha thickness, primary-alpha volume fraction, colony aspect ratio to characterized hardness microstructure. It also discuss brief work on constructing microstructure-based creep law, following Norton-Bailey creep power law.

In the second part, the crystal plasticity method undergo further expansion to accommodate the multi-scale approach in modeling Ni-superalloy response. In the lowest scale, dislocation density model developed by Samal, M.K., and Ghosh, S., used to model the sub-grain scale. Then, in the grain and polycrystalline scale, activation-energy crystal plasticity model is used with homogenization law to bridge the sub-grain and grain scale, along with asymmetry and microtwinning mechanism. This thesis will discuss heavily on the incorporation of thermally activated theory of plastic law into crystal plasticity formulation for grain and polycrystalline scale. Development of homogenization law for the grain level based on microstructure parameters; gamma prime-precipitate shape (n1), volume fraction (vf) and spacing between precipitates (lc), and microtwinning mechanism will also be discussed briefly. Finally, the thesis presents simulation validation case for the multi-scale model of polycrystalline sample for constant strain test using calibrated model from single crystal experiment of CMSX-4 Ni-based superalloys

Committee:

Somnath Ghosh, PhD (Advisor); June K. Lee, PhD (Other)

Subjects:

Engineering; Materials Science; Mechanical Engineering

Keywords:

crystal plasticity; finite element; titanium alloys; nickel superalloys; multi-scale; microstructure; creep

Ravindran, DeepakFinite Element Simulation of Hot Stamping
Master of Science, The Ohio State University, 2011, Mechanical Engineering
Hot stamping is a relatively new and upcoming technology in the automotive industry used for the manufacturing of lightweight crash resistant parts with ultra high strength. The Finite Element (FE) simulation of hot stamping has to consider the combined effects of thermal, mechanical and microstructural fields. As a result, simulation of hot stamping is a complex process and requires detailed information on various factors such as material properties, process parameters and the right choice of software code. This report presents a complete list of all the important material properties and process parameters required for FE simulation of hot stamping. A sample part geometry was obtained from an industrial partner of the Center for Precision Forming at The Ohio State University and FE simulations of the part were carried out using the FE code DEFORM. The modeling and results of these simulations are presented in this report. Based on the results, a new methodology has been proposed that makes use of two different FE codes namely, DEFORM and PAM-STAMP in order to combine the advantages of the two in simulating hot stamping processes.

Committee:

Taylan Altan (Advisor); Jerald Brevick (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

Hot stamping; simulation; quenching; microstructure; ultra high strength steels

Gonser, Matthew J.Microstructure Evolution and Material Flow Behavior in Friction-Stir Welded Dissimilar Titanium Alloys
Doctor of Philosophy, The Ohio State University, 2010, Welding Engineering

In certain aerospace structures the joining of dissimilar titanium alloys may be necessary. Fusion welding of these alloys together results in the formation of large beta grains and transformed-beta microstructures that can be deleterious to mechanical performance. Friction-stir welding (FSW) was proposed due to the reported microstructural advantages afforded by the process. The purpose of this study was to friction-stir weld dissimilar titanium alloys (Ti-6Al-4V and Timetal 21S, both 1.27 mm in thickness) together and to investigate how macroscopic flow in the stir zone and the resulting weld microstructure affect mechanical properties.

Welds were produced using a refractory tool with travel speeds from 50 to 100 mm/min and tool rotation speeds of 2000 to 3500 revolutions per minute (RPM). Basketweave and colony alpha and beta phase in the prior-beta grains formed on the Ti-6Al-4V side of the stir zone and near-HAZ. The Timetal 21S region of the stir zone consisted of refined (approximately 18 μm in diameter) metastable-beta grains compared to 30 μm diameter grains in the Timetal 21S base material. Metallurgical mixing between the two alloys resulted in a unique alpha-beta microstructure in the stir zone with high hardness (450 Vicker’s Hardness Number (VHN)). The hardness increase was attributed to a fine distribution of alpha and beta phase. The highest tensile strength (1.1 GPa, 158 ksi) and elongation (8%) for as-welded specimens occurred when lower rotation speeds (3000 RPM) and highest travel speeds (100 mm/min) were used. Placement of the Timetal 21S on the retreating side resulted in the failure of tensile samples in the Timetal 21S base material. If the Ti-6Al-4V was placed on the retreating side, the failure occurred in the SZ/TMAZ region on the Timetal 21S side of the weld.

Placement of the Ti-6Al-4V alloy on the retreating side increased the amount of metallurgical mixing between the two alloys by 40% compared to when the Timetal 21S was placed on the retreating side. Electron back-scatter diffraction (EBSD) clearly showed the presence of a TMAZ adjacent to the stir zone on the Timetal 21S side of the weld. This was confirmed by the large number of low angle subgrains within the deformed metastable-beta matrix. A series of aging and solution treatment plus aging heat treatments was given to select as-welded samples. The peak hardness for all regions was obtained for the 500°C-8 hour heat treatment, while the 600°C-8 hour heat treatment effectively equalized the hardness values across the entire weld. The heat treatment response of the metallurgically-mixed alpha-beta microstructure was dependent on alpha stabilizer content.

In terms of material flow, the material directly ahead of the tool shoulder and pin is swept into the stir zone as high-temperature beta phase, so the transport of material in the stir zone during friction-stir welding was, therefore, found to be dependent on the flow stress-dependent viscosity of the alloys. In addition, subgrain formation leads to harder microstructures, so the flow would be hindered in the TMAZ, and subsequently the SZ, upon stirring.

Committee:

Sudarsanam Suresh Babu (Advisor); William A. Baeslack III (Committee Member); John C. Lippold (Committee Member); Stanislav Rokhlin (Committee Member)

Subjects:

Engineering

Keywords:

Friction-stir welding; titanium alloys; material flow; microstructure evolution

Godbole, ChinmayThe Influence of Reinforcement on Microstructure, Hardness, Tensile Deformation, Cyclic Fatigue and Final Fracture behavior of two Magnesium Alloys
Master of Science in Engineering, University of Akron, 2011, Mechanical Engineering

The application of Metal Matrix composites (MMC) spans over a wide range of structural applications owing to its improved mechanical properties namely high specific modulus and high strength to weight ratio when compared to their monolithic metal counterparts. Magnesium having a low density of 1.73 gm/cm3 is approximately two thirds of that of aluminum, one fourth of zinc, and one fifth of steel, allows it offer a very high specific strength among conventional engineering alloys.

Three Magnesium alloys based nano reinforced metal matrix composite were fabricated using solidification technique followed by hot extrusion. Magnesium alloy AZ31 was reinforced with alumina particulate (Al2O3p) and carbon nanotubes separately to produce (1) AZ31/1.5 vol% Al2O3 and (2) AZ31/1.0 vol% CNT composites. 3 wt% aluminum was added to AZ91 Mg alloy and reinforced with alumina particulate to synthesize (3) AZ (12)1/1.5 vol% Al2O3 nanocomposite. The test specimens of the composites and the monolithic alloys were precision machined and conformed to the standards specified in ASTM E8/E466. The samples were deformed in tension under strain controlled loading at rate of 0.0001s-1 to obtain the tensile properties. Stress amplitude controlled high cycle cyclic fatigue was carried over a range of maximum stress, at frequency of 5 Hz and at load ratios of 0.1 and -1. The number of cycles to failure were recorded. In this thesis report the effect of reinforcement and processing on the microstructure modification, hardness, tensile properties, stress controlled high cycle fatigue response and micro mechanics of final fracture behavior of the magnesium alloy composite is neatly presented discussed and compared with their unreinforced monolithic alloy counterparts. The elastic modulus, yield strength, ultimate tensile strength of the reinforced magnesium alloys were compared to the unreinforced counterpart. The ductility quantified by elongation to failure over 0.5 inches (12.7 mm) gage length of the test specimen and reduction in cross-section area of the composite were compared to the monolithic alloy. A comparison of fatigue response of the reinforced magnesium alloys with unreinforced counterparts were done to observe improvement in cyclic fatigue life at load ratio of 0.1 and – 1. The key mechanisms responsible for the superior cyclic fatigue and tensile behavior of the composite are discussed.

Committee:

Tirumalai Srivatsan, Dr (Advisor); Craig Menzemer, Dr (Committee Member); Amit Prakash, Dr (Committee Member)

Subjects:

Materials Science; Mechanical Engineering

Keywords:

Magnesium alloy; aluminum oxide; particulate reinforcement; metal matrix composite; microstructure; hardness; tensile response; cyclic fatigue life; fracture

Waugh, David AUTILITY OF FOSSIL CUTICLE MORPHOLOGY APPLIED TO THE TAPHONOMY AND TAXONOMY OF DECAPOD CRUSTACEANS
PHD, Kent State University, 2013, College of Arts and Sciences / Department of Geology
The purpose of the dissertation is to examine the microstructural variations within cuticle of brachyuran decapod crustaceans, the crabs and other selected decapods, in order to identify innovative character states in extant organisms and in whole or fragmentary fossil material that can be observed, coded, and treated in phylogenetic analyses. This is an entirely new approach to the study of decapod phylogeny that will link fossil and extant decapod datasets in a manner that previously has been impossible.

Committee:

Rodney Feldmann, Dr. (Advisor)

Subjects:

Paleontology

Keywords:

cuticle, cuticle microstructure, decapods

Emge, Andrew WilliamEffect of sliding velocity on the tribological behavior of copper and associated nanostructure development
Doctor of Philosophy, The Ohio State University, 2008, Materials Science and Engineering
The unlubricated sliding of metals is important in many mechanical devices covering a wide range of sliding velocities. However, the effect of sliding velocity on the tribological behavior of unlubricated metals has not been widely studied. Similarly, the relationship between microstructures developed at high sliding velocities and tribological behavior has not been studied in depth. The current research relates two aspects of the sliding friction of ductile metals, the effect of sliding velocity and the production of nanocrystalline tribomaterial. The project focused on the effects of sliding velocity on the frictional behavior of oxygen free high conductivity (OFHC) copper sliding against 440C stainless steel, Nitronic 40 stainless steel, and copper. Low velocity tests were performed with a pin on disk tribometer. High velocity tests were performed with a rotating barrel gas gun (RBGG) which combined impact with sliding. Surface and subsurface microstructures and chemical compositions of the worn samples were characterized with a wide range of instruments. In the case of self-mated copper the sliding velocity had little effect on the coefficient of friction and associated microstructural changes. An increase in the coefficient of friction for copper sliding against stainless steel in both the pin on disk and RBGG systems was observed. For the pin on disk tests the coefficient of friction was strongly influenced by material transfer from the copper to the steel pin. The increase in the coefficient of friction for the RBGG tests was correlated to an increase in subsurface plastic deformation. The growth of the nanocrystalline tribolayer in copper after sliding against 440C stainless steel at varying times was studied at sliding velocities of 0.05 and 1.0 m/s. The 0.5 m/s sliding velocity produced a consistent nanocrystalline layer in as little as 10 s. The thickness of the nanocrystalline layer grew continuously at sliding times of up to 10 ks. The 1.0 m/s sliding velocity produced a continuous nanocrystalline layer after 10 s of sliding. Ledges developed on the wear tracks which greatly influenced the tribolayer thickness making it difficult to quantify. Dynamic recrystallization of the tribolayer also led to difficulties in measuring its thickness.

Committee:

David Rigney (Advisor)

Keywords:

Friction; Copper; Microstructure; Deformation; Sliding velocity

Riggs, Bryan EMULTI-SCALE COMPUTATIONAL MODELING OF NI-BASE SUPERALLOY BRAZED JOINTS FOR GAS TURBINE APPLICATIONS
Doctor of Philosophy, The Ohio State University, 2017, Welding Engineering
Brazed joints are commonly used in the manufacture and repair of aerospace components including high temperature gas turbine components made of Ni-base superalloys. For such critical applications, it is becoming increasingly important to account for the mechanical strength and reliability of the brazed joint. However, material properties of brazed joints are not readily available and methods for evaluating joint strength such as those listed in AWS C3.2 have inherent challenges compared with testing bulk materials. In addition, joint strength can be strongly influenced by the degree of interaction between the filler metal (FM) and the base metal (BM), the joint design, and presence of flaws or defects. As a result, there is interest in the development of a multi-scale computational model to predict the overall mechanical behavior and fitness-for-service of brazed joints. Therefore, the aim of this investigation was to generate data and methodology to support such a model for Ni-base superalloy brazed joints with conventional Ni-Cr-B based FMs. Based on a review of the technical literature a multi-scale modeling approach was proposed to predict the overall performance of brazed joints by relating mechanical properties to the brazed joint microstructure. This approach incorporates metallurgical characterization, thermodynamic/kinetic simulations, mechanical testing, fracture mechanics and finite element analysis (FEA) modeling to estimate joint properties based on the initial BM/FM composition and brazing process parameters. Experimental work was carried out in each of these areas to validate the multi-scale approach and develop improved techniques for quantifying brazed joint properties. Two Ni-base superalloys often used in gas turbine applications, Inconel 718 and CMSX-4, were selected for study and vacuum furnace brazed using two common FMs, BNi-2 and BNi-9. Metallurgical characterization of these brazed joints showed two primary microstructural regions; a soft, ductile a-Ni phase that formed at the joint interface and a hard, brittle multi-phase centerline eutectic. CrB and Ni3B type borides were identified in the eutectic region via electron probe micro-analysis, and a boron diffusion gradient was observed in the BM adjacent to the joint. The volume fraction of centerline eutectic was found to be highly dependent on the extent of the boron diffusion that occurred during brazing and therefore a function of the primary process parameters; hold time, temperature, FM/BM composition, and joint gap. Thermo-CalcTM and DICTRATM simulations were used to model the BM dissolution, isothermal solidification and phase transformations that occurred during brazing to predict the final joint microstructure based on these process parameters. Good agreement was found between experimental and simulated joint microstructures at various joint gaps demonstrating the application of these simulations for brazed joints. However, thermodynamic/kinetic databases available for brazing FMs were limited. A variety of mechanical testing was performed to determine the mechanical properties of CMSX-4/BNi-2 and IN718/BNi-2 brazed joints including small-scale tensile tests, standard-size butt joints and lap shear tests. Small-scale tensile testing provided a novel method for studying microstructure-property relationships in brazed joints and indicated that both joint strength and ductility decrease significantly with an increase in the volume fraction of centerline eutectic. In-situ observations during small-scale testing also showed strain localization and crack initiation occurs around the hard, eutectic phases in the joint microstructure during loading. The average tensile strength for standard-size IN718/BNi-2 butt joints containing a low volume fraction of centerline eutectic was found to be 152.8 ksi approximately 90% of the BM yield strength (~170 ksi). The average lap shear FM stress was found to decrease from 70 to 20 ksi for IN718/BNi-2 joints and from 50 to 15 ksi for CMSX-4/BNi-2 as the overlap was increased from 1T to 5T due to non-uniform stress/strain distribution across the joint. Digital image correlation techniques and FEA models of the lap shear brazed joints were developed to assess the strain distributions across the overlap. Results were used to validate the use of damage zone models for predicting the load carrying capacity of lap shear brazed joints and suggest that the damage zone is independent of the overlap length. To account for the presence of flaws and defects in fitness-for-service assessments of brazed joints determination of the average fracture toughness (KIC) is necessary. Currently no standard exists to measure the KIC for brazed joints, so three test methods were evaluated in this investigation on IN718/BNi-2 brazed joints. The compact tension and double cantilever beam test methods were found to give the most conservative KIC values of 16.42 and 14.42 ksivin respectively. Linear-elastic FEA models of the test specimens were used to validate the calculated KIC values. Similar to joint strength the fracture toughness appeared to be strongly influenced by the volume fraction of centerline eutectic phases. The data and methodology generated in this initial study provides validation for the proposed multi-scale computational model by demonstrating microstructure-property relationships in brazed joints and the ability to predict joint microstructure using simulation tools. Furthermore, an experimental framework and new techniques including small-scale tensile testing, digital image correlation and fracture mechanics were established to assist in future modeling efforts. Ultimately, successful development and implementation of multi-scale computational models for brazed joints will allow for the optimization of BM/FM compositions, brazing process optimization, improved reliability of brazed joints, and more efficient design and analysis of brazed components by accounting for the properties of the joint. In addition, the overall multi-scale modeling approach demonstrated in this investigation may also be applied for dissimilar joints in general, for example dissimilar metal welds used in oil and gas, petrochemical, nuclear and power generation industries.

Committee:

Boian Alexandrov, Ph.D. (Advisor); Avraham Benatar, Ph.D. (Advisor); Carolin Fink, Ph.D. (Committee Member)

Subjects:

Engineering; Materials Science

Keywords:

Brazing; Brazed Joints; Ni-base Superalloys; Microstructure; Modeling; Digital Image Correlation; Mechanical Properties

Hanson, Thomas AlanReal Effects of High Frequency Trading
PHD, Kent State University, 2014, College of Business Administration / Department of Finance
High frequency trading (HFT) has revolutionized the functioning of modern equity markets, with high-speed algorithms now constituting the majority trading volume. To date, the academic literature has focused on the microstructural effects of these changes, among them changes in liquidity, volatility, and market fragmentation. This dissertation shifts the focus to the effects that HFT can have on decisions made within firms and their subsequent valuation by the market. Specifically, I explore HFT's effects on three corporate finance variables: the quality of corporate governance, the value of the firm, and the value of cash held on the balance sheet. In all cases, the effect of HFT is detrimental to firms. Corporate governance becomes less democratic, firm value declines, and the market gives a lower value to cash on the balance sheet. Previous work has established links between market activity and decisions made at firms. Two channels have been theorized based on information aggregation and liquidity. Therefore, I also explore four possible mechanisms through which HFT could conceivably affect corporate finance: liquidity, volatility, blockholders, and corporate governance. Each of these mechanisms is significant in some cases, reinforcing the reflexive and interconnected nature of markets, corporate governance, and firm value. The overall results suggest the existence of negative externalities from HFT activity. Such effects should factor into the ongoing debate on regulation and market structure. Future research ideas are also suggested to continue exploring the relationship between market microstructure and corporate finance.

Committee:

Jayaram Muthuswamy (Advisor)

Subjects:

Finance

Keywords:

High frequency trading; real effects; agency theory; volatility; microstructure

Lolla, Sri Venkata TapasviUnderstanding Microstructure Evolution in Rapid Thermal Processing of AISI 8620 Steel
Master of Science, The Ohio State University, 2009, Welding Engineering

A new steel heat treatment process has been developed to achieve strength comparable to Advanced High Strength class of steels. This process is unique for its very short treatment time (<10s) and is hence termed Flash Process. The strength of the steel obtained from this process (ultimate tensile strength (UTS): 1694 MPa, elongation: 7.1%) showed at least 7% higher UTS and 30% greater elongation than published results of martensitic advanced high strength steel (UTS: 1585 MPa, elongation: 5.1%). Arc welding of flash processed steel resulted in softening in the heat affected zone, which could be a potential site for crack initiation. In order to devise better welding procedures to join this steel, the microstructure evolution during flash processing has to be understood.

The main goal of this thesis is to explain the formation of the flash process microstructure, which governs the strength of the steel. In order to achieve this goal the task was divided into two parts. In the first part, the thermal profile of the flash process is quantified. The second part focuses on the characterization of the microstructure in the final steel. Based on the results obtained from the two analyses, the microstructure evolution is explained. This work forms a basis for future research on the weldability of the steel produced from this process.

Initial confirmatory tests were carried out to verify the properties of the flash processed steels. The steels showed high strength (UTS > 1500 MPa) and good ductility (elongation > 8.7%). Thermocouples were attached to the steel plates to measure the thermal profile of the process. The total time of the process was recorded to be around 6s with a peak temperature of over 1000°C. Analysis of the heating curve indicated complete austenitization with Ac1 and Ac3 temperatures lying in the ranges 925- 930°C and 1045-1052°C respectively and a very short dwell time (< 2s) in the austenite region. Analysis of the thermal profile provides an insight into the microstructure evolution during the flash process.

The initial microstructure of the steel consisted of ferrite with carbides. Optical images of the flash processed steel showed very fine features that could not be clearly identified. Scanning electron microscopy of the final steel also indicated the presence of these carbides. This pointed to incomplete carbide dissolution during the process. On further analysis, transmission electron microscope images showed the presence of bainite in the steel, along with martensite.

The presence of carbides shown by microstructural characterization and the short austenite dwell time from thermal analysis, indicate a non homogenous carbon distribution in the austenite phase. This inhomogeneity is related to the formation of a mixed microstructure of bainite and martensite. Finally, the high strength observed in the flash process steel is rationalized by modifying published theoretical models on the strength of steel. It is shown that a mixed microstructure of bainite and martensite is much more stronger than completely martensitic steel.

Committee:

Sudarsanam Babu (Advisor); John Lippold (Committee Member)

Subjects:

Engineering; Materials Science; Metallurgy

Keywords:

Flash Process; Rapid Heating; Rapid Cooling; Bainite; Mixed Microstructure; AHSS; High Strength; Characterization

Foltz, John WendellThe Relationships Between Microstructure, Tensile Properties and Fatigue Life in Ti-5Al-5V-5Mo-3Cr-0.4Fe (Ti-5553)
Doctor of Philosophy, The Ohio State University, 2010, Materials Science and Engineering

β-titanium alloys are being increasingly used in airframes as a way to decrease the weight of the aircraft. As a result of this movement, Ti-5Al-5V-5Mo-3Cr-0.4Fe (Timetal 555), a high-strength β titanium alloy, is being used on the current generation of landing gear. This alloy features good combinations of strength, ductility, toughness and fatigue life in α+β processed conditions, but little is known about β-processed conditions. Recent work by the Center for the Accelerated Maturation of Materials (CAMM) research group at The Ohio State University has improved the tensile property knowledge base for β-processed conditions in this alloy, and this thesis augments the aforementioned development with description of how microstructure affects fatigue life.

In this work, β-processed microstructures have been produced in a GleebleTM thermomechanical simulator and subsequently characterized with a combination of electron and optical microscopy techniques. Four-point bending fatigue tests have been carried out on the material to characterize fatigue life. All the microstructural conditions have been fatigue tested with the maximum test stress equal to 90% of the measured yield strength. The subsequent results from tensile tests, fatigue tests, and microstructural quantification have been analyzed using Bayesian neural networks in an attempt to predict fatigue life using microstructural and tensile inputs. Good correlation has been developed between lifetime predictions and experimental results using microstructure and tensile inputs. Trained Bayesian neural networks have also been used in a predictive fashion to explore functional dependencies between these inputs and fatigue life.

In this work, one section discusses the thermal treatments that led to the observed microstructures, and the possible sequence of precipitation that led to these microstructures. The thesis then describes the implications of microstructure on fatigue life and implications of tensile properties on fatigue life. Several additional experiments are then described that highlight possible causes for the observed dependence of microstructure on fatigue life, including fractographic evidence to provide support of microstructural dependencies.

Committee:

James Williams (Advisor); Hamish Fraser (Committee Member); Katharine Flores (Committee Member)

Subjects:

Materials Science

Keywords:

titanium; fatigue; beta alloy; microstructure; neural network; fracture; tensile properties; bayesian;

Bennett, Sara E.The Order Book, Order Flow, and the Impact of Order Cancellations on Equity Index Futures
PHD, Kent State University, 2012, College of Business Administration / Department of Finance

As exchanges have provided more transparency for their products, there has been increased interest in determining how order flow and the state of the order book impact order submissions and price discovery. Studies of order books have noted the presence of orders that are fleeting and might be classified as spoofing orders.

Spoofing is a microstructure based trading strategy which has recently been classified as prohibited under the Dodd-Frank Wall Street Reform and Consumer Protection Act (2010) (“Dodd-Frank Act”). Section 4c(a)(5) of the Commodity Exchange Act in section 747 of the Dodd-Frank Act specifically prohibits trading that “is, is of the character of, or commonly known as ‘spoofing’ (bidding or offering with the intent to cancel the bid or offer before execution).”

For example, a trader who places a spoofing order on the buy side will create the illusion of excess demand over supply by placing a large limit order on the bid side. The intent of the spoof order is for the trader to actually sell contracts at a higher price than is currently available. If successful, other traders will perceive that there is real information in this large limit order and trade ahead of it, thereby raising the price. This allows the spoofer to cancel the limit order and place a market sell order at a price that is at least one tick higher than the price at the time the spoofing order was submitted.

To date, there are only two other studies that specifically study spoofing orders. This dissertation is the only study of spoofing orders that examines spoofing orders from an order book perspective rather than examining individual traders’ order submission, cancellations, and trades. This dissertation utilizes limit order book data for European equity index futures to test for possible spoofing orders. Findings indicate that approximately 15% of large imbalances in DAX futures contracts result in a price change that could be indicative of a successful spoof order. On the other hand, only about 5% of large imbalances in DJ Euro STOXX 50 futures contracts end with a change in price that could indicate a successful spoofing order.

Committee:

Mark Holder (Committee Chair); Jayaram Muthuswamy (Committee Member); Eric Johnson (Committee Member)

Subjects:

Finance

Keywords:

order book; microstructure; spoofing; order imbalance; equity index futures

YAN, ZHONGYUENHANCED CRACK DETECTION BY COMBINATION OF LASER AND ULTRASONIC TECHNIQUES
PhD, University of Cincinnati, 2001, Engineering : Aerospace Engineering
Pulsed laser irradiation of the surface of a medium can produce temporally and spatially distributed thermal stresses and deformations that can be exploited for enhanced detection of small fatigue cracks. Two such enhancement methods were investigated in this research project. First, using long-pulse laser irradiation in combination with conventional Rayleigh wave inspection, direct generation of ultrasonic Rayleigh waves can be avoided and the relatively slow laser induced crack closure can be detected as aparametric modulation effect in a manner similar to the acousto-elastic effect often used in nonlinear ultrasonic studies. This technique is particularly well suited to distinguish small fatigue cracks from nearby scattering artifacts, such as machine marks, mechanical wear, corrosion pits, etc., that could otherwise overshadow the flaw. Second, short-pulse irradiation in the thermo-elastic region, that is routinely used to generate ultrasonic Rayleigh waves, can be substantially enhanced when the irradiated area contains near-surface discontinuities. Using an expanded laser beam, direct generation of ultrasonic surface waves from intact areas can be minimized and a significant increase in amplitude occurs when a discontinuity is present in the irradiated area. This technique is better suited to distinguish small fatigue cracks from distributed material noise caused by the surrounding inhomogeneous microstructure, such as coarse grains, grain colonies, precipitations, and anomalous phases. Numerous new experimental techniques combining laser irradiation witH ultrasonic detection methods have been developed based on these two inspection principles. During the development of these techniques, in order to better understand and optimize them, several simplified models were built, numerical simulations were performed, and related optical, thermal, mechanical, and acoustical phenomena were also investigated. They include: (i) temporal and spatial distributions of temperature and thermal stress in the specimens due to repetitive long-pulse laser irradiation, (ii) photo-thermo-elastic crack closure behavior in 3-D, (iii) thermally induced refraction effects on ultrasonic Rayleigh wave propagation, and (iv) relations of the enhanced laser generated ultrasonic Rayleigh waves (amplitude, spectrum, directivity) to discontinuity parameters. This dissertation contributes to three aspects of laser-ultrasonic nondestructive evaluation (NDE), namely new enhanced inspection techniques, experimental database, and original and improved analytical models.

Committee:

Dr. Peter B. Nagy (Advisor)

Subjects:

Engineering, Aerospace

Keywords:

Rayleigh wave; inhomogeneous microstructure

Makiewicz, Kurt TimothyDevelopment of Simultaneous Transformation Kinetics Microstructure Model with Application to Laser Metal Deposited Ti-6Al-4V and Alloy 718
Master of Science, The Ohio State University, 2013, Materials Science and Engineering
Laser based additive manufacturing has become an enabling joining process for making one-of-a-kind parts, as well as, repairing of aerospace components. Although, the process has been established for more than a decade, optimization of the process is still performed by trial and error experimentation. At the same time, deployment of integrated process-microstructure models has remained as a challenge due to some of the reasons listed below: (1) lack of good process models to consider the laser-material interactions; (2) inability to capture all the heat transfer boundary conditions; (3) thermo-physical-mechanical properties; and (4) robust material model. This work pertains to the development of robust material model for predicting microstructure evolution as a function of arbitrary thermal cycles (multiple heating and cooling cycles) that can be integrated into a process model. This study focuses on the development of a material model for Ti-6Al-4V and Alloy 718. These two alloys are heavily used in turbine engines and undergo complex phase transformations, making them suited to developing a material model for laser metal deposition (LMD). The model uses simultaneous transformation kinetics (STK) theory to predict the transformation of one parent phase into several products. The model uses calculated thermodynamic properties of the alloys for portions of the respective transformation characteristics. Being a phenomenological model there are several user defined calibration parameters to fit the predicted output to experimental data. These parameters modify the nucleation and growth kinetics of the individual transformations. Analyses of experimental LMD builds are used to calibrate the material model. A Ti-6Al-4V build made on a room temperature substrate showed primarily colony alpha morphology in the bottom half of the build with a transition to basketweave alpha in the top half. An increase in hardness corresponding to the microstructural transition was observed. This sample had an average of 340 HV hardness. Analysis of the calculated thermal profiles at the location of the morphology transition showed a transition from cooling below the beta transus to cooling above the beta transus. The Ti-6Al-4V STK model was calibrated using the experimental data from this sample. The substrate of a second build was heated above the Ti-6Al-4V beta transus. This build showed predominantly basketweave alpha without a microstructural transition. Large prior beta grains (>1mm) were observed growing epitaxially from the substrate. These large grains promoted the basketweave formation. Hardness testing showed an average of 344 HV. Samples built in this way were also fatigue tested in the as built condition. Results show that they match previous builds that had been stress relieved. A third build was performed at room temperature on a substrate with large prior beta grains. This build showed basketweave morphology like the second build even though the substrate was not thermally controlled. The hardness for this build averaged 396 HV which is ~50 HV higher than the previous two. This build shows that it may be possible to produce better mechanical properties by controlling the beta grain size rather than heating the substrate. Eighteen Alloy 718 builds were made using proprietary processing conditions. All of these builds were analyzed for nano-scale γ’ and γ’’ precipitates. Two of the builds were similar but had different laser powers. The low laser power build did not show nano-scale precipitates. The higher power build did show small amounts (<3%) of nano-scale precipitates and a corresponding increase in hardness at their locations. The higher power build was used to develop the STK model for Alloy 718. Sixteen of these builds were part of a design of experiments and are referred to as DOE samples. Eight of them have a single layer while the other eight have multiple layers. They were examined for nano-scale precipitates. The amounts of precipitates were correlated to hardness values and thermal profiles.

Committee:

Sudarsanam Babu (Advisor); Wolfgang Windl (Committee Member)

Subjects:

Aerospace Materials; Materials Science; Metallurgy

Keywords:

Simultaneous Transformation Kinetics; STK; Microstructure Modeling; Laser Additive Manufacturing; Laser Metal Deposition; aerospace repair; Ti-6Al-4V; Inconel 718; Alloy 718; Additive Manufacturing; LAM; LMD;

Bathini, UdaykarA Study of Microstructure, Tensile Deformation, Cyclic Fatigue and Final Fracture Behavior of Commercially Pure Titanium and a Titanium Alloy
Master of Science in Engineering, University of Akron, 2010, Civil Engineering

Rapid industrial growth and advances in the domains of engineering and related technologies during the last fifty years have led to the extensive use of traditional metals and their alloy counterparts. Titanium is one such metal which has gained wide popularity in the aerospace and defense related applications owing to a wide range of impressive mechanical properties like excellent specific strength (σUTS/ρ), stiffness, corrosion and erosion resistance, fracture toughness and capability to withstand significant temperature variations.

Two materials, namely commercial purity titanium (Grade 2), referred to henceforth as Ti- CP (Grade 2) and the “work-horse” alloy Ti-6Al-4V have been chosen for this research study. The intrinsic influence of material composition and test specimen orientation on the tensile and fatigue behavior for both Ti- CP (Grade 2) and Ti-6Al-4V have been discussed. Samples of both Ti- CP (Grade 2) and Ti-6Al-4V were prepared from the as-provided plate stock along both the longitudinal and transverse orientations. The specimens were then deformed to failure in uniaxial tension for the tensile tests and cyclically deformed at different values of maximum stress at constant load ratio of 0.1 for the high cycle fatigue tests. The microstructure, tensile properties, resultant fracture behavior of the two materials is presented in the light of results obtained from the uniaxial tensile tests. The conjoint influence of intrinsic microstructural features, nature of loading and specimen properties on the tensile properties is discussed. Also, the macroscopic fracture mode, the intrinsic features on the fatigue fracture surface and the role of applied stress-microstructural feature interactions in governing failure for the cyclic fatigue properties for both the materials under study Ti- CP (Grade 2) and the “work-horse” alloy Ti-6Al-4V have been discussed in detail.

Careful study of the microstructure for Ti-CP (Grade 2) material at a low magnification revealed the primary alpha (α) grains to be intermingled with small pockets of beta (β) grains. Observation at the higher allowable magnifications of the optical microscope revealed very fine alpha (α) phase lamellae located within the beta (β) grain. The microhardness and macrohardness measurements were consistent through the sheet specimen for Ti- CP (Grade 2) and slightly lower compared to Ti-6Al-4V. However, the macrohardness was marginally higher than the microhardness resulting from the presence of a large volume fraction of the soft alpha phase. The hardness values when plotted reveal marginal spatial variability. Tensile fracture of Ti-CP (Grade 2) was at an inclination to the far field tensile stress axis for both longitudinal and transverse orientations. The overload region revealed a combination of fine microscopic cracks, microscopic voids of varying size and randomly distributed through the surface, and a large population of shallow dimples, features reminiscent of locally brittle and ductile failure mechanisms. The maximum stress (σmaximum) versus fatigue life (Nf) characteristics shown by this material is quite different from those non-ferrous metals that exhibit a well-defined endurance limit. When compared at equal values of maximum stress at a load ratio of 0.1, the fatigue life of the transverse specimen is noticeably greater than the longitudinal counterpart. At equivalent values of maximum elastic strain, the transverse specimens revealed noticeably improved fatigue life as compared one-on-one to the longitudinal counterparts.

Careful observations of the Ti-6Al-4V alloy microstructure over a range of magnifications spanning very low to high magnification revealed a duplex microstructure consisting of the near equiaxed alpha (α) and transformed beta (β) phases. The primary near equiaxed shaped alpha (α) grains (light in color) was well distributed in a lamellar matrix with transformed beta (dark in color). The microhardness and macrohardness values recorded for the Ti-6Al-4V alloy reveal it to be harder than the commercially pure (Grade 2) material. However, for the Ti-6Al-4V alloy the microhardness is noticeably higher than the corresponding macrohardness value that can be ascribed to the presence of a population of processing-related artifacts and the hard beta-phase. Tensile fracture of the Ti-6Al-4V alloy was macroscopically rough and essentially normal to the far field stress axis for the longitudinal orientation and cup-and-cone morphology for the transverse orientation. However, microscopically, the surface was rough and covered with a population of macroscopic and fine microscopic cracks, voids of varying size, a population of shallow dimples of varying size and shape, features reminiscent of locally brittle and ductile failure mechanisms. When compared at equal values of maximum stress at a load ratio of 0.1, there is a marginal to no influence of microstructure on high cycle fatigue life of both orientations of the alloy.

Committee:

Anil Patnaik, Dr. (Advisor)

Subjects:

Civil Engineering; Materials Science

Keywords:

Titanium Alloy; Commercially Pure Titanium; Microstructure, Tensile Properties; Tensile Fracture; Titanium; Materials tests; Cyclic tests; Fatigue life; Fracture

Corwin, Peter ESynthesis and Characterization of Titanium Zirconium Based Alloys for Capacitor Use
Master of Sciences (Engineering), Case Western Reserve University, 2013, Materials Science and Engineering
This work seeks to characterize a series of Ti-Zr binary alloys and a ternary Ti-Zr-Be alloy as anode materials for electrolytic capacitors. Specifically, this work attempts to improve the electrical behavior by reducing leakage through anodically grown oxide film on binary alloys via microstructure modifications. It also examines creep of Ti25Zr38Be37 metallic glass for surface enhancement. A variety of mechanical and electrical properties were measured. Leakage current through the anodic dielectric was found to decrease with increasing zirconium content. Alloy treatments which produce concentration gradients (a, a+ß fields) increased dielectric leakage; treatments maintaining uniform composition (as cast, wrought) retain low dielectric leakage current. UTS was fit to a solid solution model which compares well with related literature. Estimates of viscosity for Ti25Zr38Be37 metallic glass were made at various temperatures and resulting surface texturing examined. Long term creep rate change was fit to exponential decay with a time constant of 7.9x103s.

Committee:

Gerhard Welsch (Advisor); John Lewandowski (Committee Member); Mark DeGuire (Committee Member)

Subjects:

Materials Science; Metallurgy

Keywords:

Titanium; Zirconium; Beryllium; Ti; Zr; Be; Metallic Glass; Creep; Leakage; Microstructure

Harper, Megan LynnA study of the microstructural and phase evolutions in TIMETAL 555
Master of Science, The Ohio State University, 2004, Materials Science and Engineering
This study focuses on the evolution of the microstructure in TIMETAL 555. This alloy (Ti-5Al-5Mo-5V-3Cr-0.5Fe wt%) has been recently developed based on the Russian alloy VT-22 and is anticipated for use in aerospace landing gear applications. Due to TIMETAL 555's recent development, there is little information available about its microstructural evolution or response to heat treatment. In this study, the effect of cooling rate, single step, duplex, and triplex heat treatments are used to learn about the ways in which the microstructure forms. In addition, more detailed analysis of the nucleation of a phase during fast and slow cooling was performed. It has been shown that at TΒ + 50° (TΒ=860°C),at least 20 minutes is required to fully recrystallize the microstructure and eliminate any polygonization. Upon fast cooling from above TΒ, athermal ω phase is present as a precipitate in the beta phase at the nanometer scale. Upon further aging, it is shown that fine alpha laths nucleate directly from the ω particles. These laths are relatively short in their elongated direction. These results were obtained through TEM diffraction patterns and dark field images. Slow cooling allows time for nucleation of coarse alpha laths at grain boundaries and at intra-granular sites. The laths formed during slow cooling are highly elongated compared to the ω nucleated α. Micrographs are presented for a number of different multi-step heat treatments. These microstructures may contain globular alpha, if treated in the α/Β phase field, along with combinations of coarse and fine alpha laths depending on the combinations of cooling rates and aging temperatures. Room temperature tensile test results for a number of microstructures are also reported. Optimal property combinations are in the range of 170 ksi yield strength and 10% elongation.

Committee:

Hamish Fraser (Advisor)

Keywords:

heat treatment; laths; alloy; Photomicrograph; microstructure

Phillips, Peter LouisStudy of 2.5D Microstructural Modeling Techniques Used for Material Property Identification
Master of Science (M.S.), University of Dayton, 2009, Mechanical Engineering
Advances in digital image correlation (DIC) techniques allow for the study of full-field surface deformations on a microscopic scale in metal alloys. Finite element-based microstructural models can be used for property identification using numerical optimization techniques. However, the exact microstructure in the interior of the specimen is not known without destroying the material, and DIC data is available only from the visible surface for comparison. An analytical methodology has been investigated that allows for the in-situ identification of orthotropic materials properties on a microstructural level in metallic materials using finite element analysis. A study has been conducted to determine the effects that various subsurface microstructural finite element (FE) modeling techniques have on the reduction of displacement errors on the surface of microstructural models. Multiple FE models with varying geometries and properties through the thickness (2.5D Models) are developed using surface geometry from a FE model created using known three-dimensional geometry through the thickness. Displacement errors are compared and an optimization procedure has been utilized to identify the linear elastic orthotropic material properties for various 2.5D models to determine which techniques are best suited for accurate material property identification. Using the recommended modeling techniques, relative errors in the surface displacements are on the order of three to six percent, and typical errors in the estimated material properties for a linear elastic orthotropic material are approximately one to five percent.

Committee:

Robert A. Brockman, PhD (Advisor); Steven L. Donaldson, PhD (Committee Member); James M. Whitney, PhD (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

microstructure; finite element; property identification; 2.5D modeling; optimization; digitial image correlation

Payton, Eric JohnCharacterization and Modeling of Grain Coarsening in Powder Metallurgical Nickel-Based Superalloys
Doctor of Philosophy, The Ohio State University, 2009, Materials Science and Engineering
Accurate prediction of grain size as a function of processing conditions is highly sought after in many advanced alloy systems because specific grain sizes must be obtained to meet mechanical property requirements. In powder metallurgical nickel-based superalloys for turbine disk applications, physics-based modeling of grain coarsening is needed to accelerate alloy and process development and to meet demands for higher jet engine operating temperatures. Materials characterization and simulation techniques were integrated and applied simultaneously to enable quantitative representation of the microstructure, to clarify experimental results, and to validate mean-field descriptions of microstructural evolution. The key parameters controlling grain coarsening behavior were identified. A statistical-analytical mean-field model of grain coarsening with an adaptive spatio-temporal mesh was developed to enable rapid physics-based simulation of microstructural evolution. Experimental results were used as initial conditions and the model was then evaluated in the context of experimental results. Deviations of model predictions from experimental observations were then used to recommend future work to resolve remaining issues related to the microstructral evolution of powder metallurgical nickel-based superalloys, the mean-field modeling of microstructural evolution, and the quantitative characterization of materials.

Committee:

Michael Mills, PhD (Advisor); Yunzhi Wang, PhD (Committee Member); Hamish Fraser, PhD (Committee Member)

Subjects:

Aerospace Materials; Engineering; Materials Science; Metallurgy

Keywords:

physical metallurgy; microstructure; characterization; segmentation; Monte Carlo; coarsening; dissolution; grain growth; superalloy; microscopy; engineering; materials; mean field; gamma prime; nickel; backscatter; diffraction

Thomas, Joshua MichaelSimulating the mechanical response of titanium alloys through the crystal plasticity finite element analysis of image-based synthetic microstructures
Master of Science, The Ohio State University, 2012, Mechanical Engineering
Micromechanical crystal plasticity finite element simulations of the response of synthetic titanium microstructures are carried out with the goal of quantifying the effect of microstructure on mechanical properties. Two separate materials are studied: (1) an alpha-beta Ti-6Al-4V material and (2) a highly-textured, rolled alpha Ti-3Al-2.5V sheet material. Performing accurate finite element analyses begins with accurate image-based characterization of the morphological and crystallographic features of the materials at the microstructural scale. Then, statistically equivalent representative 3D microstructures are built and meshes are generated for crystal plasticity based finite element method (CPFEM) analysis. For the Ti-3Al-2.5V material, experimental results from the displacement controlled mechanical testing of dog bone shaped, rolled specimens are used for the calibration of elastic parameters as well as anisotropic crystal plasticity parameters. The inspection of micrographs of the rolled material showed elongated grain shapes which led to the updating of the crystal plasticity model to include grain aspect ratio dependence on the Hall-Petch size effect--an update of a previous size effect model which assumed spherical grains. Model validation is achieved by comparing load controlled experimental results with simulated creep results. For the Ti-6Al-4V material, the robust and validated analysis tool is used to perform sensitivity analyses and a quantitative understanding of how individual microstructural parameters affect the mechanical response properties of the alloy is developed. Functional dependencies are proposed that directly connect the metal's microstructural features to creep response, yield strength response, and tensile response.

Committee:

Somnath Ghosh, Dr. (Advisor); June Lee, Dr. (Committee Member); Reji John, Dr. (Committee Member)

Subjects:

Aerospace Materials; Engineering; Materials Science; Mechanical Engineering; Mechanics

Keywords:

finite element method; crystal plasticity; EBSD; mechanical characterization; microanalysis; micromechanics; titanium alloys; hardening; creep; microstructure-property relationships; CPFEM

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