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 res (open full item for complete abstract)

Committee: James Williams (Advisor) Subjects: Textile Technology

Master of Science, Miami University, 2018, Mechanical and Manufacturing Engineering

Titanium alloy (Ti-6Al-4V) has become an important choice of material for aerospace, automotive, and biomedical applications for its high strength-to-weight ratio and excellent corrosion resistance. However, machining of titanium alloy is still considered to be one of the major challenges to the industries. The objective of this study is to focus on two important aspects of titanium machining: tool wear and microstructural changes in the tool, workpiece, and chips during both conventional and sustainable machining processes. A series of experiments were conducted using end milling of Ti-6Al-4V with three different conditions namely dry, flood coolant, and minimum quantity lubrication (MQL). Both uncoated and titanium aluminum nitride (TiAlN)-coated carbide tools were used for machining Ti-6Al-4V at the same parameter settings. It was observed that abrasion was the dominant tool wear mechanism for all dry, flood coolant and MQL machining conditions. Adhesion and chipping were found to be two significantly dominant modes of tool failure in dry machining. The MQL machining conditions resulted in fewer occurrences of tool wear in most of the studied wear mechanisms. The chips were found to be serrated in nature regardless of the machining conditions and underwent microstructural and phase changes due to the machining conditions. The microstructure of Ti-6Al-4V workpiece revealed deformation at the subsurface of the machined face and the changes MQL machining were such that they enhance the material properties. The microstructure of the uncoated cutting tool revealed deformation near the cutting edges, and the microstructure of TiAlN-coated tool showed depletion in the coating layer from the cutting edges. Overall MQL machining condition proved to be better with respect to tool wear occurrences and machining performance.

Committee: Muhammad Jahan (Advisor); Corti Giancarlo (Committee Member); Khan Fazeel (Committee Member) Subjects: Mechanical Engineering

Master of Science in Mechanical Engineering (MSME), Wright State University, 2016, Mechanical Engineering

Ambient temperature dwell sensitivity is known to be deleterious to the fatigue response of near-alpha titanium alloys. Dwell fatigue refers to the presence of a sustained hold at peak stress as opposed to the continuous variation of normal cyclic fatigue loading. This reduction in failure life-times from dwell loading is attributed to early crack nucleation and faster crack propagation. The degradation is the result of plastic anisotropy on the microstructural scale along with tendency of titanium alloys to creep at low temperatures at stresses well below the 0.2% offset yield strength. Despite being the most widely used titanium alloy, Ti-6Al-4V has not been the subject of most dwell fatigue research. Generally, dwell sensitivity is microstructurally dependent and believed to only affect Ti-6Al-4V when severe crystallographic texture is present and under high peak stress loading. Recent studies, however, have suggested that small clusters of preferred crystal orientations, known as micro-textured regions (MTR), can have a significant effect on the dwell sensitivities in Ti-6Al-4V even without severe overall texture in the material. In this study, smooth-bar fatigue specimens were subjected to uniaxial fatigue at 20 Hz cyclic and 2-min dwell loading conditions under load-control at stresses representative of service conditions, until failure occurred. A reduction in specimen life-times by approximately a factor of three was observed under dwell conditions, which was less than for the highly susceptible near-a titanium alloys such as Ti-6Al-2Sn-4Zr-2Mo, where the dwell debit is often in excess of a factor of ten. Measurement of fatigue and dwell fatigue crack growth rates revealed a significant acceleration of the dwell crack growth rates in certain cases. Backscattered electron imaging and electron backscattered diffraction were utilized to quantify the interaction between the cracks and local microstructure. Though no correlation was found between crack growth (open full item for complete abstract)

Committee: Raghavan Srinivasan Ph.D. (Advisor); Adam Pilchak Ph.D. (Committee Member); Joy Gockel Ph.D. (Committee Member) Subjects: Materials Science; Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 2014, Materials Science and Engineering

The microstructural sources of fatigue lifetime variability were investigated for four different microstructural variations of Ti-6Al-4V. Specimens were tested at lower stresses to investigate the behavior in the HCF (high cycle fatigue) regime, which is characterized by lifetimes near or in excess of 10^6 cycles. Fractography and replication analyses confirmed that the lifetime was dominated by crack nucleation, and thus variations in the lifetime between individual test specimens are primarily attributed to variability in the time to nucleate a dominant crack. Stereology was used to quantify key microstructural features for each tested specimen. These values were used as inputs for a series of microstructurally-based fuzzy logic neural network models. Using these models, virtual experiments were conducted to investigate the individual effect of each microstructural feature on the lifetime, an investigation that is impossible to conduct empirically because of the complex microstructure in these alloy systems. These virtual experiments demonstrated that colony size and alath thickness have the greatest effect on HCF lifetime of ß-processed Ti-6Al-4V alloys, and that colony size is more important that a lath thickness. For the a/ß – processed microstructures, the volume fraction of primary a and the a lath thickness were shown to affect the lifetime, while the ap grain size was not. Defect analyses of failed specimens indicated that damage accumulation is often localized during cyclic loading, with dislocation densities varying from one a lath to another. For all specimens, a-type dislocations are seen and c+a - type dislocations were observed only in regions of localized plastic strain. Investigation of site-specific TEM foils extracted from the crack nucleation region of a/ß – processed specimens provided information about the nature and behavior of dislocations during the crack nucleation event. A comparison of short- and long- life specimens provide (open full item for complete abstract)

Committee: Hamish Fraser PhD (Advisor); Michael Mills PhD (Committee Member); Stephen Niezgoda PhD (Committee Member) Subjects: Aerospace Materials; Engineering; Materials Science

Master of Science, The Ohio State University, 0, Materials Science and Engineering

The use of quantified microstructures as inputs to neural network models for property prediction has been pioneered by Center for the Accelerated Maturation of Materials (CAMM) at The Ohio State University. Through microstructure-property correlations, neural network models provide predictive tools for mechanical properties in titanium alloys while concurrently developing phenomenological models. The output accuracy (mechanical property prediction) of such models is therefore dependent on the variance and distribution of the input data (quantified microstructures). An estimation of the true variance and distribution can be calculated if a sufficiently large sampling volume of quantifiable microstructural features is available; however, current manual image processing and segmentation techniques made attainment of large-dataset image-processing unfeasible. In this work, a new generation of automated tools has been developed by CAMM which have reduced the total analysis time, including image capture, processing, and characterization to less than 30 seconds per micrograph for optically captured micrographs. Using a comparable SEM-based technique requires less than 6 minutes per micrograph due to extended image capture times. Serial sectioning of a meso-scale 3D volume (mm3) of α+β processed Ti-6Al-4V was collected for direct 3D quantification. Images were captured two ways: (1) using a Leica optical microscope in conjunction with Clemex image analysis software and (2) using a FEI Sirion SEM. In both cases, CAMM developed image processing package MIPAR was used to calculate the spatial variation in globular a area fraction. Comparisons between the two image capturing methods reveal similar trends in spatial variation indicating SEM-based imaging is only necessary if required by the scale of the particular microstructural feature of interest. A total of 37,800 micrographs were captured and processed. The large number of micrographs allows for accurate quantifica (open full item for complete abstract)

Committee: Hamish Fraser (Advisor); William Clark (Committee Member) Subjects: Engineering; Materials Science; Metallurgy

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

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. (open full item for complete abstract)

Committee: Somnath Ghosh PhD (Advisor); June K. Lee PhD (Other) Subjects: Engineering; Materials Science; Mechanical Engineering

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 (open full item for complete abstract)

Committee: James Williams (Advisor); Hamish Fraser (Committee Member); Katharine Flores (Committee Member) Subjects: Materials Science

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 (open full item for complete abstract)

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

Doctor of Philosophy, The Ohio State University, 2009, Materials Science and Engineering

Composite α/β stereographic projections and slip system misorientation diagrams were developed and used to establish a new understanding regarding anisotropic deformation behavior of alpha and beta phases aligned according to the Burgers orientation relationship. Application of these tools showed that a 2-fold, maximum common crystal symmetry existed between single variants of Burgers oriented alpha and beta phases, which placed specific requirements on alignment and response of mating slip systems in alpha and beta phases. Of particular import were implications for crystallographic elements of mating slip systems that were found to be oriented within 180° of each other, i.e. anisotropic deformation behavior was predicted according to requirements of 2-fold maximum common crystal symmetry. Inspection of misorientation between mating slip systems established new requirements for determining the breadth of anisotropic slip behavior for a-basal, a-prism, a-pyramidal and c+a pyramidal slip systems. It was shown testing at 3, 6, 6, and 12 maximum Schmid factor loading axes was required to fully measure anisotropic deformation behavior in these slip systems, respectively. This meant each mated slip system would have 1, 2, 2 and 4 unique responses when loaded at orientations of maximum Schmid factor on the alpha slip system. The root cause of these anisotropic responses was shown to result from large changes in Schmid factor on beta slip systems when loading conditions on alpha slip systems were held constant, i.e. at maximum Schmid factor. Changes in Schmid factor on mating beta slip systems were shown to be a natural consequence of differences in crystal symmetry between alpha and beta phases oriented according to the Burgers orientation relationship. Extension of this information to titanium microstructures consisting of multiple alpha variants in a single beta grain showed that 144 possible combinations of two adjacent alpha variants could be grouped according to six c (open full item for complete abstract)

Committee: Prof. Hamish Fraser (Advisor); Prof. James Williams (Committee Member); Prof. Michael Mills (Committee Member) Subjects: Materials Science

Master of Science, The Ohio State University, 2009, Welding Engineering

Friction stir processing (FSP) can be employed to modify the grain size and microstructure of a material. In titanium alloys, the refined microstructure achieved during processing can improve the mechanical properties, such as yield stress and fatigue crack initiation resistance. Documenting the microstructural evolution of Ti-5111 (5Al-1Sn-1Zr-1V-0.8Mo) during FSP, as well as simulating the observed microstructure in a Gleeble® 3800 thermo-mechanical simulator can determine the link between strain, strain rate and temperature during processing. In this study, FSP of Ti-5111 was performed above and below the beta transus temperature allowing for investigation of the microstructural evolution in both conditions. Each processed panel was instrumented with thermocouples to record the thermal histories in the stir zone and adjacent heat-affected zone. Single sensor differential thermal analysis (SS-DTA) was used to determine the alpha-beta transformation during processing. Transverse sections of the processed panels were analyzed using optical and scanning electron microscopy, electron backscatter diffraction (EBSD) and hardness mapping. FSP produced extreme grain refinement in both processing conditions – reducing the 200-500 micron prior-beta base material grains to 1-20 microns. The stir zone in the panel processed above the transus exhibited a strong transformation microtexture, governed by the Burgers orientation relationship, while the sub-transus panel displayed a shear deformation texture. Vicker's hardness mapping revealed two distinct hardness regions: the base material and a more uniform and slightly harder stir zone. The microstructures observed in the FSP panels were simulated using hot torsion testing on a Gleeble® 3800. Ideally, the strain and strain rate data may be used to verify FSP modeling programs of titanium to reduce the parameter selection phases of future friction stir projects. However, strain localization observed during hot torsion testing (open full item for complete abstract)

Committee: John Lippold PhD (Advisor); Mary Juhas PhD (Advisor); Jim Williams PhD (Committee Chair) Subjects: Engineering; Materials Science; Metallurgy

Master of Science, The Ohio State University, 2008, Materials Science and Engineering

Titanium and its alloys are comparatively recent newcomers to the metallurgical market. They are gaining widespread acceptance for use in the recreational, aerospace, biomedical, petro-chemical, and commercial processing industries due to their combination of unique and advantageous properties, including high strength, low density, and superior corrosion resistance to most aggressive agents. The material properties of titanium and its alloys can be optimized and tailored by engineering the microstructure via control of chemistry, processing route, and heat treatment. The morphology of the two crystallographic allotropic phases can be manipulated to refine the structure and produce desirable mechanical property combinations. Microstructural constitution of the titanium alloys is classified according to the dominant phase within the alloy; alpha + beta (α + β) titanium alloys are the most widely used alloys. The temperature of the final heat treatment of the α/β components is governed by the service requirements. In order to evaluate the behavior of these alloys for future applications, it is imperative that the microstructural features and characteristics be quantified and examined on a spatial dimension. The Robo-Met.3D is a high precision robotic serial sectioning device that can fulfill this need. Initially, several months were spent resolving problems with the functioning of the Robo.Met.3D. Two-dimensional (2-D) stereology was done on Timetal 550 using automated batch processing with Adobe Photoshop and Fovea Pro. Images from different locations on the gage were obtained and compared. Final data demonstrated quantitative differences which were the result of the heat treatment. Discrepancies and inconsistencies in the data were identified as limiting factors in the reproducibility of the procedure in future work. Serial sectioning using focused ion beam (FIB) was performed using Timetal 550, and three-dimensional (3-D) reconstruction was done using IMOD. Robo-Me (open full item for complete abstract)

Committee: Hamish Fraser (Advisor); Yunzhi Wang (Committee Member) Subjects: Materials Science

Doctor of Philosophy, The Ohio State University, 2005, Materials Science and Engineering

The accelerated insertion of titanium alloys in component application requires the development of predictive capabilities for various aspects of their behavior, for example, phase stability, microstructural evolution and property-microstructure relationships over a wide range of length and time scales. In this presentation some novel aspects of property-microstructure relationships and microstructural evolution in alpha/beta Ti alloys will be discussed. Neural Network (NN) Models based on a Bayesian framework have been developed to predict the mechanical properties of alpha/beta Ti alloys. The development of such rules-based model requires the population of extensive databases, which in the present case are microstructurally-based. The steps involved in database development include producing controlled variations of the microstructure using novel approaches to heat-treatments, the use of standardized stereology protocols to characterize and quantify microstructural features rapidly, and mechanical testing of the heat-treated specimens. These databases have been used to train and test NN Models for prediction of mechanical properties. In addition, these models have been used to identify the influence of individual microstructural features on the mechanical properties, consequently guiding the efforts towards development of more robust mechanistically based models. In order to understand the property-microstructure relationships, a detailed understanding of microstructure evolution is imperative. The crystallography of the microstructure developing as a result of the solid-state beta to beta + alpha transformation has been studied in detail by employing Scanning Electron Microscopy (SEM), Orientation Imaging Microscopy (in a high resolution SEM), site-specific TEM sample preparation using focused ion beam, and TEM based techniques. The influence of variant selection on the evolution of microstructure will be specifically addressed.

Committee: Hamish Fraser (Advisor) Subjects:

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

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 re (open full item for complete abstract)

Committee: Anil Patnaik Dr. (Advisor) Subjects: Civil Engineering; Materials Science

Master of Science, University of Akron, 2006, Mechanical Engineering

In this study, two novel ceramics, namely (a) titanium boride (TiB2) with boron carbide (B4C) and (b) hafnium boride (HfB2), were consolidated to near theoretical density using the rapid consolidation technique of plasma pressure compaction. Consolidation time, temperature and pressure of compaction exert an influence on microstructure, hardness, and the presence of internal defects in the ceramic. The microstructure of the ceramic sample, in the as-consolidated condition, was characterized by both optical microscopy and scanning electron microscopy observations. The nature, morphology and distribution of intrinsic microstructural features, processing-related artifacts and other microstructural defects are presented, and their influence on microhardness of the ceramic samples is highlighted. Image analyses of the as consolidated samples was done to determine the volume fraction of the each of the constituents with the intent of establishing the influence of sintering parameters on size, distribution, volume fraction of the constituent phases, and grain size. The magnitude and severity of residual stress induced and present in the microstructure of the Titanium Boride (TiB2)-Boron Carbide (B4C) composite upon cooling from the consolidation temperature was determined using finite element analysis. The stress levels were found to be high enough for the initiation of fine microscopic cracks. A final correlation between temperature of consolidation, composition and time for obtaining optimum favorable mechanical properties was found out. The hafnium boride powder was consolidated with different amount of carbon. The presence of carbon and its influence on microstructural development, grain size and hardness was determined and is presented and discussed. The optimum composition of carbon required for maximum density and hardness was found and reported.

Committee: Tirumalai Srivatsan (Advisor) Subjects: Engineering, Materials Science

Doctor of Philosophy, The Ohio State University, 2013, Materials Science and Engineering

Due to a great combination of physical and mechanical properties, beta titanium alloys have become promising candidates in the field of chemical industry, aerospace and biomedical materials. The microstructure of beta titanium alloys is the governing factor that determines their properties and performances, especially the size scale, distribution and volume fraction of precipitate phase in parent phase matrix. Therefore in order to enhance the performance of beta titanium alloys, it is critical to obtain a thorough understanding of microstructural evolution in beta titanium alloys upon various thermal and/or mechanical processes. The present work is focusing on the study of nucleation mechanisms of refined alpha microstructure and super-refined alpha microstructure in beta titanium alloys in order to study the influence of instabilities within parent phase matrix on precipitates nucleation, including compositional instabilities and/or structural instabilities. The current study is primarily conducted in Ti-5Al-5Mo-5V-3Cr (wt%, Ti-5553), a commercial material for aerospace application. Refined and super-refined precipitates microstructure in Ti-5553 are obtained under specific accurate temperature controlled heat treatments. The characteristics of either microstructure are investigated in details using various characterization techniques, such as SEM, TEM, STEM, HRSTEM and 3D atom probe to describe the features of microstructure in the aspect of morphology, distribution, structure and composition. Nucleation mechanisms of refined and super-refined precipitates are proposed in order to fully explain the features of different precipitates microstructure in Ti-5553. The necessary thermodynamic conditions and detailed process of phase transformations are introduced. In order to verify the reliability of proposed nucleation mechanisms, thermodynamic calculation and phase field modeling simulation are accomplished using the database of simple binary Ti-Mo system. There (open full item for complete abstract)

Committee: Hamish Fraser (Advisor); Yunzhi Wang (Committee Member); Michael Mills (Committee Member) Subjects: Materials Science

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