Master of Science in Materials Science and Engineering (MSMSE), Wright State University, 2018, Materials Science and Engineering

Laser based additive manufacturing of Ti-6Al-4V components is under consideration for aerospace applications. The mechanical properties of the finished components depend on their microstructure. Process mapping compares process variables such as heat source power, heat source travel speed, material feed rate, part preheat temperature and feature geometry to process outcomes such as microstructure, melt pool geometry and residual stresses. In this work, the microstructure of two-dimensional pads, multilayer pads, thin walls, and structural components at the steady state location was observed. A method for measuring β grain widths that allows for the calculation of standard deviations, confidence intervals, and variances in grain size was developed. This represents an improvement over the commonly used line-intercept method. The method was used to compare variability of β grain widths across different part geometries. It was found that thin wall parts have the highest β width variability and that the width of the β grains varies more towards the top of multi-layered samples than towards the bottom. Experimental results for α and β grain size across multiple deposit geometries are presented that offer new insight into the effect of process variables on microstructure. β grain widths are also compared for different deposit geometries with the same power, velocity, and feed rate. Single layer pad geometries were found to have the smallest β grain widths, multi-layer pads had larger β grain widths, and thin wall samples had the largest β grain widths. Trends in α width with Vickers hardness were also considered in the context of thermal gradient measurements. Hardness maps were created for the structural component samples. Optical microscopy was used to observe a layering effect in structural component samples. It was found that light and dark bands had different Vickers microhardness values.

Committee: Nathan Klingbeil Ph.D. (Advisor); Joy Gockel Ph.D. (Committee Member); Raghavan Srinivasan Ph.D. (Committee Member) Subjects: Engineering; Materials Science; Mechanical Engineering; Metallurgy

Master of Science in Mechanical Engineering, Cleveland State University, 2018, Washkewicz College of Engineering

In metallurgy, Titanium has been a staple for biomedical purposes. Its low toxicity and alloying versatility make it an attractive choice for medical applications. However, studies have shown the difference in elastic modulus between Titanium alloys (116 GPa) and human bone (40-60 GPa) contribute to long term issues with loose hardware fixation. Additionally, long term studies have shown elements such as Vanadium and Aluminum, which are commonly used in Ti-6Al-4V biomedical alloys, have been linked to neurodegenerative diseases like Alzheimers and Parkinsons. Alternative metals known to be less toxic are being explored as replacements for alloying elements in Titanium alloys. This research will focus on advanced processing and characterization of beta-phase Titanium alloys for biomedical applications. The microstructure, mechanical and electrochemical properties of these alloys have been analyzed and compared with C.P. Titanium. The main objective is to study the effect of different alloying elements on microstructure, phase transformation and mechanical properties of these newly developed beta-phase Titanium alloys and establish new avenues for the future development of biocompatible Titanium alloys with optimum microstructure and properties.

Committee: Tushar Borkar Ph.D (Committee Chair); Taysir Nayfeh Ph.D (Committee Member); Jason Halloran Ph.D (Committee Member) Subjects: Biomedical Research; Design; Materials Science; Mechanical Engineering

MS, University of Cincinnati, 2016, Engineering and Applied Science: Materials Science

In this study, the effects of Ultrasonic Nanocrystal Surface Modification (UNSM) on residual stress, near surface microstructure, hardness, high cycle fatigue, biocompatibility and corrosion behaviour of a low-modulus beta Ti-35Nb-7Zr-5Ta-0.3O (wt %) was studied. The UNSM is novel mechanical surface treatment which effectively improves mechanical properties, fatigue life and wear of engineering components. UNSM causes severe plastic deformation on the surface, thus inducing deep compressive stresses and a surface nano-crystalline layer in the component which improves hardness, yield strength and fatigue life. At first, the as-received specimens were solution treated at 850º C for 1 hour and water quenched to obtain a single phase ß structure. The solution treated specimens were then subjected to UNSM treatment with two different static (20N and 50N) and dynamic loads (20% and 50%). The microstructure after UNSM was characterized by optical, scanning electron microscope (SEM), Electron Backscattered Diffraction (EBSD) and transmission electron Microscopy (TEM). Nanoindentation test was also performed to determine local properties like hardness with distance from the treated surface. The UNSM treated specimen induces compressive residual stresses as high as -1600 MPa and shows significant increase in surface hardness from 4.5 GPa to 6 GPa. The residual compressive stresses and hardness increases with increase in static load. The severe plastic deformation caused by UNSM produces nanocrystalline layer of about 1 µm from the treated surface and a gradient microstructure of deformation bands and high dislocation density which was confirmed by transmission electron microscopy. The deformation mechanism after UNSM was also studied. The deformation mechanism in this alloy is dominated by dislocation movement and occurrence of deformation bands with high dislocation density. Three-point bending fatigue tests were also performed to study improvement in fatigue life (open full item for complete abstract)

Committee: Vijay Vasudevan Ph.D. (Committee Chair); Ashley Paz y Puente Ph.D. (Committee Member); Rodney Roseman Ph.D. (Committee Member) Subjects: Materials Science

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

One of the greatest challenges in additive manufacturing is fabricating titanium structures with consistent and desirable microstructure. To date, fully columnar deposits have been achieved through direct control of process variables. However, the introduction of external factors appears necessary to achieve fully equiaxed grain morphology using existing commercial processes. This work introduces and employs an analytic model to relate process variables to solidification thermal conditions and expected beta grain morphology at the surface of and at the deepest point in the melt pool. The latter is required in order to ensure the deposited microstructure is maintained even after the deposition of subsequent layers and, thus, the possibility of equiaxed microstructure throughout. By exploring the impact of process variables on thermal, morphological, and geometric trends at the deepest point in the melt pool, this work evaluates four commercial processes, estimates the range of process variables capable of producing fully equiaxed microstructure, and considers the expected size of the resultant equiaxed melt pool.

Committee: Nathan Klingbeil Ph.D. (Advisor); Joy Gockel Ph.D. (Committee Member); Raghavan Srinivasan Ph.D. (Committee Member) Subjects: Aerospace Materials; Engineering; Materials Science; Mechanical Engineering; Metallurgy; Morphology

Doctor of Philosophy (PhD), Wright State University, 2008, Engineering PhD

One of the main objectives during primary processing of titanium alloys is to reduce the prior beta grain size. Producing an ingot with smaller prior beta grain size could potentially eliminate some primary processing steps and thus reduce processing cost. Trace additions of boron have been shown to decrease the as-cast grain size in alpha + beta titanium alloys. The primary focus of this dissertation is to investigate the effect of boron on microstructural stability and thermomechanical processing in beta titanium alloys.Two metastable beta titanium alloys: Ti-15Mo-2.6Nb-3Al-0.2Si (Beta21S) and Ti-5Al-5V-5Mo-3Cr (Ti5553) with 0.1 wt% B and without boron additions were used in this investigation. Significant grain refinement of the as-cast microstructure and precipitation of TiB whiskers along the grain boundaries was observed with boron additions. Beta21S and Beta21S-0.1B alloys were annealed above the beta transus temperature for different times to investigate the effect of boron on grain size stability. The TiB precipitates were very effective in restricting the beta grain boundary mobility by Zener pinning. A model has been developed to predict the maximum grain size as a function of TiB size, orientation, and volume fraction. Good agreement was obtained between model predictions and experimental results. Beta21S alloys were solution treated and aged for different times at several temperatures below the beta transus to study the kinetics of alpha precipitation. Though the TiB phase did not provide any additional nucleation sites for alpha precipitation, the grain refinement obtained by boron additions resulted in accelerated aging. An investigation of the thermomechanical processing behavior showed different deformation mechanisms above the beta transus temperature. The non-boron containing alloys showed a non-uniform and fine recrystallized necklace structure at grain boundaries whereas uniform intragranular recrystallization was observed in boron containing (open full item for complete abstract)

Committee: Raghavan Srinivasan PhD (Advisor); H. Daniel Young PhD (Committee Member); Nathan W. Klingbeil PhD (Committee Member); Sharmila Mukhopadhyay PhD (Committee Member); Daniel Eylon PhD (Committee Member); Seshacharyulu Tamirisakandala PhD (Committee Member) Subjects: Aerospace Materials; Engineering; Materials Science; Metallurgy

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, 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

Doctor of Philosophy, The Ohio State University, 2008, Materials Science and Engineering

Microstructural evolution in Beta Titanium alloys is an important factor that governs the properties exhibited by them. Intricate understanding of complex phase transformations in these alloys is vital to tailor their microstructures and in turn their properties to our advantage. One such important subject of study is the nucleation and growth of alpha precipitates triggered by the compositional instabilities in the beta matrix, instilled in them during non equilibrium heat treatments. The present work is an effort to investigate such a phenomenon.Here studies have been conducted primarily on two different Beta-Titanium alloys of commercial relevance- Ti5553 (Ti-5Al-5Mo-5V-3Cr-0.5Fe), an alloy used in the aerospace industry for landing gear applications and, TNZT (Ti-35Nb-7Zr-5Ta), a potential load bearing orthopedic implant alloy. Apart from the effect of thermal treatment on these alloys, the focus of this work is to study the interplay between different alpha and beta stabilizers present in them. For this, advanced nano-scale characterization tools such as High Resolution STEM, High Resolution TEM, EFTEM and 3D Atom Probe have been used to determine the structure, distribution and composition of the non equilibrium instabilities such as beta phase separation and omega phase, and also to investigate the subsequent nucleation of stable alpha. Thus in this work, very early stages of phase separation via spinodal decomposition and second phase nucleation in titanium alloys are successfully probed at an atomic resolution. For the first time, atomically resolved HRSTEM Z-contrast image is recorded showing modulated structures within the as-quenched beta matrix. Also in the same condition HRTEM results showed the presence of nanoscale alpha regions. These studies are revalidated by conventional selected area diffraction and 3D atom probe reconstruction results. Also TEM dark field and selected area diffraction studies are conducted to understand the effect of quenching (open full item for complete abstract)

Committee: Hamish L. Fraser PhD (Advisor); Rajarshi Banerjee PhD (Committee Member); James C. Williams PhD (Committee Member); William A.T. Clark PhD (Committee Member) Subjects: Materials Science

Doctor of Philosophy, The Ohio State University, 2006, Physics

Microstructure features in TiB-reinforced titanium alloys are correlated with mechanical properties. Both laser deposition and arc melting are used to fabricate test alloys where microstructure evolution with heat treatment is examined. SEM and TEM investigations of microstructure are coupled with 3D reconstruction to provide an adequate picture of phases in these alloys. Mechanical properties are then studied. Wear testing of several test alloys is presented, followed by hardness and modulus measurements of individual phases via micro- and nano-indentation as well as a novel micro-compression technique. Bulk mechanical properties are then tested in Ti-6Al-4V and Ti-555 (Ti-5Al-5V-5Mo-3Cr-1Fe) with varying amounts of boron. Image processing methods are then applied to high resolution back-scattered scanning electron microscope images to quantify microstructure features in the tensile test specimens, and these values are then correlated with mechanical properties.

Committee: John Wilkins (Advisor) Subjects:

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 relationship between the texture and the microstructure of both beta processed and alpha/beta processed Ti alloys has been examined in this study. In the beta-processed microstructures, it has been shown that two sets of alpha colonies sharing a common {0001} plane and rotated by ~10.5° from each other may have growth directions which have very large angles of about ~80.7° between them. Moreover, it was observed that alpha laths growing from certain prior beta grain boundaries sometimes shared common basal planes. In some special cases, the alpha laths growing into two different prior beta grains from the grain boundary between them had almost exactly the same orientation, although they had vastly different growth directions. Additionally, there were some cases in which alpha laths growing into different prior beta grains not only had the same crystallographic orientation, but also had the same growth direction. Scanning Electron Microscopy (SEM), Orientation Imaging Microscopy (OIM) and Transmission Electron Microscopy (TEM) have been used to investigate these phenomena and the existing theories of growth directions have been used in conjunction with the results obtained to explain them. In alpha/beta-processed alloys, the phenomenon of globularization of alpha laths breaks down the beta-processed microstructure and modifies the texture of these alloys. Samples of Ti-6Al-4V having colony and basketweave microstructures were hot deformed in the high alpha/beta temperature range and their microstructure and microtexture were examined by the use of SEM and OIM. It is shown that the samples which had a colony microstructure had greater “clustering” of grains with similar orientations than those having a basketweave microstructure. The mode of transformation on heating from the alpha to the beta phase was investigated by measuring the texture of both phases at different temperatures, in situ, using the HIPPO instrument at the Los Alamos Neutron Science Center. A com (open full item for complete abstract)

Committee: Hamish Fraser (Advisor) Subjects: Engineering, Materials Science