Master of Science (MS), Ohio University, 2014, Geological Sciences (Arts and Sciences)

Impact craters in carbonate rock account for ~30% of all impacts on Earth, yet little research has been conducted to study shock metamorphism on such targets. This study examined the petrography of carbonate rocks at Jeptha Knob, a 4.26 km diameter structure east of Shelbyville, KY, to investigate whether evidence of shock metamorphism is present and to determine peak pressures experienced by a previously postulated impact event. Petrographic observations and XRD analyses of calcite/dolomite have the potential to confirm shock metamorphism and constrain peak pressures in impact sites because certain petrographic features are correlated with increasing shock pressures. Specifically, XRD peaks in calcite/dolomite broaden and peak intensities decrease, mechanical twin density increases, and twin spacing decreases. Thin section petrography and XRD analysis of samples collected from the JK78-3 core show that the structure is highly deformed and provide evidence of these rocks experiencing pressures of > 4.6 GPa. The results collected from JK78-3 are consistent with that of an impact origin and provide insight into the development of other impact craters in carbonate rocks.

Committee: Keith Milam (Advisor); Doug Green (Committee Member); Alycia Stigall (Committee Member) Subjects: Geological; Geology

Doctor of Philosophy, The Ohio State University, 2023, Materials Science and Engineering

This thesis explores effects of local phase transformation at stacking faults and microtwins in nickel-based superalloys, expanding upon prior knowledge of this subject. The alloys investigated are primarily used in the hot sections of jet turbine engines, specifically for the polycrystalline disk components; a goal of this research is to increase the service temperature of these parts and therefore the operating temperature of the engine. An increase in temperature has significant effect on promoting efficiency (lowering fuel consumption) of the turbine engine, which ultimately leads to cost savings and carbon emissions of these aircraft. Initially two nominally similar polycrystalline alloys were investigated, with a subtle difference in Nb content, intended to elucidate its effect on local phase transformation strengthening during high temperature creep. Tensile creep tests were conducted at 750 °C and 600 MPa to target the creep regime dominated by superlattice intrinsic and extrinsic stacking faults, as well as microtwinning (i.e. planar defects). Alloy A, with higher Nb and lower Al, was found to be superior in creep strength to Alloy B, with lower Nb and higher Al, as well as previously investigated commercial alloys ME3 and LSHR. ME3 was the prototypical local phase transformation softening alloy, by which Co/Cr segregation to planar defects enabled deleterious microtwinning; it was previously assumed that preventing these features completely via η local phase transformation on superlattice extrinsic stacking faults would lead to enhanced creep properties. Atomic resolution scanning transmission electron microscopy and energy dispersive spectroscopy found that this increased creep strength was not due to η, as Alloy A exhibited microtwins with Nb segregation and ordering. It was hypothesized that increased Nb content was the cause of increased creep strength exhibited by Alloy A (RRHT5) relative to Alloy B (RRHT3), but differing strain levels between alloy (open full item for complete abstract)

Committee: Michael Mills (Advisor); Maryam Ghazisaeidi (Committee Member); Steve Neizgoda (Committee Member); Yunzhi Wang (Committee Member) Subjects: Materials Science

Doctor of Philosophy, The Ohio State University, 2019, Materials Science and Engineering

Titanium (Ti) and its alloys have a wide range of applications in biomedical, automotive and aerospace industries due to their excellent strength to weight ratio and corrosion resistance. Alpha phase Ti has hexagonal closed packed (hcp) structure that shows anisotropic plastic deformation; 〈 a 〉 type slip on prism planes is the easiest to activate but cannot accommodate deformation along the 〈 c 〉 axis. The low temperature ductility of Ti is linked to twinning. Therefore, understanding the mechanisms behind the twin nucleation and growth in Ti alloys is important from both theoretical and industrial application points of view. To that end, the present study seeks a better understanding of the atomic scale processes involved in twin nucleation mechanisms and the effect of alpha-stabilizing solutes such as interstitial oxygen, substitutional aluminum and rare earth elements on twinning. Systematic molecular dynamics (MD) simulations are used to identify the underlying mechanism of twin nucleation from dislocation/grain boundary interactions. Density functional theory (DFT) simulations are employed to examine the effect of oxygen interstitials on the twinning behavior of Ti. A systematic framework has been developed to predict the diffusion of interstitial elements near the twin boundaries in hcp alloys. Next, uncertainty that arises from first-principles calculations in predicting diffusion coefficients are quantified. Finally, solute segregation to the twin boundaries as a new mechanism for dynamic strain aging (DSA) is investigated in Ti and other hcp alloys.

Committee: Maryam Ghazisaeidi (Advisor); Michael Mills (Committee Member); Wolfgang Windl (Committee Member) Subjects: Computer Science; Materials Science

Doctor of Philosophy, The Ohio State University, 2017, Physics

Classical interatomic potentials are an important bridge between nano-scale and meso-scale properties of materials, and facilitate an understanding of deformation and phase transitions at the atomistic level. This work presents the development of two semi-empirical interatomic potentials, one for the body-centered cubic metal tungsten and another for multi-phase titanium-niobium alloys. Accurate density functional theory calculations constitute large databases of forces, stresses and energies to which the empirical models are fit using an evolutionary algorithm. Accuracy of the potentials is verified by comparison with experiment and first-principles calculations for numerous structural, elastic and thermal properties. The models are used to investigate structural phase transitions under high pressure, in the case of tungsten, and chemical disorder, in the case of titanium-niobium. The presented models provide improved descriptions of these technologically important metals over existing classical potentials.

Committee: John Wilkins (Advisor); Maryam Ghazisaeidi (Committee Member); Richard Kass (Committee Member); Fengyuan Yang (Committee Member) Subjects: Condensed Matter Physics; Materials Science; Metallurgy

Master of Science, The Ohio State University, 2017, Materials Science and Engineering

Shape memory alloys (SMAs) are a class of materials that undergo a diffusionless, martensitic transformation. Due to their unique shape memory and superelastic properties, there is interest in implementing these alloys as lightweight, power dense actuators in high temperature environments like aerospace. The traditional SMA NiTi will undergo stable transformation up to 100C, but will not sustain a dependable lifetime beyond this. One method of extending the transformation temperatures is creating a ternary system based on NiTi. With the proper heat treatment and aging conditions, this third element will form optimally sized nanoprecipitates. In this work, the NiTiHf precipitation phase of interest is the H-phase. These precipitates work in two-fold: they stabilize the material over its lifetime by suppressing fatigue and ratcheting and raising transformation temperatures, yet they allow twinned martensite lathes to form. The mechanisms of this duality are not well defined. This work seeks to understand this phoneme using phase field finite element analysis. Phase field modeling incorporates the interfacial energy between the austenite and developing martensite front into the free energy. From this, the relationship between the martensite and austenite over time can be explored spatially and temporally to determine how the twinned martensite microstructure develops over time. This work incorporates several new features into the phase field investigations of the NiTiHf system. These simulations quenched the austenite system from 350K to 150K. The first feature is the introduction of an elliptical precipitate as non-transforming elements within the mesh. These inclusions influence show some templating of the microstructure, depending on the ratio of their major and minor axes. A precipitate with major axis 0.40 and minor axis 0.05 was chosen to represent the H-phase. It was rotated from 0 to 90 degrees within the matrix to investigate the interaction of orientat (open full item for complete abstract)

Committee: Peter Anderson (Advisor); Michael Mills (Committee Member) Subjects: Materials Science

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 h (open full item for complete abstract)

Committee: William Brantley (Advisor); William Johnston (Committee Member); Sarandeep Huja (Committee Member); John Nusstein (Committee Member) Subjects: Dental Care; Materials Science

Doctor of Philosophy, The Ohio State University, 2006, Materials Science and Engineering

Magnesium has reasonable intrinsic cost and exceptional specific strength and stiffness, making it attractive for vehicle structural application. Its adoption in wrought form is limited by room-temperature formability issues related to complex plastic behavior caused by its HCP crystal structure and the presence of twinning. In order to potentially exploit these characteristics in designing a novel forming operation, an efficient material model suitable for finite element implementation would be helpful. Such a model has been constructed based on three phenomenological deformation modes: S (slip), T (twinning) and U (untwinning), corresponding respectively to in-plane tension, initial in-plane compression and initial tension following compression. A von Mises yield surface with initial non-zero back stress was employed to account for plastic yielding asymmetry. The model was formulated with combined isotropic and nonlinear kinematic hardening. Texture was quantified using a weighted discrete probability density function of c-axis orientations. Starting from an assumed perfectly basal-textured sheet, in-plane compression causes reorientation of c-axes toward the compression direction by twinning, and subsequent tension returns them to a sheet normal direction by untwinning. The orientation of c-axes evolves with twinning/untwinning deformation using explicit rules incorporated in the model. The proposed model was implemented in ABAQUS/Standard through UMAT. Constitutive parameters were calibrated from in-plane tension/compression and reversal tests. Simple shear tests were simulated and compared with experiments with good agreement obtained. Tests under a complex loading path, such as orthogonal compressions, tension following orthogonal compression and tension following biaxial compression, were also simulated.

Committee: Robert Wagoner (Advisor) Subjects: