Doctor of Philosophy (PhD), Wright State University, 2023, Engineering PhD

The as-printed surfaces of parts produced through laser powder bed fusion are significantly rougher than surfaces produced through traditional manufacturing processes. This increased roughness can have a significant impact on mechanical properties, with perhaps the most notable detriment in the fatigue life of the part. Therefore, the as-printed surface roughness in additively manufactured materials must be studied more extensively to determine its impact on fatigue performance. This work investigates the surface roughness of additively manufactured specimens through the investigation of processing parameters and their effects on surface roughness in metal additive manufacturing. Furthermore, the relationships between as-printed surface roughness and fatigue behavior in AM materials are studied using both axial fatigue testing and vibration bending fatigue testing of nickel superalloy 718. Following the results of these initial tests, this work also examines the relationships between sharp corners and fatigue life in as-printed additively manufactured alloy 718. Results suggest that both rougher surfaces and sharp corner geometries cause decreases in fatigue life, thus providing guidelines for the design of additively manufactured components under fatigue loading conditions.

Committee: Nathan Klingbeil Ph.D. (Committee Co-Chair); Joy Gockel Ph.D. (Committee Co-Chair); Luke Sheridan Ph.D. (Committee Member); Henry D. Young Ph.D. (Committee Member) Subjects: Mechanical Engineering

PhD, University of Cincinnati, 2022, Engineering and Applied Science: Materials Science

Hollow superalloy structures provide several advantages, but are difficult to fabricate, particularly on the micro-scale. The Kirkendall effect offers a novel route to produce such hollowed micro-objects using an inside-out diffusion approach instead of traditional template-removal methods. By depositing the desired alloying additions on Ni-based wires via a chemical vapor deposition technique called pack cementation, multi-component microtubes can be fabricated by taking advantage of the radial symmetry and spatial confinement present within the structure, which under the appropriate annealing conditions, leads to formation of a central, continuous Kirkendall channel internally throughout the wire, converting it into a microtube. To conduct this work, the pack cementation method, under the conventional open and a proposed closed system, were first used to deposit Ti or to co-deposit Al and Ti on Ni-based wires (pure Ni or Ni-20 wt.% Cr), to form the prerequisite core/shell structure for inside-out Kirkendall diffusion. The central coalesced Kirkendall porosity was successfully observed after homogenization, with different area fractions ranging from 3% to 20%, in the Ni-Cr-Ti, Ni-Al-Ti, and Ni-Cr-Al-Ti systems studied in this research. The correlation between deposition time and phase formation in each alloy system was established. Also, the elemental effect on the diffusion kinetics was studied and utilized to explain the porosity evolution. The results indicate the chromium addition could further facilitate the interdiffusion between Ni and Ti leading to larger central porosity, while the interaction of Ti and Al introduce Kirkendall porosity in the Ni-Al system. The quaternary system generates more discreetly dispersed Cr-rich precipitates, but still develops a decent area fraction of central Kirkendall porosity. Subsequently, a dealloying process, which extracts the alloying elements (Cr, Al, and Ti) from the hollowed quaternary wire substrate and depos (open full item for complete abstract)

Committee: Ashley Paz y Puente Ph.D. (Committee Member); Matthew Steiner Ph.D. (Committee Member); Jing Shi Ph.D. (Committee Member); Dinc Erdeniz Ph.D. (Committee Member) Subjects: Materials Science

Doctor of Philosophy, The Ohio State University, 2020, Materials Science and Engineering

The realization of advanced alloy compositions in service relies on a thorough understanding of metallurgical processing variables. Within this work, the gamma prime precipitation of an advanced powder metallurgy nickel-base superalloy during controlled cooling from supersolvus temperatures is compared to prior alloy generations using a complement of characterization and modeling approaches. The on-cooling precipitation of the alloy is studied and characterized to calibrate a multi-scale precipitation model. The proposed framework incorporates a computationally efficient addition to the mean-field modeling approach that increases its ability to model dynamic, multi-modal gamma prime burst events. The gamma prime size predicted by the model shows good agreement with experimental results. The precipitation calculation is applied to the element integration points of a continuum Finite Element heat conduction simulation, where the latent heat generated from the precipitation is accounted for. The results are compared to experimental findings and indicate potential use of the model for evaluating precipitation effects at multiple length scales. The lattice misfit evolution of two commercial PM nickel superalloys during cooling from supersolvus temperatures is also characterized, using in-situ synchrotron X-Ray Diffraction (XRD). The diffraction pattern deconvolution necessary for quantifying misfit was accomplished by combining observation of the superlattice peak intensities with thermodynamic modeling to quantify the intensity relationship between the overlapping phases. The misfit from the XRD measurements was compared to the Scanning Electron Microscopy observations of gamma prime particle shapes for a subset of the experimental conditions. The trend of measured misfit agreed with the microstructural characterization. Time-resolved observations of the on-cooling lattice parameter suggest that lower-temperature changes to the peak intensity characteristics coinc (open full item for complete abstract)

Committee: Michael Mills (Advisor); Wei Zhang (Advisor); Yunzhi Wang (Committee Member); Stephen Niezgoda (Committee Member) Subjects: Aerospace Materials; Engineering; Materials Science

Doctor of Philosophy, The Ohio State University, 2020, Materials Science and Engineering

Much effort has been directed over the past several decades towards net-shape manufacturing of nickel-base superalloys which are difficult to conventionally process because of their compositional complexity. Net-shape hot isostatic pressing (NS-HIP) has been employed for the last fifty years for consolidation of superalloys and has numerous advantages when compared with traditional processing. These include very efficient use of material, refined microstructure without prominent texture or residual stresses, and the absence of porosities. Disadvantages include the cost of sacrificial tooling and the presence of defect networks in consolidated material. These networks of defects, designated as prior particle boundaries (PPB) because they originate at the surfaces of powder stock, contribute to a scatter or deficit in mechanical properties, which in the past has disqualified NS-HIP materials from use in critical aerospace applications. Studies in this work have shown that the HIP:ing of superalloy powders of the same nominal composition, but processed with different conditions, yield quite different microstructures and properties. The origins of these differences, such as atomization method, cleanliness of the material, powder size and shape, and grain structure have been examined using various characterization techniques. This work has focused on the characterization of microstructure and defects of IN-718 in rapidly solidified powders, at intermediate stages during HIP, and in the as-HIP condition. Novel methods for characterizing and quantifying defects in superalloys, with an emphasis on the contrast mechanisms in low voltage in-column scanning electron microscopy are presented. An approach has been developed for net-shape HIP processing of IN-718 with improved microstructure and tolerance for defects.

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

Doctor of Philosophy (PhD), Wright State University, 2020, Engineering PhD

In many structural applications void-like defects cause significant performance debits which call for component redesign or post-processing to account for or remove the defects. For laser powder bed fusion (LPBF) processes, it has been shown that many of these features and their size and shape characteristics are controllable through LPBF process parameter manipulation. For design efforts, however, it is necessary to understand the direct influences of processing on the formation of porosity and the role that individual pores and porosity distributions have on the properties and performance of AM components. Additionally, design criteria must be established to facilitate implementation of AM components into structurally critical applications. To this end, the investigations that have been performed here relate the AM material processing of alloy 718 to the pore structure, crack growth properties and fatigue performance. This dissertation first explores the influence of four key process parameters and scan strategies on the formation and characteristics of porosity distributions in AM material. Then, based on the porosity distributions observed via non-destructive inspection techniques, a crack-growth based life prediction method was developed to accurately predict fatigue lives of AM components. Additionally, fatigue limit models were modified based on experimental data to explore the interactions of defect size and applied stress with respect to both finite and "infinite" fatigue life which enables defect tolerant design for components manufactured via AM. Finally, a novel compliance-based method for crack initiation detection was developed and used to assess some of the assumptions made in the prior investigations. The connections made through the work presented herein link AM processing to potential design requirements which will facilitate faster, safer design efforts for implementation of AM components into structurally critical applications.

Committee: Joy E. Gockel Ph.D. (Advisor); Nathan W. Klingbeil Ph.D. (Committee Member); Ahsan Mian Ph.D. (Committee Member); Onome Scott-Emuakpor Ph.D. (Committee Member); Anthony Rollett Ph.D. (Committee Member) Subjects: Materials Science; Mechanical Engineering; Mechanics; Metallurgy

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

Nickel-base superalloys are routinely employed for structural components within the late-stage compressor and turbine sections of gas turbine propulsion engines due to their unique combination of ductility and strength at elevated temperatures. The desirable performance of this material class is a direct consequence of an aggregate microstructure containing a disordered γ-FCC phase strengthened by ordered γ'-L12 precipitates. In recent years, a novel alloying system with microstructural characteristics analogous to nickel-base superalloys has garnered significant interest within the aerospace community. At specific compositions, the ternary Co-Al-W system exhibits similar L12 precipitates within a FCC matrix, but with the added advantage of a solidus temperature approximately 100 - 150 °C higher than observed in nickel-based systems. This effort adds to other alloy development investigations assessing the potential of this new alloy class for commercial transition into aerospace propulsion applications. Two aspects of Co-Al-W-base alloys were probed in detail: (i) microstructural stability after exposure to an elevated temperature for extended times and (ii) the hot deformation behavior of polycrystalline alloys under conditions relevant to the industrial thermo-mechanical processes necessary for component fabrication. In many previous publications, the Co3 (Al,W)- γ' strengthening phase in the Co-Al-W ternary system has been proposed as thermodynamically metastable at desired application temperatures. Bulk specimens of five Co-rich compositions of the Co-Al-W ternary and Co-Al-W-Ni quaternary systems were characterized after isothermal aging near 850 °C to assess previously unevaluated γ' W:Al ratios and confirm the effect of Ni alloying at exposure times up to 5000 hours. The aged microstructures, phase fractions, and phase compositions were evaluated with the intent of informing computational thermodynamic simulations for the Co-rich end of Co-Al-W and Co-Al- (open full item for complete abstract)

Committee: Michael Mills (Advisor); Stephen Niezgoda (Advisor); Yunzhi Wang (Committee Member); Kiran D'Souza (Committee Member) Subjects: Materials Science; Metallurgy

Doctor of Philosophy, The Ohio State University, 2018, Industrial and Systems Engineering

A novel class of gamma-prime strengthened cobalt-based superalloys may enable a significant temperature and efficiency capability improvement relative to nickel-base superalloys for future generation turbine engine hardware. However, little information exists regarding deformation processing of these novel Co-Al-W alloys into useable product forms with the necessary microstructure refinement at an industrially relevant scale with industrially relevant processes. To address this need, an ingot metallurgy thermomechanical processing sequence was demonstrated for a novel class of cobalt-base gamma-prime containing superalloys. From an as-cast ingot, the material was characterized and a homogenization heat treatment was developed and executed to reduce residual segregation from casting. Representative ingot conversion steps using extrusion were evaluated and performed followed by a recrystallization heat treatment to produce the desired fine-grain, wrought microstructure. Deformation processing of wrought material was completed at supersolvus hot-working temperatures using both cylindrical upset specimens to establish flow-stress behavior and custom-designed double-cone upset specimens to experimentally quantify the effect of strain, strain-rate, and temperature in microstructure evolution during hot-working, including the dynamic recrystallization and grain growth. All upset testing was completed at two supersolvus temperatures (1149 °C or 1204 °C) and one of three strain-rates (0.01/s, 0.1/s, or 1.0/s) depending on the type of testing completed. Required thermophysical and thermomechanical data was determined for material property inputs to a finite element model which was used to correlate observed microstructures to location-specific thermomechanical processing history. As part of this development, a significant effort was undertaken at each stage of processing to sufficiently characterize the microstructure through optical microscopy, electron microscopy, (open full item for complete abstract)

Committee: Rajiv Shivpuri (Advisor); Jerald Brevick (Committee Member); Hamish Fraser (Committee Member) Subjects: Aerospace Materials; Engineering; Industrial Engineering; Materials Science; Metallurgy

Doctor of Philosophy (Ph.D.), University of Dayton, 2017, Materials Engineering

Compressive surface residual stresses achieved by mechanical surface treatments (shot peening, low-plasticity burnishing, or laser shock peening) typically extend component life under fatigue loading. Designers are unable to include this surface residual stress benefit in design life predictions due to the absence of detailed and accurate models of the behavior, and to the uncertainty of residual stress profiles that exist both before and after service. Generalization of coupled creep-plasticity models to incorporate microstructural features, size distributions, and volume fractions can provide an analytical description of the relevant relaxation mechanisms, allowing engineered residual stress to be taken into account in design. The current study combines observations from microstructural characterization and mechanical testing with the development of numerical models to predict surface residual stress relaxation following exposure to load and temperature. The grain size and γ' precipitate distributions are quantified for two different IN100 microstructures, and experimental measurements of the yield strength and creep behavior of the materials are obtained. The microstructural data are incorporated into a coupled creep-plasticity modeling framework which describes how the prior plastic deformation affects creep response. Residual stress relaxation predictions are validated against measured residual stress profiles from shot-peened laboratory scale experiments for both heat treatments of IN100 under service-relevant conditions. The resulting numerical model accurately predicts relaxation of engineered residual stresses under expected service conditions, using only data from standard mechanical tests and microstructural characterization methods, thereby enabling favorable residual stresses to be considered in the design process.

Committee: Robert Brockman Ph.D. (Advisor); Dennis Buchanan Ph.D. (Committee Member); Paul Murray Ph.D. (Committee Member); Michael Caton Ph.D. (Committee Member); Reji John Ph.D. (Committee Member) Subjects: Materials Science

Doctor of Philosophy, The Ohio State University, 2016, Materials Science and Engineering

Alloy 718 is a γ''-strengthened Ni-base disk superalloy used in the aerospace industry, and it has been used prominently for decades. Though there has been extensive research into the processing/property relationships, there is very little known about the intermediate microstructure and mechanisms that are common in commercial 718 that dictate the yield strength. A variety of analytical techniques, including scanning transmission electron microscopy (STEM) and energy dispersive spectroscopy (EDS) were employed to investigate the microstructure of alloy 718 after various deformed conditions and heat treatment conditions. The γ'' in alloy 718 following a commercial heat treatment was found to have both monolithic γ'' particles as well as composite particles in which γ' and γ'' share a planar phase boundary. Isothermal heat treatments were applied to solutionized samples, and it was found that low heat treatment temperatures produced a large percentage of composite particles, but high temperatures led to the formation of a primarily monolithic structure. Additionally, these composite particles were shown to have a desirable stabilizing effect at high temperatures, and they were seen to grow much more slowly than their monolithic counterparts. STEM analysis was able easily show the morphology of any edge-on γ'' particles, and EDS was capable of determining the general morphology of in-plane particles. EDS was also useful in determining a rough volume fraction of each phase in tin foils, and it was found that the volume fraction of γ' was slightly larger than that of γ'' after commercial heat treatments. Deformation mechanisms were also characterized using STEM. Diffraction STEM was used on isolated dislocations and it was determined that isolated dislocations do not have contrast consistent with 1/2<110> dislocations, so some form of <112> dislocation was thought to be operative. Atomic resolution STEM analysis uncovered a variety of mechanisms present in (open full item for complete abstract)

Committee: Michael Mills (Advisor); Hamish Fraser (Committee Member); Maryam Ghazisaeidi (Committee Member); Mo-How Shen (Committee Member); Yunzhi Wang (Committee Member) Subjects: Aerospace Materials; Materials Science; Metallurgy

Doctor of Philosophy, The Ohio State University, 2015, Welding Engineering

Abstract Increasing the steam temperature and pressure in coal-fired power plants is a perpetual goal driven by the pursuit of increasing thermal cycle efficiency and reducing fuel consumption and emissions. The next target steam operating conditions, which are 760°C (1400°F) and 35 MPa (5000 psi) are known as Advanced Ultra Supercritical (AUSC), and can reduce CO2 emissions up to 13% but this cannot be achieved with traditional power plant construction materials. The use of precipitation-strengthened Nickel-based alloys (superalloys) is required for components which will experience the highest operating temperatures. The leading candidate superalloys for power plant construction are alloys 740H, 282, and 617. Superalloys have excellent elevated temperature properties due to careful microstructural design which is achieved through very specific heat treatments, often requiring solution annealing or homogenization at temperatures of 1100 °C or higher. A series of postweld heat treatments was investigated and it was found that homogenization steps before aging had no noticeable effect on weld metal microhardness, however; there were clear improvements in weld metal homogeneity. The full abstract can be viewed in the document itself.

Committee: John Lippold (Advisor); Boian Alexandrov (Committee Member); Antonio Ramirez (Committee Member) Subjects: Materials Science; Metallurgy

Master of Science, The Ohio State University, 2015, Mechanical Engineering

The objective of this research is to generate experimental data that can be used to calibrate and validate constitutive models for plastic deformation and failure that are implemented in numerical simulations. In the first part of the research, tension tests are conducted at elevated temperatures on notched and unnotched thin flat specimens made of a nickel-based superalloy. The geometry of each sample is designed to induce various states of stress inherent in original jet engine components. Three-dimensional Digital Image Coorelation (3D-DIC) is used to measure the full-field deformations. The results of these tests show the setup is successful in capturing displacements and strains on the surface of each sample at elevated temperatures for ductile materials. The force versus displacement curves reveal that the nickel-based superalloy being tested exhibits thermal softening and serrated flow due to strain localizations at elevated temperatures. The second part of the research introduces a method to characterize the thermally induced strain behavior of a 6000 series aluminum alloy during the car manufacturing process. To simulate the stamping process, dogbone and rectangular strip specimens are subject to uniaxial and bending strains. A method for measuring strains on the surface of specimens during bend tests is established. Following deformation, specimens are subjected to thermal heating cycles that simulate the paint-bake cycle. The thermally iii induced strains during the heating cycle are measured for the each of the specimens. In addition, the material properties and thermal buckling behavior of the aluminum at various temperatures are investigated. The results show that specimens subject to different bending strains display elevated coefficients of thermal expansion and residual strain after being rendered to a heating cycle. The measurements from the material property and thermal buckling testing can be used to calibrate a thermally dependent material mo (open full item for complete abstract)

Committee: Amos Gilat (Advisor); Brian Harper (Committee Member) Subjects: Mechanical Engineering

MS, University of Cincinnati, 2011, Engineering and Applied Science: Materials Science

Precipitation behavior in IN718Plus superalloy has been studied after heat treatment between 923K to 1123K upto 1000h. Spherical gamma prime (γ') precipitates transform to plate shaped eta phase (η) upon aging which is important for fatigue design of this alloy. The as received microstructure shows plate shaped grain boundary delta phase but after thermal aging titanium containing eta plates appear within the grains by transformation of γ' along the close-packed plane through a faulting mechanism. Confirmation was obtained using transmission electron microscopy (TEM) through convergent beam electron diffraction, orientation analysis and energy dispersive X-Ray measurements. The activation energy for γ' precipitate coarsening was calculated based on precipitate size data obtained through analysis of TEM micrographs. Coarsening kinetics of γ'and γ' are coupled and were analyzed based on activation energy analysis and are compared with microhardness results. Coarsening of γ' precipitates is found to deviate from LSW theory at small sizes but matches well for intermediate and bigger particles. In this study, a procedure to identify the range of validity of operative rate law is found based on JMA model. Small particle coarsening is found to exhibit a radius square dependence with time proposed [Ardell et al, Nature Materials 2005] for ordered alloys. Since number density of plate shaped precipitates increases progressively at expense of spherical γ' with time, a general condition is derived for such systems where the content of stable daughter phase is proportional to the metastable γ' phase which it is forming from. Gamma double prime (γ'') phase was not observed during above aging treatments. Finally, thermal relaxation of Laser Shock induced stresses in this material is compared based on SXRD, XRD and microhardness data. The main features of this study are: A. Advances knowledge of precipitation in this class of superalloy upon thermal ageing at and above service te (open full item for complete abstract)

Committee: Vijay Vasudevan PhD (Committee Chair); Rodney Roseman PhD (Committee Member); Raj Singh ScD (Committee Member) Subjects: Materials Science

PhD, University of Cincinnati, 2010, Engineering and Applied Science: Materials Science

Nickel-base superalloys are used at high temperature applications in aerospace and power generation. The objective of present work is to gain a better understanding of microstructural evolution and deformation mechanisms in Nickel-base superalloys. The microstructure of the Nickel base superalloy is basically composed of gamma matrix with eta, delta, gamma prime or gamma double prime/or both, carbides and nitrides. Cold working of IN 718 and Waspaloy to 50% reduction led to an increase in hardness. This hardening was related to the continuous increase in dislocation density in both alloys. Cold working of IN 718 to levels of 10% and higher also led to shearing of the gamma double prime precipitates present initially, leading to their dissolution and the redistribution of the alloying elements into the matrix. Shot peening of both alloys introduces near surface compressive residual stresses and a significant increase in the surface and near-surface hardness to a distance of ~200-400 µm in both alloys. Shot peening of both alloys followed by aging at 900°C quickly led to a large drop in hardness to near that of the bulk material. Aging shot peened the IN 718 at 700°C led to an increase in the hardness throughout the sample. Microstructural characterization revealed that this hardening is related to the formation of new precipitates of gamma prime or gamma double prime or both within the gamma matrix. Aging shot peened Waspaloy at 700°C led to an increase or decrease at near surface region at short time, depending on the Almen intensity. Microstructural characterization shows that these changes are related to partial reduction in % cold work by recrystallization and/or new gamma prime precipitation, depending on the Almen intensity. The hardening, microstructural evolution and stress rupture behavior of IN 740 were studied. Aging of the IN 740 alloy led to significant hardening due to the gamma prime precipitation. The gamma prime coarsening in aged and tensile tested (open full item for complete abstract)

Committee: Vijay Vasudevan PhD (Committee Chair); Rodney Roseman PhD (Committee Member); Jainagesh Sekhar PhD (Committee Member); Raj Singh ScD (Committee Member) Subjects: Materials Science

Doctor of Philosophy, The Ohio State University, 2012, Welding Engineering

Friction stir processing (FSP), a derivative of the recently developed friction stir welding process, is a localized solid-state thermomechanical process used for microstructural modification. Location-specific microstructural enhancement resulting from FSP can be used to improve the properties and performance of a component only where necessary thus reducing cost associated with producing a component entirely from a high-performance material. FSP has been limited to relatively low temperature alloys systems, e.g., aluminum and magnesium-base alloys. Recent tool material developments have enabled FSP to be applied to more refractory materials, such as Ni-base alloys. This study focuses on the unique microstructures in Ni-alloys including Alloy 201 (Ni-201), Inconel¿¿¿¿ Alloy 600 (IN600), Haynes Alloy 282, Haynes Alloy 214, Mar-M247, and Ren¿¿¿¿ 41 that result from FSP and additive friction stir processing (AFSP), an adaptation of FSP in which material additions are incorporated to further enhance site-specific properties. Comprehensive autogenous FSP process parameter development was performed for Ni-201 and IN600 to determine process windows. Analysis of process force output revealed defect-free runs were characterized by pseudo-steady state process forces. For both materials, overall measured process forces decreased as the FSP heat input was increased. Microstructural examination of both alloys after FSP reveals considerable microstructural refinement in the stir zone (SZ). Equiaxed, recrystallized grains, with average grain sizes ranging from 8 to 27 ¿¿¿¿m were observed. Grain refinement along with the extent of recovery and recrystallization varied based on the applied process parameters. Additive friction stir processing was used to locally create ¿¿¿¿'-strengthened surface layers on non-age hardenable substrates. Superalloys with varied ¿¿¿¿'-former contents were deposited on IN600 substrates using fusion-based deposition techniques and subsequently frict (open full item for complete abstract)

Committee: John Lippold PhD (Advisor); S. Suresh Babu PhD (Committee Member); Mary Juhas PhD (Committee Member); Mills Michael PhD (Committee Member) Subjects: Materials Science

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

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

Small scale mechanical testing affords the possibility of measuring constitutive materials properties and studying intrinsic size effects on mechanical behavior. Accordingly, Uchic and colleagues developed a microcompression testing technique about five years ago. Published studies employing this technique have clearly demonstrated that size-scale effects exist independently of other previously known size effects such as nucleation-controlled deformation (whiskers) or the presence of imposed strain gradients (nanoindentation). The overwhelming majority of these studies have focused on metals with the FCC crystal structure. In this study, the microcompression test methodology was employed to evaluate Rene N5, an engineering alloy with a heterogeneous microstructure containing features which vary over a range of length scales. Compression samples were tested using sample diameters in a range from 2.5 to 80 um, while selectively testing samples from both dendrite core and interdendritic regions. A size-dependent flow response, consistent with exhaustion hardening, was observed. Region-specific properties were also observed, where samples tested from interdendritic regions had varied and on average decreased flow stress values compared to those tested from dendrite core regions. A representative sample diameter, where bulk properties would be matched, was extrapolated to be 400 um.Additionally, a custom in-situ SEM testing device for performing uniaxial mechanical tests on micrometer-scale samples was employed. This device allows one to access both tensile and compressive test modes, and also directly observe the spatial and temporal distribution of deformation events through continuous recording of SEM images. Microcompression experiments with this device demonstrated that the degree of lateral constraint imposed by the compression platen affects many aspects of the observed response, such as the strain hardening rate, crystal rotations, elastic modulus, dislocation in (open full item for complete abstract)

Committee: Hamish L. Fraser PhD (Advisor); Michael D. Uchic PhD (Committee Member); Peter M. Anderson PhD (Committee Member); Michael J. Mills PhD (Committee Member) Subjects: Materials Science; Metallurgy

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

Ni-base superalloys are an important class of high temperature structural materials that are used in the hot section of aircraft gas turbine engines since they possess the inherent capability to retain strength and resistance to creep, fatigue, and oxidation at elevated temperature. These components are subjected to elevated temperatures and complex stress states where time dependent creep deformation is of utmost concern and needs to be accounted for from a design criteria point of view. In this study, the creep deformation mechanisms of a newer generation turbine disk alloy, Rene 104, was investigated using transmission electron microscopy characterization techniques. The creep deformation behavior and underling creep deformation mechanisms were found to be highly dependent upon stress, temperature and microstructure.Microtwinning was found to be the dominant deformation mechanism following creep at an intermediate temperature and stress regime. Microtwins form by the motion of paired a/6<112> Shockley partial dislocations that shear both the γ matrix and γ' precipitates. The rate limiting process in this mechanism is diffusion mediated atomic reordering that occurs in the wake of the shearing, twinning partial dislocations in order to maintain the ordered L12 structure of the γ' precipitates. To determine the salient microstructural features that lead to microtwinning specimens with varying γ' precipitate size scale, volume fraction and γ channel width spacing were crept at the same temperature and stress (677°C and 724MPa). The most creep resistant microstructure consisted of a bimodal distribution of γ' precipitates with a finer secondary γ' precipitate size, low volume fraction of γ' and narrow γ channel width spacing. Due to the combined effects of narrow γ channel width spacing, stacking fault energy, and resolved shear stress the a/2<110> dislocations dissociate into leading and trailing a/6<112> Shockley partial dislocations at low strain, which was determ (open full item for complete abstract)

Committee: Michael Mills PhD (Advisor); Glenn Daehn PhD (Committee Member); James Williams PhD (Committee Member); Robert Hamlin PhD (Committee Member) Subjects: Engineering; Materials Science; Metallurgy

PhD, University of Cincinnati, 2013, Engineering and Applied Science: Materials Science

Laser shock peening (LSP) for improving fatigue life of IN718Plus superalloy is investigated. Fatigue geometry and LSP parameters were optimized using finite element method (FEM). Residual stress distributions estimated by FEM were validated using Synchrotron XRD and line focus lab XRD, and correlated with microhardness. An eigenstrain analysis of LSP induced edge deflections (measured with optical interferometry) was also conducted. Transmission electron microscopy (TEM) of single-spot LSP coupons shows sudden increase in dislocation density under LSP treated region. Total life fatigue was conducted at R=0.1 at 298K and 923K, with and without LSP. S-N curve endurance limit increases at both temperatures with FEM optimized LSP samples. Based on TEM of fatigue microstructure and LSP coupons, mechanistic description of observed fatigue improvement is attempted. Often need arises to weld components, and weld heat-affected-zone reaches near-solvus temperatures. To simulate this treatment, sub-solvus hot-rolled IN718Plus is aged at 923K. We observe precipitation of thin eta-Ni3(Al, Ti) plates after 1000 hours, making the material susceptible to cracks, and lowering fatigue life. Effect of LSP on fatigue crack growth (FCG) is studied following ASTM guidelines on M(T) geometry at R=0.1. Acceleration in FCG rate with LSP is observed for this geometry and LSP condition. Prior FEM optimization was not conducted for FCG tests, and may account for lower FCG resistance after LSP. FCG results were corroborated with COD compliance based analysis. Crack measurements were done using potential drop method, and crack closure was analyzed. Effect of LSP on overload FCG was investigated by single-cycle 100% overload followed by single-spot LSP on the crack-tip plastic zone. Crack retardation occurs after application of overload+LSP. Effective contribution of overload+LSP to crack retardation is estimated. Fractographic analysis of LSP treated fatigue samples suggests sub-surface (open full item for complete abstract)

Committee: Vijay Vasudevan Ph.D. (Committee Chair); Kristina Langer Ph.D. (Committee Member); Dong Qian Ph.D. (Committee Member); Rodney Roseman Ph.D. (Committee Member); Dale Schaefer Ph.D. (Committee Member) Subjects: Materials Science