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

Background: A land-based Industrial gas turbine (9HA) lies at the heart of two world records for efficient power generation. Based on thermodynamic principles, the efficiency of gas turbines is dictated by their operating temperatures. Thus, the drive for more efficient power generation ultimately revolves around increasing the operating temperature of gas turbine engines. Specifically, developing a more efficient powerplant requires a gas turbine wheel operating at or above 1200°F (649°C). However, because of the massive size of such turbine wheels (average reported diameters are about 40''), no current superalloy can meet the above temperature goals. In fact, because of its large size, maintaining microstructural stability during the thermomechanical processing of gas turbine wheels is a herculean task. The Unknown: However, most polycrystalline superalloys, including the current state-of-the-art wheel material (Alloy 706), exhibit a hierarchy of microstructures spanning multiple length scales. In that case, microstructural optimization reliant on intragranular precipitate phases alone may not achieve the desired high-temperature performance. Objectives and Findings: The present research focused on optimizing the microstructure of polycrystalline superalloys through concurrent multi-scale structure-property correlation studies. Specifically, I looked at three aspects of the hierarchical nature of the microstructure observed in any typical polycrystalline superalloy: (1) intragranular precipitate distribution, (2) precipitation and consequent precipitate-free zones near annealing twin boundaries, and (3) secondary precipitate evolution on high angle grain boundaries. Our results indicate that unless alloy development strategies utilize a simultaneous optimization approach for these three aspects, achieving the desired high performance in Ni-Fe-based superalloys is difficult. Results from several advanced characterization experiments using various in-si (open full item for complete abstract)

Committee: Michael J. Mills (Advisor) Subjects: Materials Science

Master of Science (MS), Ohio University, 2013, Mechanical Engineering (Engineering and Technology)

The purpose of this work was to develop a die wear model for superalloy tooling used in multi-channel copper tube extrusion and determine the feasibility of coating superalloys with a chemical vapor deposition (CVD) thin-film for wear improvement. Samples were produced from commercial alloys ATI 720, Inconel 718, and Rene 41 alloys and coated with a multi-layer aluminum oxide CVD coating, Bernex 29 CVD coating offered by Ionbond. Post-coating heat treatment processes for the superalloys were modified to maintain coating adhesion. Pin on disc wear tests were conducted on coated and non-coated samples. The CVD coating provided a decrease in wear rate by at least 70%. Based on the wear test results, a die wear model to predict the service life of extrusion tooling was developed.

Committee: Frank Kraft Dr. (Advisor); John Cotton Dr. (Committee Member); Greg Kremer Dr. (Committee Member); David Ingram Dr. (Committee Member) Subjects: Engineering; Materials Science; Mechanical Engineering; Metallurgy

Master of Science, The Ohio State University, 2024, Welding Engineering

Haynes 282 is a precipitation strengthened Ni-based superalloy known for its combination of creep strength, thermal stability, and weldability, which make it a promising alloy for additive manufacturing (AM). Processing conditions such as scan strategy and component geometry result in unique thermal histories that can impact the final microstructure and mechanical properties. In this work, two pyramidal Haynes 282 components were fabricated via electron beam powder bed fusion (EBM-PBF) using linear raster and random spot scan strategies. Systematic microstructural characterization of gamma grain orientation, gamma prime precipitation, grain boundary character, processing defects, and microhardness measurements were used to determine the process-property-structure relationship of the as-built components. In both scan strategies, hardness increased by ~3% along the build direction and columnar gamma grains showed a pronounced texture in the direction. The random spot scan strategy showed higher local gamma grain misorientation, more porosity and elongated defects, and increased volume fraction and gamma prime size. The random spot scan strategy had intergranular cracking while the linear scan strategy was unaffected. Further analysis was done on boride particles along grain boundaries through scanning transmission electron microscopy and atom probe tomography. Along grain boundaries, there were enriched liquid films consisting of B, Cr, and Mo. These elements were linked to a local decrease in melting temperature to form liquid films during the last stage of solidification. Hot cracking occurred due to these liquid films being present along columnar high angle grain boundaries and high enough thermal stresses. Understanding the cracking mechanism and its relation to the process allows for suggestions of guidelines to prevent cracking in Haynes 282 produced by EBM-PBF.

Committee: Carolin Fink (Advisor); Joerg Jinschek (Committee Member); Gopal Viswanathan (Committee Member) Subjects: Materials Science

MS, University of Cincinnati, 2024, Engineering and Applied Science: Materials Science

The pursuance of innovative materials has resulted in the development of high-entropy alloys (HEAs) that contain precipitate-reinforcing phases, which are referred to as high-entropy superalloys (HESAs). The primary goal of this investigation is to provide a thorough comprehension of the impact of the nickel:cobalt ratio on the microstructure and morphological changes of precipitate phases, as well as its impact on creep resistance. This required the development of six distinct alloys with varied Ni and Co contents. Initially, the CALPHAD methodology was employed to investigate the phases present in these alloys by plotting the phase diagrams. The resulting data were subsequently compared to the experimental results. Vacuum arc melting (VAM) was employed to fabricate these alloys in the experimental phase. Cast alloys were subjected to heat treatment at 850°C from their homogenized state with varying aging periods in the range of 0 to 720 hours. The alloys were characterized and the gamma (?) and gamma prime (?') phase structures were examined using optical microscopy, scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), and X-ray diffraction (XRD) analyses. Vickers microhardness experiments were implemented to assess the alloys' mechanical properties in both their as-cast and aged states. Compressive creep experiments were conducted at 850°C with varying stresses and the specimens were deformed up to 10% strain. The collection of creep data and the creation of Norton plots were conducted. The presence of ? and ?' phases, as predicted by PANDAT, was confirmed for all alloys by imaging via SEM in back-scattered mode, contingent upon the Ni: Co ratio. The FCC matrix phase is verified through XRD, and the ?' phase is verified through TEM electron diffraction patterns. The partitioning behavior of the elements into various phases was observed using EDS in both SEM and TEM. By increasi (open full item for complete abstract)

Committee: Dinc Erdeniz Ph.D. (Committee Chair); Matthew Steiner Ph.D. (Committee Member); Eric Payton Ph.D. (Committee Member) Subjects: Materials Science

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, Case Western Reserve University, 2022, Materials Science and Engineering

Nickel-based superalloys are strategically important materials for all high-value industries that support a sustainable safe society, amounting to an annual $7 billion global market. They are uniquely qualified to operate at intermediate temperatures (<900°C) and in corrosive environments, making them a choice material for turbines for propulsion and energy generation, as well as pressure vessels for reactors that manufacture radioactive therapies for cancer treatments. Their performance hinges on the careful design of processing routes that govern microstructure features; primarily nanoscale precipitates. The exploitation of processing-structure-property (PSP) models to tailor materials for device performance represent the heart of materials science and engineering. The development of the quantitative PSP model requires the collection of disparate datasets that involves standard experiments (data-driven methods) or physics-based computational methods that simulate required output data. Data-driven approaches involve an immense amount of time and cost-consuming data collection processes. Physics-based computational simulations are not capable of simulating property metrics and are also required to be experimentally calibrated due to their less fidelity. Because of these reasons, both approaches are challenging and motivate an enormous demand for accelerated, high-throughput, approaches. Further, in previous studies, both approaches (data-driven and simulations) functioned as two separate branches and were not integrated. Materials informatics, enabled by high-throughput approaches could overcome these challenges and corporate the data-driven and computational simulations that allow engineers to quantitatively measure the critical microstructure properties and design the processing routes to improve the material performance of Ni-based superalloys. This approach effectively provides a value-added proposition to be better stewards of the manufacturing digital thread (open full item for complete abstract)

Committee: Jennifer Carter (Committee Chair); Roger French (Committee Member); Francis Merat (Committee Member); John Lewandowski (Committee Member) Subjects: Engineering; Materials Science

PhD, University of Cincinnati, 2021, Engineering and Applied Science: Materials Science

Porous materials, including foams and lattice structures, are used in many applications such as biomedical implants, heat exchangers, catalysts, and batteries due to their light weight, high surface area and energy absorption properties. Lattice structures, specifically, are of great interest since their properties can be tailored by employing various design methodologies (e.g., topology optimization). On the other hand, Ni-based superalloys are used in many applications where high-temperature and oxidation/corrosion resistance are important such as in gas turbine components. The advantageous properties of these Ni-Cr-Al-based alloys with the geometry and tailored mechanical properties of lattice structures can be combined through a number of different additive manufacturing modalities (e.g., laser powder-bed fusion). However, this can result in residual stresses and potential crack formation that degrade mechanical properties. Therefore, there is a need for the development of an alternative approach. One such approach would be the decoupling of the printing and alloying steps. Beyond preventing residual stresses and crack formation, one extra benefit would be the ability to introduce a second level of porosity via the Kirkendall effect. This would further increase the surface area and reduce the weight per volume. This work focuses on this decoupled approach via studying i) the sintering kinetics of printed Ni strands to understand if the polymer binder and extrusion pressure affect the sintering behavior compared to pressure-less sintering of loosely poured and pressed Ni powders ii) the sintering kinetics of printed strands made from either pre-alloyed Ni-Cr or Ni and Cr mixed elemental powders to determine differences in sintering rates and final densification iii) alloying Ni-Cr strands with Al via pack cementation and homogenization to investigate how the microstructure influences Kirkendall pore development and evolution and iv) alloying Ni-Cr strands with (open full item for complete abstract)

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

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

Vacancy-mediated diffusion along dislocations, often referred to as pipe diffusion, can contribute to creep deformation of metals in many engineering applications. This process is studied along an a/2⟨1 -1 0⟩ screw dislocation in fcc Ni using a density functional theory approach. An accurate geometrical configuration of the screw dislocation core, dissociated into Shockley partial dislocations and separated by a stacking fault, was previously derived using a lattice Green's function technique. Activation energies and frequencies are calculated for atom-vacancy exchanges that contribute to diffusion around and along one of the partial cores. This analysis reveals the significant role of the sites within the compressive component of the dislocation, the dominant contribution of the hops around the screw geometry rather than directly along the dislocation line, and the importance of including the stacking fault sites. Kinetic Monte Carlo simulations use these frequencies to generate diffusion coefficients that account for correlation effects. Near 80% of the melting temperature, these pipe diffusivities are an order of magnitude higher than those found in fcc regions, and they are eight orders higher at room temperature. Calculations are compared to experimental results and the differences are discussed. While pipe diffusion is unlikely to contribute to isotropic mass flux at low dislocation densities, it will accelerate dislocation mechanisms controlling creep, particularly when alloying elements are involved. To quantify the effects of chemistry on diffusivity in addition to geometry, the described techniques also analyze the pipe diffusivity of several important alloying elements to Ni-base superalloys: Co, Cr, Al, Ti, Mo, W, and Re. Calculations quantify how the partial core enhances diffusivities of each element relative to isotropic diffusion in fcc Ni. Advances in first principles methods provide a means to explore the rate constants of mechanisms that cons (open full item for complete abstract)

Committee: Amir A. Farajian Ph.D. (Advisor); Raghavan Srinivasan Ph.D., P.E. (Committee Member); H. Daniel Young Ph.D. (Committee Member); Dallas R. Trinkle III. Ph.D. (Committee Member); Christopher Woodward Ph.D. (Committee Member) Subjects: Engineering; Materials Science

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

Conventional metal alloys are made up of one predominant base metal and smaller quantities of one or more additional alloying elements. Although this strategy has successfully produced all metals currently in use, a new multi-principal element paradigm is emerging in which highly-alloyed compositions lack a clear base metal. This foray into previously unexplored composition space has produced new alloys with excellent combinations of strength, ductility, and other properties, particularly when using 3d transition metal elements. This work concerns multi-principal element alloys comprising transition metals chromium, cobalt, and nickel. Key findings include a more detailed understanding of heterogeneous microstructures and their effect on alloy strength and ductility; new indications that deformation twinning may be unnecessary for high strain-hardening rates and good ductility; and elucidation of the discontinuous coarsening effect along grain boundaries in highly-alloyed CrCoNi-base materials like commercial superalloy 740H. These results can be broadly applied to other multi-principal element alloys and austenitic steels.

Committee: Michael Mills (Advisor); Stephen Niezgoda (Committee Member); Maryam Ghazisaeidi (Committee Member); Boyd Panton (Committee Member) Subjects: Materials Science

Master of Sciences, Case Western Reserve University, 2018, Materials Science and Engineering

This thesis investigates the interaction between gamma' precipitates and grain boundaries in a Ni-Al system during deformation. This interaction is investigated using molecular dynamics, and gamma'/boundary configurations were built to investigate how the orientation, size, and interaction of gamma' change the deformation behavior of the grain boundary. The gamma' aided in nucleating defects (i.e., dislocations) that contributed to the boundary sliding mechanism. By increasing the size of precipitates that bisect the boundary, the boundary becomes stronger, whereas increasing the size of precipitates adjacent to the boundary makes the boundary weaker. Additionally, the interaction of multiple gamma' plays a role in grain boundary sliding behavior. Low concentrations of gamma' produce sliding dominated by atomic shuffling, whereas high concentrations of gamma' produce sliding dominated by dislocation emission. More work is needed to investigate the effects of temperature, initial defects, and different grain boundary configurations on sliding behavior.

Committee: Jennifer Carter (Committee Chair); John Lewandowski (Committee Member); David Matthiesen (Committee Member) Subjects: Materials Science

PhD, University of Cincinnati, 2017, Engineering and Applied Science: Mechanical Engineering

Fatigue failure is a major reason behind the failure of mechanical components and machine parts. In turbine engines and related applications, the components are subjected to cyclic loading at elevated temperatures. Superalloys have high strength and environmental resistance to perform under extreme high temperatures and stress conditions. Improvement in the strength, fatigue life, and/or temperature capabilities of these superalloys will yield huge economic benefits. To address these challenges, surface treatment techniques are implemented to improve the fatigue behavior of currently used superalloys at elevated temperatures. This study investigates Ultrasonic Nano-crystal Surface Modification (UNSM) and Laser Shock Peening (LSP) as techniques to improve strength and fatigue behavior of ATI 718 Plus (718Plus) at room and elevated temperatures. The effect of temperature and strain rate on the strength, ductility, and failure behavior of 718Plus was investigated. The results showed that with the increase of temperature at slow strain rate, there is a small reduction in the yield strength, a large drop in ductility, and a change in fracture mode from ductile transgranular to brittle intergranular cracking. Analysis of the microstructure showed that the driving mechanism at higher temperatures and slower strain rates is oxygen-induced intergranular cracking, a dynamic embrittlement mechanism and that the d precipitates on the grain boundaries are facilitators. Increase of strain rate at 704 °C caused a small increase in the yield strength, a huge increase in the ductility, and a change in fracture mode from brittle to ductile failure. This showed that the driving mechanism at higher strain rates was Portevin–Le Chatelier effect. Finally, 718Plus has superior fatigue behavior at its operation temperature (650 °C) compared to room temperature due to the strengthening of the ?' precipitates which increased its endurance limit by ~20% (~145 MPa). The repetitive strikes (open full item for complete abstract)

Committee: Vijay Vasudevan Ph.D. (Committee Chair); Woo Kyun Kim Ph.D. (Committee Member); Yijun Liu Ph.D. (Committee Member); Dong Qian Ph.D. (Committee Member); Jing Shi Ph.D. (Committee Member) Subjects: Mechanical Engineering; Mechanics

Doctor of Philosophy, The Ohio State University, 2017, Welding Engineering

Brazed joints are commonly used in the manufacture and repair of aerospace components including high temperature gas turbine components made of Ni-base superalloys. For such critical applications, it is becoming increasingly important to account for the mechanical strength and reliability of the brazed joint. However, material properties of brazed joints are not readily available and methods for evaluating joint strength such as those listed in AWS C3.2 have inherent challenges compared with testing bulk materials. In addition, joint strength can be strongly influenced by the degree of interaction between the filler metal (FM) and the base metal (BM), the joint design, and presence of flaws or defects. As a result, there is interest in the development of a multi-scale computational model to predict the overall mechanical behavior and fitness-for-service of brazed joints. Therefore, the aim of this investigation was to generate data and methodology to support such a model for Ni-base superalloy brazed joints with conventional Ni-Cr-B based FMs. Based on a review of the technical literature a multi-scale modeling approach was proposed to predict the overall performance of brazed joints by relating mechanical properties to the brazed joint microstructure. This approach incorporates metallurgical characterization, thermodynamic/kinetic simulations, mechanical testing, fracture mechanics and finite element analysis (FEA) modeling to estimate joint properties based on the initial BM/FM composition and brazing process parameters. Experimental work was carried out in each of these areas to validate the multi-scale approach and develop improved techniques for quantifying brazed joint properties. Two Ni-base superalloys often used in gas turbine applications, Inconel 718 and CMSX-4, were selected for study and vacuum furnace brazed using two common FMs, BNi-2 and BNi-9. Metallurgical characterization of these brazed joints showed two primary microstructural regions; a s (open full item for complete abstract)

Committee: Boian Alexandrov Ph.D. (Advisor); Avraham Benatar Ph.D. (Advisor); Carolin Fink Ph.D. (Committee Member) Subjects: Engineering; Materials Science

Doctor of Philosophy, The Ohio State University, 2017, Materials Science and Engineering

High entropy alloys (HEAs) are a relatively new class of materials that have garnered significant interest over the last decade due to their intriguing balance of properties including high strength, toughness, and corrosion resistance. In contrast to conventional alloy systems, HEAs are based on four or more principal elements with near equimolar concentrations and tend to have simple microstructures due to the preferential formation of solid solution phases. HEAs appear to offer new pathways to lightweighting in structural applications, new alloys for elevated temperature components, and new magnetic materials, but more thorough characterization studies are needed to assess the viability of the recently developed multicomponent materials. One such HEA, AlMo0.5NbTa0.5TiZr, was selected to be the basis for this characterization study in part due to its strength at elevated temperatures (s0.2 = 1600 MPa at T = 800 ºC) and low density compared with commercially available Ni-based superalloys. The refractory element containing HEA composition was developed in order to balance the high temperature strength of the refractory elements with the desirable properties achieved by the high entropy alloying design approach for potential use in aerospace thermal protection and structural applications. Ingots of AlMo0.5NbTa0.5TiZr were cast by vacuum arc melting followed by hot isostatic pressing (HIP) and homogenization at 1400 ºC for 24 hrs with a furnace cool of 10 ºC/min. The resulting microstructure was characterized at multiple length scales using x-ray diffraction (XRD), scanning transmission electron microscopy (SEM), conventional and scanning transmission electron microscopy (TEM and STEM), and x-ray energy dispersive spectroscopy (XEDS). The microstructure was found to consist of a periodic, coherent two phase mixture, where a disordered bcc phase is aligned orthogonally in an ordered B2 phase. Through microstructural evolution heat treatment stud (open full item for complete abstract)

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

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

The creep deformation mechanisms present during creep at intermediate stress and temperatures in ME3 were further investigated using diffraction contrast imaging. Both conventional transmission electron microscopy and scanning transmission electron microscopy were utilized. Distinctly different deformation mechanisms become operative during creep at temperatures between 677-815ºC, and at stresses ranging from 274-724MPa. Both polycrystalline and single crystal creep tests were conducted. The single crystal tests provide new insight into grain orientation effects on creep response and deformation mechanisms. Creep at lower temperatures (760C) resulted in the thermally activated shearing modes such as microtwinning, stacking fault ribbons and isolated superlattice extrinsic stacking faults (SESFs). In contrast, these faulting modes occurred much less frequently during creep at 815ºC under lower applied stresses. Instead, the principal deformation mode was dislocation climb bypass. In addition to the difference in creep behavior and creep deformation mechanisms as a function of stress and temperature, it was also observed that microstructural evolution occurs during creep at 760C and above, where the secondary coarsened and the tertiary precipitates dissolved. Based on this work, a creep deformation mechanism map is proposed, emphasizing the influence of stress and temperature on the underlying creep mechanisms. Next, the effects of varying crystal orientation and composition on active deformation modes are explored for two different, commercially used Ni-base disk alloys, ME3 and ME501. Understanding these effects will allow for improved predictive deformation modeling and consequently faster advancements in Ni-base alloy development. In order to investigate these effects, compression creep tests were conducted on [001] and [110] oriented single crystal specimens of the disk alloys ME3 and ME501, at different stress/temperature regimes. At 760 C and below, a pr (open full item for complete abstract)

Committee: Michael Mills (Advisor) Subjects: Materials Science

Master of Science, The Ohio State University, 2016, Welding Engineering

Giant oil reservoirs were found off the coast of Brazil in the Santos Basin Bay. They are located under a geological salt layer; thus they are called pre-salt. The fact that these reservoirs are located far from shore, has presented some technological challenges. Currently, X65 pipes internally clad with Alloy 625 are used for traditional subsea oil extraction. Such pipes are joined using Alloy 625 filler material. Since the depth and the amount of risers and pipelines to be installed in the pre-salt oil reserves are very large, pipe reeling is considered the most efficient technology for pipeline installation. For pipe reeling, it is necessary for the welds joining the pipes to overmatch the X65 base metal yield strength by 100 MPa, without any post wed heat treatment, thus Alloy 625 does not meet those requirements. The objective of this project was to explore the applicability of precipitation strengthened nickel-based filler metals for welding of internally clad X65 pipes for application in pipelines and risers for oil extraction from pre-salt subsea oil fields. Ni-base super alloys 718 and 282 were considered as potential welding consumables for this application. The solidification behavior in the weld metal of these alloys diluted with the Alloy 625 pipe cladding was evaluated using thermodynamic simulations with Scheil-Gulliver module of Thermo-Calc™. The Alloy 718 / Alloy 625 system exhibited almost constant solidification temperature range of about 250 oC with formation of Laves phase at the end of solidification throughout the whole dilution range. The Alloy 282 / Alloy 625 system exhibited potential for lower susceptibility to solidification cracking with the solidification temperature range gradually degreasing to 150 oC and no formation of Laves at dilutions lover than 70%. The compatibility of Alloy 718 and Alloy 625 filler metals with alloy the 625 cladding and X65 steel base metal was evaluated by performing bead-on-plate welding and producing mu (open full item for complete abstract)

Committee: Boian Alexandrow Dr. (Advisor); Avraham Benatar Dr. (Committee Member) Subjects: Engineering

Doctor of Philosophy, The Ohio State University, 2016, Industrial and Systems Engineering

Inertia friction welding of Ni-base superalloys is of increasing importance for future designs of turbine engines where the overall pressure ratio and operating temperatures are increased in an effort to improve efficiency. Ni-base superalloys are chosen for the hottest sections due to their inherent ability to retain good strength levels to very high fractions of their melting point. As model systems, the Ni-base superalloys LSHR and Mar-M247 were chosen for further welding and investigation. The LSHR is a powder-metallurgy alloy that is a surrogate for a bore material in a multi-alloy disk system. The Mar-M247 is a coarse-grain cast alloy that is a surrogate for a rim alloy. As the strength retention at elevated temperatures is advantageous for engine applications, it is disadvantageous for the production of sound welds. This effectively reduces the size of the processing window for these alloys. These alloys that are high in gamma prime formers are also prone to weld cracking issues. These factors make it necessary to utilize a process model to improve weld outcomes and reduce the amount of trial-and-error experimentation. Finite element modeling techniques and experimental weld process examination have led to insights into the complex interplay between the weld process parameters and their impact on post-weld microstructure, bond quality & character as well as mechanical properties. In effort to improve upon the predictive capability of the finite element process model, a number of weld parameters were examined. A minimum energy input bond criteria was formulated along with empirical relationships between the process parameters and weld behavior. Key input data for finite element process modeling was shown to include flow stress, coefficient of friction, and weld process efficiency. Strategies to estimate appropriate values for these key parameters were demonstrated and validated within the process model. The methodology to identify appropriate welding p (open full item for complete abstract)

Committee: Rajiv Shivpuri (Advisor); Hamish Fraser (Committee Member); Wei Zhang (Committee Member) Subjects: Industrial Engineering; Metallurgy

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

Polycrystalline Ni-base superalloys are used as turbine disks in the hot section in jet engines, placing them in a high temperature and stress environment. As operating temperatures increase in search of better fuel efficiency, it becomes important to understand how these higher temperatures are affecting mechanical behavior and active deformation mechanisms in the substructure. Not only are operating temperatures increasing, but there is a drive to design next generation alloys in shorter time periods using predictive modeling capabilities. This dissertation focuses on mechanical behavior and active deformation mechanisms found in two different advanced polycrystalline alloy systems, information which will then be used to build advanced predictive models to design the next generation of alloys. The first part of this dissertation discusses the creep behavior and identifying active deformation mechanisms in an advanced polycrystalline Ni-based superalloy (ME3) that is currently in operation, but at higher temperatures and stresses than are experienced in current engines. Monotonic creep tests were run at 700°C and between 655-793MPa at 34MPa increments, on microstructures (M1 and M2) produced by different heat treatments. All tests were crept to 0.5% plastic strain. Transient temperature and transient stress tests were used determine activation energy and stress exponents of the M1 microstructure. Constant strain rate tests (at 10-4s-1) were performed on both microstructures as well. Following creep testing, both microstructures were fully characterized using Scanning Electron Microscopy (SEM) for basic microstructure information, and Scanning Transmission Electron Microscopy (STEM) to determine active deformation mechanism. It was found that in the M1 microstructure, reorder mediated activity (such as discontinuous faulting and microtwinning) is dominant at low stresses (655-724 MPa). Dislocations in the ¿ matrix, and overall planar dislocation activi (open full item for complete abstract)

Committee: Michael Mills PhD (Advisor); Glenn Daehn PhD (Committee Member); Yunzhi Wang PhD (Committee Member) Subjects: Materials Science; Metallurgy

PhD, University of Cincinnati, 2006, Engineering : Materials Science

The goal of improving the efficiency of pulverized coal powerplants has been pursued for decades. The need for greater efficiency and reduced environmental impact is pushing utilities to ultra supercritical conditions (USC), i.e. steam temperatures approaching 760°C under a stress of 35 MPa. The long-term creep strength and environmental resistance requirements imposed by these conditions are clearly beyond the capacity of the currently used ferritic steels and other conventional alloys. As part of a large DOE-funded consortium, new and existing materials based on advanced austenitic stainless steels and nickel base superalloys are being evaluated for these very demanding applications.In the present work, the nickel base superalloys of Inconel 617, CCA617, Haynes 230 and Inconel 740, and austenitic alloys Super 304H and HR6W, were evaluated on their microstructural properties over elevated temperature ageing and creep rupture conditions. The materials were aged for different lengths of time at temperatures relevant to USC applications, i.e., in the range from 700 to 800°C. The precipitation behaviors, namely of the γ', carbides and γ phase in some conditions in nickel base superalloys, carbides in Haynes 230, Cu-rich precipitates in Super 304H and Laves phase particles in HR6W, were studied in detail using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and related analytical techniques. Particular attention has been given on the structure, morphology and compositional distinctiveness of various phases (including γ, γ', carbides, secondary phase precipitates, and other types of particles) and their nature, dislocation structures and other types of defects. The results were presented and discussed in light of associated changes in microhardness in the cases of aged samples, and in close reference to mechanical testing (including tensile and creep rupture tests) wherever available. Several mechanical strengthening mechanisms were proposed an (open full item for complete abstract)

Committee: Dr. Vijay Vasudevan (Advisor) Subjects: Engineering, Materials Science

Doctor of Philosophy, The Ohio State University, 2012, Materials Science and Engineering

Ni-based superalloys continue to be used in the hot sections of turbine engines due to their superior high temperature properties and retained strength. The present document will focus specifically on the polycrystalline alloy R104, and the deformation substructure observed during and following cyclic mechanical testing. Both low cycle fatigue (LCF) and sustained peak low cycle fatigue (SPLCF) tests are considered. Two chapters on electron microscopy technique development follow a brief introduction on general properties of Nickel superalloys. Almost exclusively, scanning transmission electron microscopy (STEM) was performed for defect characterization. Furthermore, through a systematic study of STEM-based diffraction contrast methods, including experimental and computational results, STEM is presented as a valid means of defect analysis. The second chapter in this set also uses STEM, but in a non-traditional setting: the microscope is configured for high resolution imaging, i.e., the sample is aligned along a low index zone axis and a large convergence angle is used. In this low angle annular dark field (LAADF) mode, an annular detector accepts low-angle scattering, which allows one to obtain atomic resolution images while retaining defect contrast. Both techniques described in these two chapters were used extensively throughout this research. The remaining chapters discuss the application of the microscopy techniques developed in the proceeding chapters to cyclically deformed specimens of R104. Both interrupted and failed samples were deformed in LCF at 427C and 704C, and interrupted SPLCF samples were tested at 704 and 760C. The deformation mechanisms observed will be discussed at length in this document. In general, dislocation activity dominates under LCF conditions while stacking faults and stacking fault ribbons are most prominent under SPLCF conditions. Time and temperature components will be discussed in regards to the operative mechanisms. A point of emph (open full item for complete abstract)

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

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