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, 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, 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

Master of Science (MS), Wright State University, 2014, Mechanical Engineering

The advancement of laser or electron beam-based additive manufacturing requires the ability to control solidification microstructure. Previous work combined analytical point source solutions and nonlinear thermal finite element analysis (FEA) to explore the effects of deposition process variables on Ti-6Al-4V solidification microstructure. The current work seeks to extend the approach to Inconel 718, with the addition of Cellular Automaton-Finite Element (CAFE) models. Numerical data from finite element results are extracted in order to calculate accurate melt pool geometry, thus leading to corresponding cooling rates and thermal gradients. The CAFE models are used to simulate grain grown and nucleation, providing a link between additive manufacturing process variables (beam power/velocity) and solidification microstructure. Ultimately, a comparison of results between Ti-6Al-4V and Inconel 718 is expected to lay the ground work for the integrated control of melt pool geometry and microstructure in other alloys.

Committee: Nathan Klingbeil Ph.D. (Advisor); Raghavan Srinivasan Ph.D., P.E. (Committee Member); Jaimie Tiley Ph.D. (Committee Member) Subjects: Aerospace Engineering; Aerospace Materials; Engineering; Materials Science; 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

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