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

The development of viable high-temperature shape memory alloys (HTSMAs) demands a coordinated multimodal characterization effort linking nanoscale crystal structure to macroscale thermomechanical properties. In this work, several high performance NiTi-based shape memory alloys are comprehensively explored with the goal of gaining insight into the complex transformation and deformation mechanisms responsible for their remarkable behavior. Through precise control of alloying and aging parameters, microstructures are optimized to enhance properties such as high-temperature strength and stability. These are crucial requirements for the development of advanced applications such as actuators and adaptive components that operate in demanding automotive and aerospace environments. An array of NiTiHf and NiTiAu alloys are at the core of this effort, offering the possibility of increased capability over traditional pneumatic and hydraulic systems, while simultaneously reducing weight and energy requirements. NiTi-20Hf alloys exhibit a favorable balance of properties, including high strength, stability, and work output at temperatures in excess of 150 °C. The raw material cost of Hf is also much lower compared with Pt, Pd, and Au containing counterparts. Advanced scanning transmission electron microscopy (STEM) and synchrotron X-ray characterization techniques are used to explore unusual nanoscale effects of precipitate-matrix interactions, coherency strain, and dislocation activity in these alloys. Novel use of the 4D STEM strain mapping technique is used to quantify strain fields associated with precipitates, which are being coupled with new phase field modeling approaches to particle/defect interactions. Volume fractions of nanoscale precipitates are measured using STEM-based tomography techniques, atom probe tomography, and synchrotron diffraction of bulk samples. Plastic deformation of the HTSMA austenite phase is shown to occur through <100>B2 type slip for the f (open full item for complete abstract)

Committee: Michael Mills (Advisor); Yunzhi Wang (Committee Member); Peter Anderson (Committee Member); Ronald Noebe (Committee Member); David Wood (Committee Member) Subjects: Materials Science; Metallurgy

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

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

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

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

Targeted creep strength and low-temperature toughness are required to qualify 2.25Cr- 1Mo steel welding consumables relevant to the power generation industries. Submerged arc welding of 2.25Cr-1Mo steels using the same base metal, filler, welding parameters and post weld heat treatment but different fluxes was conducted to make multipass welds. These welds exhibit a drastic variation in low temperature Charpy impact toughness in the post weld heat treated condition (114 ft-lbs to 17 ft-lbs at testing temperature of -40°F). The difference in toughness behavior was investigated using computational modeling and multi-scale microstructure characterization. The effect of flux on the recovery of alloying elements including Cr, Mo, Mn and Si into the weld metal region was analyzed using computational thermodynamic models. Single and two pass welds were made to study the microstructures as a function of thermal cycling. Another study was conducted to control and physically simulate the tempering response of the weld metal by simulating a weld cooling rate of 5 °C/s and 30 °C/s (¿t8-5 of 60 s and 10 s) on welds. Phase transformation analysis was done. Phase identification, packet size determination and precipitate characterization were done. Charpy toughness and SEM fractography was conducted to determine fracture mode. The calculated weld composition as a function of weight % ratio of filler to flux composition was found to be in good agreement with the measured composition. The predicted alloying element recovery was very sensitive to the concentration of deoxidizers in the system including Al, Si and Mn. The single and two pass welds had a solidification substructure of “ghost” delta ferrite, which led to segregation of Cr and Mo away from the ghost delta ferrite structure. The as-solidified microstructure was found to be granular bainite. The precipitates M3C, M7C3 and M23C6 formed during post weld heat treatment. Charpy toughness variation was found in the (open full item for complete abstract)

Committee: John Lippold (Advisor); Boian Alexandrov (Committee Member); Wei Zhang (Committee Member); Suresh Babu (Committee Member) Subjects: 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, 2005, Engineering : Materials Science

In this project, phase transformations and precipitation behavior of Al-Cu nanoparticles were first studied. The nanoparticles were synthesized by a Plasma Ablation process and found to contain a 2∼5 nm thick adherent aluminum oxide scale, which prevented further oxidation. On aging, a precipitation sequence consisting of, nearly pure Cu precipitates to the metastable θ′ to equilibrium θ was observed. The structure of θ′ and its interface with the Al matrix has been characterized. Ultrafine Al-Cu nanoparticles (5∼25 nm) were also synthesized by inert gas condensation and their aging behavior was studied. These particles were found to be quite stable against precipitation. Secondly, pure Al nanoparticles were prepared by the Exploding Wire process and their sintering and consolidation behavior were studied. It was found that Al nanopowders could be processed to bulk structures with high hardness and density. Sintering temperature was found to have a dominant effect on density, hardness and microstructure. Sintering at temperatures >600 degree C led to breakup of the oxide scale, leading to an interesting nanocomposite composed of 100∼200 nm Al oxide dispersed in a bimodal nanometer-micrometer size Al matrix grains. And the randomly dispersed oxide fragments were quite effective in pinning the Al grain boundaries, preventing excessive grain growth and retaining high hardness. Cold rolling and hot rolling were effective methods for attaining full densification and high hardness. Thirdly, the microstructure evolution and mechanical behavior of Al-Al2O3 nanocomposites were studied. The composites can retain high strength at elevated temperature and thermal soaking has practically no detrimental effect on strength. Although the ductility of the composite remains quite low, there was substantial evidence for high localized plasticity. The strengthening mechanisms of the composite include: Orowan strengthening, grain size strengthening and forest strengthening. Finally, (open full item for complete abstract)

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

Doctor of Philosophy, The Ohio State University, 2012, Chemical and Biomolecular Engineering

Fumaric acid is an important specialty chemical with wide applications as a food acidulant and as the chemical feedstock for the production of resins, plasticizers, and miscellaneous industrial products. Currently, fumaric acid is produced through a petroleum-based chemical method. Fungal fermentation provides a promising alternative method due to its advantages in environmental friendliness and availability of widespread renewable biomass feedstock. However, conventional fumaric acid fermentation processes suffer from low product yield, low productivity, and high production cost, whereas the competing chemical production methods are currently more economical. Therefore, the goal of this research was to develop effective and economic fermentation and recovery methods for improved fumaric acid production by Rhizopus oryzae. One of the major technical challenges is to control the fungal morphology and physiology for the overproduction of fumaric acid in a sustainable and scalable way. First, an effective fermentation process with good morphology control was developed using soybean meal hydrolysate as the nitrogen source for cell growth and fumaric acid production by R. oryzae. Uniformly dispersed mycelial clumps with a diameter of ~0.1 mm were formed with enhanced subsequent fumaric acid production. The fermentation reached a product titer of 50.2 g/L with yield of 0.72 g/g glucose. SMH with a high protein content was demonstrated as a good nitrogen source and the formation of protein precipitate acted as the immobilization carriers for cells. The solid-phase protein also provided a novel method for slow/controlled release that allowed the utilization of the nitrogen source by cells for an extended period without losing cell activity. The fermentation was then studied in a 5-L stirred tank bioreactor (STB) and the results also showed that using SMH as the nitrogen source improved fumaric acid production with increased yield and productivity compared to urea. Aeration (open full item for complete abstract)

Committee: Shang-tian Yang (Advisor); Jeffrey Chalmers (Committee Member); David Wood (Committee Member); G&#246;n&#252;l Kaletun&#231; (Committee Member) Subjects: Chemical Engineering; Microbiology

Master of Science, The Ohio State University, 2008, Materials Science and Engineering

Microstrucures play an important role in determining the properties of titanium alloys. This work employs the phase field method to study diffusional phase transformation and the accompanying microstructural evolutions in Ti-6Al-4V. The phase field method used in this work incorporated real thermodynamic and mobility data base and adopts the model proposed by Kim et al. to eliminate the inherent length scale problem in traditional phase field method. It also uses the sparse data structure and algorithm to achieve more efficient computing.Different initial microstructures are used to study how spatial and size distribution of the precipitate phase may influence the kinetics of precipitate growth. Compared to the case with particles of identical size, a normal size distribution of the alpha precipitates has little influence on the kinetics of precipitate growth. However, a uniform spatial distribution of the precipitates will slightly speed up the kinetics, compared to a system with a spatially random distributed precipitates. Grain boundary diffusion becomes dominant below to . It can speed up the kinetics much more and even alters the shapes of the grain boundary alpha phase during evolution. The grain boundary alpha layers developed by grain boundary diffusion may serve as nucleation sites for the secondary plate-like alpha precipitates. In this present study, a model is developed, which can capture the evolution of grain boundary alpha phase and its kinetics.

Committee: Yunzhi Wang (Advisor); Suliman Dregia (Committee Member) Subjects: Materials Science

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

The objective of this study was to quantitatively study localized corrosion, especially exfoliation corrosion (EFC) of high strength aluminum alloys and to investigate the mechanism of exfoliation corrosion with a focus on the effects of alloy temper, microstructure, relative humidity (RH) and mechanical stress. A new technique, Exfoliation of Slices in Humidity (ESH), was developed for the determination of exfoliation corrosion (EFC) susceptibility and quantification of EFC kinetics. This technique involves in exposing properly oriented and unconstrained samples to high humidity following an electrochemical pretreatment. The EFC kinetics was determined by measuring the width of the central unattacked region of the samples. The ESH results show the capability of the ESH test to discriminate between plates of varying susceptibility and to determine EFC rates quantitatively. Optical microscopy and analytical TEM were used to investigate the effects of microstructure and local chemistry at grain boundary on EFC susceptibility. Alloys with more elongated grain shape are more susceptible to EFC and a high Zn content in grain boundary precipitate free zone relates to a high susceptibility. The effects of RH, temper and applied stress on EFC kinetics of AA7178 were investigated by ESH tests. The critical RH for EFC propagation in AA7178 was found to be about 56% and the EFC kinetics increased with RH. ESH tests provide a quantitative description of the temper effect on EFC kinetics. The effects of applied compressive and tensile stresses on EFC kinetics were studied using a four-point bending jig. Compression accelerated EFC significantly and tension reduced kinetics. An equation describing the effects of RH, stress and time on EFC kinetics was developed based on the ESH results using Eyring model. In situ X-ray radiography was used to characterize intergranular and exfoliation corrosion in high strength Al alloys. The samples were either exposed to sodium chloride solutio (open full item for complete abstract)

Committee: Gerald Frankel (Advisor) Subjects: Engineering, Materials Science

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

The focus of this work is to study the precipitation hardening behavior of three Al-Ge-Si alloys and the effect of precipitates on their mechanical responses under forward and reverse loading conditions. Because of the misfit strain cancellation of Ge and Si elements in the aluminum matrix the ternary alloys have finer, denser and more evenly distributed precipitates than in the Al-Ge or Al-Si binary alloys. For the same total alloy content the ternary alloys have smaller precipitate spacing and therefore higher yield stress according to the Orowan theory [1], make them candidates for structural applications. Experimental and analytical work has been conducted to understand the effect of precipitates on the mechanical behavior of the Al-Ge-Si alloys. The material samples were first solution heat treated and then artificially aged. Their precipitate characteristics, such as precipitate shape and size, were examined through TEM. Tensile and tension/compression tests of the aged materials were conducted to obtain their mechanical properties. Based on the measured precipitate information a combined hardening model was constructed to explain the mechanical strengthening mechanisms in these alloys. Although the yield stress modeling result is satisfactory, the combined model over-predicts the monotonic strain hardening of the materials. Various sources affecting material response during reverse loading were also analyzed. Besides the precipitate effect grain boundary and grain texture are the other important factors. For the Al-1%Ge-Si material in this study, grain boundary obstacles accounts for 20% of the backstress while precipitates contribute the remaining 80%. It was found that the magnitude of the Bauschinger factor relates to the volume fraction of precipitate, while the Bauschinger strain correlates better with particle spacing.

Committee: Robert Wagoner (Advisor) Subjects:

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

Allotriomorphic or grain-boundary alpha (GBA) is an unavoidable and important microstructural feature of the diffusional phase transformations in beta-processed titanium alloys. This phase has a negative influence on various mechanical properties, such as high cycle fatigue, fracture toughness, ductility etc. and has a profound influence on the overall microstructural evolution. In the present work, the evolution of GBA has been in explored relative to the grain-boundary parameters namely, misorientation angle and axis, and the GB plane. While the misorientation angle and axis relate to the crystallography of adjacent crystals and can easily be determined using diffraction based techniques, an accurate determination of the local crystallographic orientation of the GB plane is difficult because of its three-dimensional nature. To address this issue, two independent experimental approaches have been developed and validated by utilizing a combination of dual beam focused ion beam (FIB), SEM and electron back-scattered diffraction (EBSD) methods. Both these approaches considerably simplify the problem because of a relatively easier experimental set-up and a versatile methodology. The crystallographic variant selection and morphology of GBA have been subsequently evaluated relative to the quantified GB parameters. Results indicate that in alpha/beta- and beta-titanium alloys, the crystallographic variant selection and GB parameters control the evolution of the allotriomorphic alpha phase. In particular, misorientation angle/ axis parameters control the early precipitation of GBA. The grain-boundaries of those adjacent beta-grains that produce nearly parallel and/or poles are the preferred sites for the early nucleation of GBA. In addition, these closely related poles significantly influence the crystallographic variant selection criterion in a majority of cases and contribute to a considerable short-listing of the allowed variants of GBA. The morphology of GBA is prima (open full item for complete abstract)

Committee: Hamish L. Fraser PhD (Advisor); William A. T. Clark PhD (Committee Member); Yunzhi Wang PhD (Committee Member); Patrick M. Woodward PhD (Committee Member) Subjects: Aerospace Materials; Engineering; Metallurgy