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

Ni-based and Fe-based alloys are widely used structural materials for critical applications such as jet engines and load-bearing components. Materials design and optimization has been accelerated through the Integrated Computational Materials Engineering (ICME) approach. ICME relies on high fidelity thermodynamic and kinetic databases to simulate the composition-processing-properties relationships in materials. A novel extension to the diffusion multiple approach was developed exploring vapor-based surface treatments including aluminization of Ni-based systems and low-pressure carburization of Fe-based systems to generate composition profiles in multiple dimensions. The vapor-reacted treatments allow for high-throughput data generation across wide temperature-composition regimes that aren't achievable using traditional diffusion couple experiments. The Ni-Cr-Pt-Al and Fe-Cr-Mo-Ni-V-C alloy systems were studied for improved understanding of Pt-modified aluminides in thermal barrier coatings on airfoils and carbide formation in steels, respectively. A diffusion multiple of Ni, Cr, and Pt was annealed at 1200 °C for 100 hours for the establishment of the first reliable isothermal section phase diagram of the Ni-Cr-Pt ternary system using electron probe microanalysis (EPMA). Interdiffusion coefficients were also extracted from the measured diffusion concentration profiles using a forward simulation analysis (FSA) for the Ni-Cr, Ni-Pt and Cr-Pt binary systems. The impurity diffusion coefficient data obtained were combined with literature data to assess reliable Arrhenius equations for Pt in Ni, Ni in Pt, Cr in Ni, and Cr in Pt. Other pieces from the annealed Ni-Cr-Pt diffusion multiple were subjected to vapor phase aluminization at 1010 °C for 9 hours and pack aluminization at 1150 °C for 3 hours. Al pickup drastically increased from Cr to Ni to Pt. The Pt and Cr rich part of an isothermal section of the Pt-Cr-Al ternary system at 1010 °C and the Ni and Pt (open full item for complete abstract)

Committee: Ji-Cheng Zhao (Advisor); Alan Luo (Committee Member); Maryam Ghazisaeidi (Committee Member) Subjects: Materials Science

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

Pipes of solid solution strengthened Ni-based alloys, as Alloy 826 and Alloy 800H, have been used for high temperature service in once through steam generators (OSTGs) on off-shore platforms. The oil and gas industry is seeking to increase service temperature, improve service reliability, and extend service life to 40 years of such installations. Alloy 230 has better-high temperature stability and mechanical properties, and higher service temperature than Alloys 825 and 800H, and is therefore considered as a potential replacement of these alloys in newly built OTSGs. However, the weldability and the high temperature service behavior in welds of Alloy 230 have not been thoroughly investigated yet. This study is a comprehensive comparative research focused on susceptibility to solidification cracking and stress relief cracking in Alloys 800H, 825, and 230. To evaluate the solidification behavior and solidification cracking susceptibility in these alloys, the Cast Pin Tear Test (CPTT), thermodynamic simulations with Thermo-Calc, and the technique of Single-Sensor Differential Analysis (SS-DTA) were used. The results revealed that Alloy 230 and Alloy 825 were more resistant to solidification cracking than Alloy 800H, due to narrower solidification temperature range and crack back filling with eutectic constituents. The OSU Stress Relief Cracking (SRC) Test was applied to evaluate the susceptibility to stress relief cracking in autogenous gas tungsten arc welds of the investigated alloys. None of the three alloys failed by stress relief cracking mechanism while loaded at stress equal to 90% of the high temperature yield strength at 650 oC for 8 hours.. Alloys 825 and 800H showed significant amount of stress relief, while Alloy 230 sustained the applied load at 650C with almost no stress relief. Tensile testing at 650 oC after the 8 hours SRC test showed that the autogenous weld in Alloy 230 had significantly higher yield and tensile strength and slightly lower el (open full item for complete abstract)

Committee: Boian Alexandrov (Advisor); Avraham Benatar (Advisor) Subjects: Materials Science; Metallurgy

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, 2021, Welding Engineering

This study focuses on the development of low alloy steel (LAS) girth welds on internally clad API 5L Grade X65 steel by investigating the metallurgical phenomena of welds made using a high melting temperature consumable over a low melting temperature substrate. The metallurgical phenomena of welds made using a low melting temperature consumable over a high melting temperature substrate have been widely reported in literature. The solidification behavior of alloy 625 overlays on high-strength steel (HSS) has been reported in works pertaining to the oil and gas, petrochemical, and power generation industries. Extensive investigations have been conducted analyzing microstructural and compositional gradients along the fusion boundary and transition zone that degrade the mechanical properties of such welds. Alloy 625 girth welds on internally clad HSS have also become a topic of continued discussion as premature failures have been associated to the fusion boundary between the Ni-based alloy weld metal and HSS pipe. The oil and gas industry is investigating the potential replacement of alloy 625 girth welds with LAS girth welds. LAS girth welds could possibly reduce susceptibility to premature failures while also reducing pipeline manufacturing and installation expenses. Reel pipelay is a method of installing pipelines to the ocean floor from giant reels mounted on an offshore vessel. Reel pipelay is known to increase installation rates and reduce manufacturing expenses since welding and inspection is performed onshore. DNV-OS-F101, however, states that girth welds intended for reel pipelay applications shall overmatch the base metal yield strength by 100 MPa. Careful consideration is also required during girth welding to ensure that the corrosion resistant properties of the internally clad layers remain intact. Such girth welds have been challenging to develop due to poor weldability. Dilution from the low melting temperature substrate into the high melting temper (open full item for complete abstract)

Committee: Boian Alexandrov Dr. (Advisor); Carolin Fink Dr. (Committee Member); Dennis Harwig Dr. (Committee Member) Subjects: Composition; Design; Engineering; Materials Science; Metallurgy; Ocean Engineering; Petroleum Engineering

Master of Science, Miami University, 2017, Physics

Ni2MnGa is a Heusler alloy that exhibits a first order martensitic phase transition near 200 K upon cooling. Recent study showed that when Mn was partially replaced with Cr in Ni2Mn1-xCrxGa, the martensitic transformation temperature increased with increasing Cr concentration. At the same time, a sharp drop in electrical resistivity was observed in the system, and the magnitude of the drop increased with increasing Cr content. In an attempt to better understand these observations, in this thesis a series of Heusler alloys that include Ni2-xCrxMnGa, Ni1.75Cr0.25Mn0.75Co0.25Ga, Ni1.9Cr0.1Mn0.75Co0.25Ga, and Ni2.15Cr0.1MnGa have been investigated. The samples were fabricated by conventional vacuum arc melting technique in argon atmosphere. The characterization of the samples were performed by x-ray diffraction, dc magnetization, differential scanning calorimetry, and electrical resistivity measurements. The experimental results revealed interesting properties of the alloys that include existence of multiple magnetic and structural phases, coupled and decoupled phase transitions, large change in electrical resistivity in the vicinity of the martensitic phase transformation, and large magnetocaloric effects in selected compounds. The experimental results have been discussed in detail considering the intrinsic properties of the materials.

Committee: Dr Mahmud Khan (Advisor); Dr. Khalid F. Eid (Committee Member); Dr. Herbert Jaeger (Committee Member) Subjects: Physics