Doctor of Philosophy, The Ohio State University, 2019, Welding Engineering

This work builds upon the previous research regarding hydrogen assisted cracking (HAC) of low alloy steel to nickel-base filler dissimilar metal welds (DMWs). In particular, this work is focused on DMWs commonly experienced in offshore oil and gas production systems in subsea use. The HAC tendency of these welds has been attributed to formation of susceptible microstructures at the fusion boundary during welding. As such, a post-weld heat treatment (PWHT) is utilized to temper these microstructures as well as relieve residual stresses. However, these microstructures can persist even after PWHT due to the steep compositional gradient driving migration of carbon from the base metal toward the fusion boundary and into the partially mixed zone (PMxZ) of the weld. The degree to which this migration occurs is a function of materials selection (base metal and filler metal) as well as weld and PWHT procedure. Due to this phenomenon, there is a balance that must be found to provide tempering of the susceptible microstructures that form during welding and limiting the formation of new susceptible microstructures during PWHT. Previous research has established a test method in the form of the delayed hydrogen cracking test (DHCT) which can delineate the effects of materials combination, weld procedure, and PWHT on HAC of DMWs. This test's qualitative ranking of susceptibility agreed well with industry experience. The current study worked towards refining the test methodology investigating the effects of test parameter influence on realized results. Of the investigated variables, it was found that how the test samples are coated is of primary importance where a consistently exposed fusion boundary scheme providing the most repeatable result in test. Additionally, a comparison was made between the test hydrogen charging condition which uses a dilute acid and constant current density of 10mA/cm2 and the service environment which is seawater with a constant potential (-8 (open full item for complete abstract)

Committee: Boian Alexandrov Ph.D (Advisor); Antonio Ramirez Ph.D (Committee Member); Carolin Fink Ph.D (Committee Member); Matthew Hamilton Ph.D (Committee Member) Subjects: Engineering; Materials Science

Master of Science, Miami University, 2012, Chemical, Paper and Biomedical Engineering

Gallium nitride semiconductors are of great interest as high power/temperature transistors due to their wide band gaps and high electron mobility. However, AlGaN/GaN transistors have shown device instability at higher temperatures. In this thesis, Thermo Calc© and DICTRA© software were used to investigate the defect chemistry of the Al-Ga-N material system and the diffusion kinetics of nickel into the AlGaN layer of the device by the Computer Coupling of Phase Diagrams and Thermochemistry methodology. Using this methodology, both a thermodynamic and kinetic database need to be developed. A Ga-N thermodynamic database was first built and the phase diagram and defect concentration were calculated to ensure its accuracy in diffusion simulations. The kinetic simulation results indicated temperature activated diffusion of nickel as a possible mechanism for device failure.

Committee: Lei Kerr PhD (Advisor); Doug Coffin PhD (Committee Member); Shashi Lalvani PhD (Committee Member) Subjects: Chemical Engineering; Electrical Engineering; Engineering; Materials Science; Mechanical Engineering; Nanoscience; Nanotechnology