UC Santa Barbara

Titanium is a lightweight and attractive metal for use in aerospace, chemical processing, and biomedical industries due to its high strength-to-weight ratio, good oxidation and corrosion resistance, as well as biocompatibility. One important problem with the use of titanium is the embrittlement that accompanies its incorporation of interstitials, most notably oxygen and nitrogen. Since titanium readily dissolves both nitrogen and oxygen, understanding these reactions is fundamental to develop strategies to minimize contamination or to take advantage of these reactions to form nitrides, oxides, or oxynitrides that may exhibit interesting properties for energy storage, biomedical, coatings, or other applications. This research is motivated by the need for improving the fundamental understanding of titanium interactions with nitrogen and oxygen and the nature of its developed phases.Reaction studies were performed at 800°C (to remain in the hcp α-Ti phase) using argon-based environments with three distinct low partial pressures of nitrogen and oxygen. These experiments were performed in two furnace configurations, one of which allowed monitoring of scale evolution in situ using Raman spectroscopy. Characterization of the developed scales relied on scanning and transmission electron microscopy, energy dispersive spectroscopy, focused ion beam, X-ray diffraction, electron diffraction, and Raman spectroscopy. Across environments used, the microstructures show the formation of layered structures with varying porosity throughout. When fast-cooling specimens in the gettered Ar environment (the lowest partial pressure used), the scale consists of mainly nitride phases. Slow-cooling enables the formation of a N-rich hcp-based phase, an oxynitride (TiNxOy) phase, Ti2O3 in a corundum structure, Magnéli phases, and rutile. Ultra-high purity Ar studies show an additional range of oxygen orderings in the substrate when slow-cooling the specimen after the desired dwell time. Analysis of the specimens exposed to gettered and ultra-high purity Ar also shows the formation of twinned fcc TiN or oxynitride structures and an orientation relationship between the fcc phase and the underlying hcp Ti substrate. Partial dislocations are proposed to play a role in the hcp Ti to fcc TiN/TiNxOy transformation and potentially in the formation of voids at the metal-scale interface. Reacting titanium to a 1%O2-Ar environment hinders the formation of nitride or oxynitride phases and develops multiple layers of rutile. In situ experiments proved rutile formation occurs at the exposure temperature rather than during cooling and shows coloration changes in the surface throughout the reaction. Blisters also appeared in between the outermost rutile scales and developed at temperature rather than from CTE mismatch between the oxide and metal during cooling. This dissertation has contributed to the understanding of titanium reactions with nitrogen and oxygen at high temperatures and low partial pressures. The experimental approaches used can be applied towards studying further complex systems either in Ti-based alloys or in other systems such as complex concentrated alloys (CCAs).