Role of Oxygen on Phase Stability, Precipitation, Deformation, and Oxidation in Pure Titanium and Beta Titanium Alloys

Abstract

Titanium and its alloys exhibit many desirable properties, such as a high strength-to-weight ratio and excellent corrosion resistance, which result in their continued importance as structural materials particularly for aerospace and biomedical industries. However, titanium’s reactivity with oxygen presents significant challenges to mechanical performance, including embrittlement caused by oxygen in solid solution and fast oxidation during high temperature exposures. Oxygen is therefore typically considered a detrimental element for titanium alloys. Commercial alloys commonly require strict limits on oxygen impurities to prevent embrittlement and are used at relatively low service temperatures to prevent material loss by oxidation. These challenges present opportunities for titanium alloy development. Oxygen has been shown to modify phase formation and precipitation sequences in metastable beta titanium alloys containing high amounts of beta-stabilizing elements, which resulted in novel mechanical behavior suggestive of potential new application spaces. Regarding oxidation, while the development of protective coatings has shown significant reductions in oxidation kinetics for Ti alloys, limited understanding still exists on how alloying elements might provide protection. Consequently, this thesis is organized in two parts. First, it argues that oxygen is not always detrimental by providing advancements in our understanding of the role of oxygen as an alloying element in beta alloys. Second, when oxygen concentrations and oxidation need to be controlled, it details a possible approach to creating effective coatings using silicon.

The role of oxygen was demonstrated in a model beta Ti-Nb alloy and commercial Ti-15-333 and Ti-15Mo alloys. Compositionally-graded microstructures were created using high temperature oxidation followed by ageing to understand oxygen’s influence on metastable omega and stable alpha phase precipitation kinetics and morphologies. Multi-scale microstructural characterization methods including scanning electron microscopy, transmission electron microscopy, wavelength dispersive spectroscopy, atom probe tomography, and micropillar compression were utilized to evaluate microstructural evolution and mechanical behavior as a function of oxygen content. Elevated oxygen levels induced morphology, number density, and size changes for the metastable omega phase and accelerated alpha nucleation rate. Notably, oxygen partitioning to omega during ageing resulted in increased resistance of omega to precipitate shearing and suppression of catastrophic failure during micropillar compression. While both oxygen and omega are known embrittlement risk factors, the stabilization of omega with oxygen leads to promising microstructures and mechanical properties. Furthermore, oxygen-induced refinement of alpha precipitates provides an additional pathway to obtain fine alpha laths that enable precipitation strengthening of beta Ti alloys and very high strengths required for structural components. Finally, mechanistic understanding of Si’s improvement of titanium oxidation resistance using Si-coated Ti specimens showed that Ti5Si3 silicide formation during oxidation exposures inhibited inward oxygen diffusion and formation of fast growing internal TiO2 scales. This understanding may inform not only the design of better protective coatings for alloys used at elevated temperatures but also the tailoring of alloy chemistries leading to similar oxidation mechanisms.

In conclusion, the results detailed in this thesis address existing severe limitations associated with oxygen in titanium alloys. These findings directly impact commercial applications by providing design strategies to mitigate detrimental effects from interstitial oxygen, omega precipitation, and environmental degradation. This knowledge will contribute to future titanium alloy chemistry and processing development that utilizes beneficial impacts of elevated oxygen to enable new microstructures, properties, industrial material reuse, and commercial material specifications.