High-Temperature Oxidation Mechanisms of Titanium and Titanium Alloys

Abstract

To help combat climate change, we need to develop durable, lightweight structural materials for transportation, especially in aerospace where minor weight reductions can dramatically reduce fuel consumption and CO2 emissions. Among candidate materials, titanium alloys are particularly promising because of their high strength-to-weight ratio, fracture toughness, and resistance to fatigue and creep. Although widely used for aircraft structural components, titanium alloys remain poorly suited to the hottest areas in jet engines because they oxidize rapidly above 550 °C. During oxidation, oxygen reacts with titanium, readily diffusing into the metal and also forming (rutile TiO2) oxide scale. Both phenomena are undesirable, with oxygen embrittling the metal and oxide growth reflecting a loss of load-bearing metal. The oxidation behavior of titanium can be improved by alloying it with small amounts of other elements. In particular, Si and Nb are known to be beneficial. Additionally, nitrogen found in air can also influence the oxidation behavior, typically with positive effects of reducing overall oxidation rates, oxide thickness, and the amount of oxygen dissolved in the metal. Similarly to oxygen, nitrogen diffuses into the metal and can form nitride phases at the oxide-metal interface. Despite the known benefits of Si and Nb alloying additions and nitrogen, their influence on the oxidation mechanisms remains poorly understood. Notably, it remains challenging to distinguish the role of nitrogen from potential synergy with alloying additions. Therefore, this thesis investigates the mechanisms by which Si, Nb, and atmospheric nitrogen influence the oxidation response of titanium. To this end, I systematically characterized and compared the behavior of pure titanium and titanium alloys containing Si (0.8 at.%) or Nb (2 at.%) during oxidation at 800 °C for up to 100 h in O2 or N2-O2 atmospheres. Without atmospheric nitrogen, Si and Nb had no effect on oxygen dissolution in the metal. Instead, they slowed oxide scale growth via two different mechanisms previously unidentified in the literature. Si initially formed a protective Ti5Si3 layer at the scale-metal interface that prevented inward oxide growth. In contrast, Nb doped the oxide grain boundaries, slowing outward oxide growth presumably by slowing outward Ti transport along the oxide grain boundaries. With atmospheric nitrogen, nitrogen in the surface of the metal can play a key role in reducing oxygen concentrations throughout the metal, as evidenced using thermodynamic calculations and diffusion simulations. Notably, the mechanistic role of nitrogen in the metal appears to be the same independent of alloying elements in the metal. Synergistically with alloying, nitrogen helps the oxide scales remain compact and protective over extended oxidation time. In this thesis, I discredited many mechanisms repeated throughout the literature, evidenced distinct and previously unidentified mechanisms by which Si and Nb influence scale growth, and conclusively demonstrated the beneficial role of nitrogen in the metal in reducing oxygen concentrations in the metal. For the first time in the titanium oxidation literature, I showed that oxygen transport through the oxide primarily takes place along oxide grain boundaries, rather than through the lattice as previously assumed. Finally, I demonstrated a null effect of alloying additions on the behavior of O and N in the metal, suggesting in contrast to the literature that dilute alloying is ineffective for influencing O and N solubilities or diffusivities in the metal. These new findings provide a critical scientific basis for engineering long-lasting, oxidation-resistant titanium alloys.