Alloying elements can be divided into three categories according to their influence on the phase transition temperature:
1. The elements that stabilize the α phase and increase the phase transition temperature are α stabilizing elements, such as aluminum, carbon, oxygen, and nitrogen. Among them, aluminum is the main alloy element of titanium alloy, which has obvious effects on improving the alloy’s normal temperature and high-temperature strength, reducing specific gravity, and increasing elastic modulus.
2. The element that stabilizes the β phase and reduces the phase transition temperature is the β-stabilizing element, which can be divided into two types: isomorphic and eutectoid. Products using titanium alloys include molybdenum, niobium, and vanadium; the latter include chromium, manganese, copper, iron, and silicon.
3. The elements that have little effect on the phase transition temperature are neutral elements, such as Zr, Sn, etc.
Oxygen, nitrogen, carbon, and hydrogen are the main impurities in titanium alloys. Oxygen and nitrogen have greater solubility in the α phase, which has a significant strengthening effect on the titanium alloy, but it reduces the plasticity. It is usually stipulated that the oxygen and nitrogen content in titanium should be below 0.15-0.2% and 0.04-0.05% respectively. The solubility of hydrogen in the α phase is very small, and too much hydrogen dissolved in the titanium alloy will produce hydrides, which will make the alloy brittle. Generally, the hydrogen content in titanium alloys is controlled below 0.015%. The dissolution of hydrogen in titanium is reversible and can be removed by vacuum annealing.