How much do you know about the principles of titanium alloys?

Titanium alloys are composed of titanium combined with other elements. Titanium has two allotropic forms: below 882°C, it is the closely packed hexagonal α-titanium, and above 882°C, it is the body-centered cubic β-titanium.

Alloy elements can be categorized into three types based on their influence on the phase transition temperature.

Elements that stabilize the α phase and raise the phase transformation temperature are known as α-stabilizing elements, including aluminum, carbon, oxygen, and nitrogen. Among them, aluminum is a primary alloying element in titanium alloys, significantly enhancing the alloy's strength at both room and high temperatures, reducing its density, and increasing its modulus of elasticity.

Elements that stabilize the β phase and lower the phase transition temperature are known as β-stabilizing elements, which can be further categorized into two types: eutectoid and peritectoid. The former includes molybdenum, niobium, vanadium, etc.; the latter includes chromium, manganese, copper, iron, silicon, and so on.

Elements that have little effect on the phase change temperature are neutral elements, such as zirconium and tin.

Oxygen, nitrogen, carbon, and hydrogen are the main impurities in titanium alloys. Oxygen and nitrogen have a high solubility in the α phase, significantly strengthening the alloy but reducing its ductility. Typically, the oxygen and nitrogen content in titanium is regulated to be below 0.15% to 0.2% and 0.04% to 0.05%, respectively. Hydrogen has a very low solubility in the α phase, and excessive hydrogen in titanium alloys can form hydrides, making the alloy brittle. Generally, the hydrogen content in titanium alloys is controlled to be below 0.015%. The solubility of hydrogen in titanium is reversible and can be removed through vacuum annealing.