Titanium alloy is based on titanium to join other elements of the alloy. Titanium has two kinds of homogeneous amorphous: 882 ℃ below the closed hexagonal structure of α titanium, 882 ° C above the body of the heart of the β titanium. Alloy elements according to their impact on the phase transition temperature can be divided into three categories: ① stable α phase, to improve the phase transition temperature of the elements of α stable elements, aluminum, carbon, oxygen and nitrogen. Aluminum is the main alloy of titanium alloy elements, it is to improve the alloy at room temperature and high temperature strength, reduce the specific gravity, increase the elastic modulus has a significant effect. ② stable β phase, lower phase transition temperature of the elements for the β-stable elements, but also sub-crystal and eutectoid two. The former are molybdenum, niobium, vanadium, etc .; the latter are chromium, manganese, copper, iron, silicon and so on. ③ little effect on the phase transition temperature elements for the neutral elements, zirconium, tin and so on.

Oxygen, nitrogen, carbon and hydrogen are the main impurities in titanium alloys. Oxygen and nitrogen have a greater solubility in the α phase, the titanium alloy has a significant strengthening effect, but the plasticity decreased. It is generally stipulated that the oxygen and nitrogen contents in titanium are 0.15 to 0.2% and 0.04 to 0.05%, respectively. Hydrogen in the α phase solubility is very small, too much hydrogen dissolved in the titanium alloy will produce hydride, the alloy brittle. Usually the hydrogen content in titanium alloy is controlled below 0.015%. The dissolution of hydrogen in titanium is reversible and can be removed by vacuum annealing.