Titanium alloys are alloys based on titanium and added to other elements. Titanium has two isomorphous crystals: below 882 ° C is a close-packed hexagonal structure α titanium, and above 882 ° C is a body-centered cubic β titanium.Oxygen, nitrogen, carbon and hydrogen are the main impurities of titanium alloys. Oxygen and nitrogen have a large solubility in the α phase, and have a significant strengthening effect on the titanium alloy, but the plasticity is lowered. The content of oxygen and nitrogen in the titanium is usually specified to be 0.15 to 0.2% and 0.04 to 0.05% or less, respectively. Hydrogen has a low solubility in the alpha phase, and excessive hydrogen is dissolved in the titanium alloy to produce a hydride which makes the alloy brittle. Usually, the hydrogen content in the titanium alloy is controlled to be less than 0.015%. The dissolution of hydrogen in titanium is reversible and can be removed by vacuum annealing.