Titanium Alloy Elements

Titanium alloys are alloys consisting of titanium as a base with other elements added. Titanium has two kinds of homogeneous and heterogeneous crystals: α-titanium with dense hexagonal structure below 882℃, and β-titanium with body-centred cubic structure above 882℃. Alloying elements can be divided into three categories according to their effect on the phase transition temperature: ① stabilising the α phase and increasing the phase transition temperature of the elements for the α-stabilising elements, such as aluminium, carbon, oxygen and nitrogen. Among them, aluminium is the main alloying element of titanium alloy, which has obvious effects on improving the strength of the alloy at room temperature and high temperature, reducing the specific gravity and increasing the elastic modulus. ② Stabilisation of β-phase, reduce the phase transition temperature of the elements for the β-stabilising elements, and can be divided into homocrystalline and eutectic type two. The former has molybdenum, niobium, vanadium, etc.; the latter has chromium, manganese, copper, iron, silicon, etc.. ③ The elements that have little effect on the phase transition temperature are neutral elements, such as zirconium and tin.

Oxygen, nitrogen, carbon and hydrogen are the main impurities in titanium alloys. Oxygen and nitrogen in the α-phase has a greater solubility, titanium alloy has a significant strengthening effect, but the plasticity is reduced. The content of oxygen and nitrogen in titanium is usually 0.15-0.2% and 0.04-0.05% respectively. Hydrogen in the α-phase solubility is very small, titanium alloys dissolved in excess of hydrogen will produce hydride, so that the alloy becomes brittle. Normally, the hydrogen content in titanium alloys is kept below 0.015%. The dissolution of hydrogen in titanium is reversible and can be removed by vacuum annealing.