Titanium Alloying

Titanium alloys are alloys composed of titanium with the addition of other elements. Titanium has two types of homogeneous heterocrystals: densely arranged hexagonal structure α-titanium below 882 °C, and body-centred cubic β-titanium above 882 °C.
Alloying elements can be divided into three categories according to their effect on the phase transition temperature:
① The elements that stabilise the α phase and increase the phase transition temperature are α-stabilising elements, such as aluminium, carbon, oxygen and nitrogen. 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 modulus of elasticity.
The elements that stabilise the β-phase and reduce the phase transition temperature are the β-stabilising elements, which can be divided into homocrystalline and eutectic types. Application of titanium alloy products 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.