Alloying Elements Classification

The alloying element 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.
According to their effect on the phase transition temperature
Alloying elements can be classified into three categories according to their effects on the phase transition temperature.
① Stabilizing α-phase, increase the phase transition temperature of the elements for the α-stabilizing elements, aluminium, carbon, oxygen and nitrogen, etc.. 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. It is usually stipulated that the content of oxygen and nitrogen in titanium is 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.
Depending on the phase composition
Titanium alloys can be divided into three categories according to the composition of the phase: α-alloys, (α + β) alloys and β-alloys, which are expressed as TA, TC and TB in China respectively.

① α-alloys contain a certain amount of stable α-phase elements, the equilibrium state is mainly composed of α-phase. α-alloys have a small specific gravity, good heat strength, good weldability and excellent corrosion resistance, the disadvantage of the room temperature strength is low, usually used as a heat-resistant materials and corrosion-resistant materials. α-alloys can be divided into full-α-alloys (TA7), near-α-alloys (Ti-8Al-1Mo-1V) and a small number of compounds of the α-alloys (Ti-2.0%) and α-alloys (Ti-2.2%), α-alloys (Ti-2.2%) and α-alloys (Ti-2.2%). (Ti-2.5Cu). ② (α+β) alloys contain a certain amount of elements that stabilise the α- and β-phases, and in equilibrium the alloy is organised in the α- and β-phases. (α+ β) alloy has medium strength, and can be heat-treated to strengthen, but the welding performance is poor. (α + β) alloys are widely used, of which the production of Ti-6Al-4V alloys accounted for more than half of all titanium materials.
③ β alloys contain a large number of stable β-phase elements, high temperature β-phase can be retained to room temperature. β alloys can be divided into heat-treatable β alloys (sub-stable β alloys and nearly sub-stable β alloys) and heat-stable β alloys. Heat-treatable β-alloys have excellent plasticity in the quenched state and can be aged to a tensile strength of 130-140kgf/mm2. β-alloys are usually used as high-strength, high-toughness materials. Disadvantages are high specific gravity, high cost, poor welding performance, cutting and machining difficulties.