Titanium alloys are alloys based on titanium with other elements added. There are two kinds of homogenous crystals in titanium: α titanium with dense hexagonal structure below 882°C and β titanium with body centered cubic structure above 882°C. The alloying elements can be divided into three categories according to their effects on phase transition temperature :① α -stable elements, which increase phase transition temperature, are α -stable elements, including aluminum, carbon, oxygen and nitrogen. Aluminum is the main alloying element in titanium alloy, which has obvious effect on improving the strength of alloy at room temperature and high temperature, reducing the specific gravity and increasing the elastic modulus. (2) β -stable elements can be divided into two types: isocrystalline and eutectoid.
α titanium alloy: it is a single-phase alloy composed of α phase solid solution. It is α phase either at general temperature or at high practical application temperature. Its structure is stable, wear resistance is higher than pure titanium, and oxidation resistance is strong. At 500°C~600°C, it still retains its strength and creep resistance, but it cannot be strengthened by heat treatment, and its strength at room temperature is not high.
β titanium alloy: it is a single phase alloy composed of β phase solid solution. It has high strength without heat treatment. After quenching and time effect, the alloy is further strengthened and its strength can reach 1372~1666MPa at room temperature. But poor thermal stability, not suitable for use at high temperature.
α+β titanium alloy: it is a duplex alloy, with good comprehensive performance, good microstructure stability, good toughness, plasticity and high temperature deformation performance, can be better for hot pressure processing, quenching, aging to strengthen the alloy. The strength after heat treatment is about 50 %~100 % higher than that after annealing. It has high high temperature strength and can be operated at 400°C~500°C for a long time. Its thermal stability is inferior to α titanium alloy. The most commonly used of the three titanium alloys are α titanium alloy and α+β titanium alloy; The cutting machinability of α titanium alloy is the best, followed by α+β titanium alloy, and β titanium alloy is the worst. α titanium alloy is code-named TA, β titanium alloy is code-named TB, α+β titanium alloy is code-named TC.
