Titanium is an allotropic isomer with a melting point of 1720°C. It has a dense hexagonal lattice structure, called α-titanium, when it is lower than 882°C, and a body-centered cubic lattice structure, called β-titanium, when it is higher than 882°C. The use of titanium's different characteristics of the above two structures, add the appropriate alloying elements, so that its phase transition temperature and phase content gradually change and get different organizations of titanium alloys (titanium alloys). At room temperature, titanium alloys have three types of matrix organization, titanium alloys are also divided into the following three categories: α-alloys, (α + β) alloys and β-alloys. In China, these alloys are referred to as TA, TC and TB respectively.
Alpha titanium alloy
It is a single-phase alloy composed of α-phase solid solution, which is α-phase, stable organization, higher wear resistance than pure titanium, and strong oxidation resistance, both at general temperature and at higher temperature of practical application. Under the temperature of 500℃～600℃, it still maintains its strength and creep resistance, but it cannot be strengthened by heat treatment, and its room temperature strength is not high.
β Titanium Alloy
It is a single-phase alloy composed of β-phase solid solution, which has high strength without heat treatment, and the alloy is further strengthened after quenching and aging, and the room-temperature strength can be up to 1,372～1,666MPa; however, it has poor thermal stability, and it is not suitable to be used at high temperatures.
α+β Titanium Alloy
It is a dual-phase alloy with good comprehensive performance, good organizational stability, good toughness, plasticity and high-temperature deformation properties, can be better hot pressure processing, can be quenched, aging to make the alloy strengthened. After heat treatment, the strength is about 50% higher than the annealed state; high-temperature strength, can work for a long time at a temperature of 400 ℃ to 500 ℃, and its thermal stability is second to α titanium alloy.
Three kinds of titanium alloys commonly used are α titanium alloy and α + β titanium alloy; α titanium alloy has good machinability, α + β titanium alloy is second, β titanium alloy is poor. α titanium alloy code for TA, β titanium alloy code for TB, α + β titanium alloy code for TC.
Titanium alloys can be divided into heat-resistant alloys, high-strength alloys, corrosion-resistant alloys (titanium-molybdenum, titanium-palladium alloys, etc.), low-temperature alloys, and special functional alloys (titanium-iron hydrogen storage materials and titanium-nickel memory alloys). The composition and properties of typical alloys are shown in the table.
Heat treatment Titanium alloys can obtain different phase compositions and organizations by adjusting the heat treatment process. It is generally believed that the fine isometric organization has better plasticity, thermal stability and fatigue strength; needle-like organization has higher endurance strength, creep strength and fracture toughness; isometric and needle-like mixed organization has better overall performance.
