Compared with other metal materials, titanium alloys have the following advantages:
1. The specific strength (tensile strength/density) is high (see figure), the tensile strength can reach 100~140kgf/mm2, and the density is only 60% of steel.
2. Good medium temperature strength, the use temperature is hundreds of degrees higher than that of aluminum alloy, it can still maintain the required strength at medium temperature, and can work for a long time at a temperature of 450~500 °C.
3. Good corrosion resistance, a uniform and dense oxide film is immediately formed on the surface of titanium in the atmosphere, and it has the ability to resist the erosion of a variety of media. In general, titanium has good corrosion resistance in oxidizing and neutral media, and better corrosion resistance in seawater, wet chlorine and chloride solutions. However, in reducing media, such as hydrochloric acid, titanium has poor corrosion resistance.
4. Titanium alloys with good low temperature performance and very low interstitial elements, such as TA7, can maintain a certain plasticity at -253 °C.
5. Low elastic modulus, small thermal conductivity, no ferromagnetism.
6. High hardness.
7. Poor stamping and good thermoplasticity.
Heat treatment Titanium alloys can obtain different phase compositions and microstructures by adjusting the heat treatment process. It is generally believed that the fine equiaxed structure has good plasticity, thermal stability and fatigue strength, the acicular structure has high durability strength, creep strength and fracture toughness, and the equiaxed and acicular mixed structure has good comprehensive properties.
Commonly used heat treatment methods are annealing, solution and aging treatment. Annealing is to eliminate internal stress, improve plasticity and microstructure stability to obtain better comprehensive properties. Generally, the annealing temperature of α alloy and (α+β) alloy is selected at 120~200 °C below the transition point of (α α+β)-→β phase β α; Generally, the quenching of (α+β) alloys is carried out at 40~100 °C below the (α+β)-→β phase transition point, and the quenching of sub-stable β alloys is carried out at 40~80 °C above the (α+β)-→β phase transition point. The aging temperature is generally 450~550°C. In addition, in order to meet the special requirements of the workpiece, metal heat treatment processes such as double annealing, isothermal annealing, β heat treatment, and deformation heat treatment are also used in the industry.