Titanium is an element with an atomic number of 22 in the periodic table, a subgroup element of the fourth period, which includes zirconium and hafnium in addition to titanium, and is characterized by a high melting point and a stable oxide film on its surface at room temperature.
Characteristics of Titanium
1.Small density, large specific strength
The density of titanium is 4.51g/cm3, which is 57% of steel, titanium is less than twice as heavy as aluminum, and its strength is three times that of aluminum. The specific strength of Ti- alloy (strength/density ratio) is 3.5 times that of stainless steel; and is 1.3 times of aluminum alloy,1.7 times the magnesium alloy.
2.Excellent corrosion resistance
The passivity of titanium depends on the existence of oxide film, and its corrosion resistance in oxidizing media is much better than in reducing media. High rates of corrosion occur in reducing media. Titanium is not corroded in some corrosive media, such as seawater, wet chlorine, chlorite and hypochlorite solutions, nitric acid, chromic acid, metal chlorides, sulfides and organic acids. However, in the medium that reacts with titanium to produce hydrogen (such as hydrochloric acid and sulfuric acid), titanium usually has a high corrosion rate. However, if a small amount of oxidizing agent is added to the acid, a passivation film will be formed on the surface of titanium. Therefore, in the mixture of strong sulfuric acid – nitric acid or hydrochloric acid – nitric acid, and even in hydrochloric acid containing free chlorine, titanium is corrosion resistant. Titanium’s protective oxide film is often formed when the metal hits water, even in small amounts of water or water vapor. If titanium is exposed to a strong oxidizing environment in which there is no water at all, violent reactions will occur, and even spontaneous combustion will often occur.
3.Good heat resistance
Usually aluminum at 150 ～ C, stainless steel at 310 ～ C that lost the original properties, while titanium alloy at about 500 ～ C still maintains good mechanical properties. When the aircraft speed reaches 2.7 times the speed of sound, the surface temperature of the aircraft structure reaches 230⊥, aluminum alloy and magnesium alloy can not be used, and titanium alloy can meet the requirements. Titanium has good heat resistance, and it is used in the disc and blade of the aeroengine compressor and the skin of the aircraft rear fuselage.
4 Good low temperature performance
The strength of some titanium alloys (such as Ti-5AI-2.5SnELI) increases with the decrease of temperature, but the plasticity is not reduced much, and it still has good ductility and toughness at low temperatures, which is suitable for use at ultra-low temperatures. It can be used in dry liquid hydrogen and liquid oxygen rocket engines, or in manned spacecraft as ultra-low temperature containers and storage tanks.
5. Non-magnetic
Titanium is non-magnetic, it is used in submarine hull, will not cause mine explosion.
6. Small thermal conductivity
The thermal conductivity of titanium is small, only 1/5 of steel, 1/13 of aluminum, and 1/25 of copper. Poor thermal conductivity is a drawback of titanium, but in some cases this feature of titanium can be used.
7. Low elastic modulus
The elastic modulus of titanium is only 55% of that of steel, and low elastic modulus is a disadvantage when used as a structural material.
8.Tensile strength and yield strength is very close
The tensile strength of Ti-6AI-4V titanium alloy is 960MPa and the yield strength is 892MPa, and the difference between the two is only 58MPa, as shown in Table 2-4.
9.Titanium is easy to be oxidized at high temperatures
Titanium has a strong binding force with hydrogen and oxygen, and attention should be paid to preventing oxidation and hydrogen absorption. Titanium welding should be carried out under the protection of argon to prevent pollution. Titanium tube and sheet should be heat treated under vacuum, and micro-oxidizing atmosphere should be controlled during heat treatment of titanium forgings.
10. Low damping performance
With titanium and other metal materials (copper, steel) made of the same shape and size of the clock, with the same force to knock each bell will find that the clock made of titanium oscillation sound lasts a long time, that is, the energy given by knocking the clock is not easy to disappear, so we say that the damping performance of titanium is low.
There are three special functions of titanium
1) Shape memory function
This function refers to Ti-50% Ni (atomic) alloy, under certain temperature conditions, can restore its original shape ability, called this material shape memory alloy.
2) Superconducting function
This function of Ti refers to NbTi alloy, when the temperature drops to close to absolute zero, the wire made of NbTi alloy, will lose resistance, any large current through, the wire will not heat, no energy consumption, NbTi is called superconducting material.
3) Hydrogen storage function
Ti-50% Fe (atomic) alloy has the ability to absorb a large amount of hydrogen. Using this feature of TiFe, hydrogen can be safely stored, that is, hydrogen storage does not necessarily use steel high-pressure cylinders. Under certain conditions, hydrogen can also be released by TiFe, which is called energy storage material.
