The characteristics and functions of titanium
Titanium is an element with atomic number 22 in the periodic table. The fourth period element subgroup, the IVB designation, includes zirconium and hafnium in addition to titanium. The common feature is a high melting point, which is formed on its surface at room temperature. stable oxide film.
1. Low density, high strength, high specific strength
The density of titanium is 4.51g/cm3, which is 57% of steel. Titanium is less than twice the weight of aluminum, yet three times stronger. The specific strength (strength/density ratio) of titanium alloy is the largest among commonly used industrial alloys (see Table 2-1). The specific strength of titanium alloy is 3.5 times that of stainless steel; 1.3 times that of aluminum alloy; 1.7 times that of magnesium alloy. It is an indispensable structural material for the aerospace industry.
Table 2-1 Density and specific strength comparison of titanium and other metals
2. Excellent corrosion resistance
The passivation of titanium depends on the presence of oxide films, which are more resistant to corrosion in oxidizing media than in reducing media. High-speed corrosion occurs in reducing media. Titanium is not corroded in some corrosive media, such as sea water, wet chlorine, chlorite and hypochlorite solutions, nitric acid, chromic acid, metal chlorides, sulfides and organic acids. But in the medium (such as hydrochloric acid, sulfuric acid) that reacts with titanium to generate hydrogen, titanium usually has a large corrosion rate. However, if a small amount of oxidant is added to the acid, a passivation film will be formed on the titanium surface. Therefore, titanium has corrosion resistance in concentrated sulfuric acid-nitric acid or hydrochloric acid-nitric acid mixture, even in hydrochloric acid containing free chlorine. A protective oxide film of titanium usually forms when the metal comes into contact with water, even in small amounts or in water vapor. If titanium is exposed to a strong oxidizing environment without water, rapid oxidation and violent reactions will occur, and often even spontaneous combustion will occur. This phenomenon occurs when titanium reacts with fuming nitric acid containing excess nitrogen oxides and when titanium reacts with dry chlorine gas. Therefore, in order to prevent such reactions, there must be a certain amount of water. Even in small amounts or in water vapor. If titanium is exposed to a strong oxidizing environment without water, rapid oxidation and violent reactions will occur, and often even spontaneous combustion will occur. This phenomenon occurs when titanium reacts with fuming nitric acid containing excess nitrogen oxides and when titanium reacts with dry chlorine gas. Therefore, in order to prevent such reactions, there must be a certain amount of water. Even in small amounts or in water vapor. If titanium is exposed to a strong oxidizing environment without water, rapid oxidation and violent reactions will occur, and often even spontaneous combustion will occur. This phenomenon occurs when titanium reacts with fuming nitric acid containing excess nitrogen oxides and when titanium reacts with dry chlorine gas. Therefore, in order to prevent such reactions, there must be a certain amount of water.
3. Good heat resistance
Usually aluminum loses its original properties at 150°C, stainless steel loses its original properties at 310°C, and titanium alloys still maintain good mechanical properties at around 500°C. When the speed of the aircraft reaches 2.7 times the speed of sound, the surface temperature of the aircraft structure reaches 230°C, and aluminum and magnesium alloys cannot be used, while titanium alloys can meet the requirements. Titanium has good heat resistance and is used in the disks and blades of aero-engine compressors and the skin of the aircraft's 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 does not decrease much. It still has good plasticity and toughness at low temperature, suitable for use at ultra-low temperature. It can be used for dry liquid hydrogen and liquid oxygen rocket engines, and can also be used for ultra-low temperature containers and storage tanks of manned spacecraft.
5. non-magnetic
Titanium is non-magnetic and is used in submarine shells and will not cause mine explosions.
6. Small thermal conductivity
The comparison of thermal conductivity between titanium and other metals is shown in Table 2-2.
Table 2-2 Comparison of thermal conductivity between titanium and other metals
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 disadvantage of titanium, but this property of titanium can be exploited in some cases.
7. low elastic modulus
The elastic modulus of titanium is compared with other metals in Table 2-3.
Table 2-3 Elastic modulus comparison of titanium and other metals
The modulus of elasticity of titanium is only 55% of that of steel, and its low modulus of elasticity is a disadvantage when it is used as a structural material.
8. Tensile strength and yield strength are very similar
The tensile strength of Ti-6AI-4V titanium alloy is 960MPa, yield strength is 892MPa, the difference between the two is only 58MPa, see Table 2-4.
Table 2-4 Comparison of tensile strength and yield strength of titanium and other metals
9. Titanium is easily oxidized at high temperature
Titanium has strong hydrogen and oxygen bonds, and care must be taken to prevent oxidation and hydrogen absorption. Titanium welding should be carried out under argon protection to prevent contamination. Titanium tubes and thin plates should be heat treated under vacuum, and the heat treatment of titanium forgings should control the micro-oxidative atmosphere.
10 .Low damping resistance
Use titanium and other metal materials (copper, steel) to make bells of exactly the same shape and size, and strike each bell with the same force, and you will find that the bell made of titanium vibrates and the sound lasts, that is, by ringing the bell. The given energy is not easy to disappear, so we say that the damping performance of titanium is low.
