Ten Characteristics and Three Special Functions of Titanium

Ten Characteristics of Titanium 

(1) Low Density, High Strength, and High Specific Strength

The density of titanium is 4.51g/cm³, which is 57% of that of steel. Titanium is less than twice as heavy as aluminum, but its strength is three times greater than that of aluminum. The specific strength (the ratio of strength to density) of titanium alloys is the highest among commonly - used industrial alloys (see Table 2 - 1). The specific strength of titanium alloys is 3.5 times that of stainless steel, 1.3 times that of aluminum alloys, and 1.7 times that of magnesium alloys. Therefore, it is an essential structural material in the aerospace industry. (See Table 2 - 1 for the comparison of density and specific strength between titanium and other metals) 

(2) Excellent Corrosion Resistance

The passivity of titanium depends on the presence of an oxide film. Its corrosion resistance in oxidizing media is much better than that in reducing media. High - rate corrosion occurs 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 media that react with titanium to produce hydrogen (such as hydrochloric acid and sulfuric acid), titanium usually has a relatively high corrosion rate. But if a small amount of an oxidant is added to the acid, a passive film will be formed on the surface of titanium. Therefore, titanium is corrosion - resistant even in a mixture of strong sulfuric - nitric acid or hydrochloric - nitric acid, or even in hydrochloric acid containing free chlorine. The protective oxide film of titanium is often formed when the metal comes into contact with water, and it can be formed even in a small amount of water or water vapor. If titanium is exposed to a strongly oxidizing environment without any water, rapid oxidation and violent reactions will occur, and even spontaneous combustion often occurs. Such phenomena have occurred in the reactions of titanium with fuming nitric acid containing excessive nitrogen oxides and the reaction of titanium with dry chlorine. Therefore, to prevent such reactions, a certain amount of water must be present. 

(3) Good Heat Resistance

Usually, aluminum loses its original properties at 150°C, and stainless steel loses its original properties at 310°C. However, titanium alloys still maintain good mechanical properties at around 500°C. When the speed of an aircraft reaches 2.7 times the speed of sound, the surface temperature of the aircraft structure reaches 230°C. At this time, aluminum and magnesium alloys can no longer be used, but titanium alloys can meet the requirements. Titanium's good heat - resistance property enables it to be used in the disks and blades of aero - engine compressors and the skin of the rear fuselage of an aircraft. 

(4) Good Low - Temperature Performance

The strength of some titanium alloys (such as Ti - 5AI - 2.5SnELI) increases as the temperature decreases, but the plasticity does not decrease much. They still have good ductility and toughness at low temperatures and are suitable for use at ultra - low temperatures. They can be used in liquid - hydrogen - liquid - oxygen rocket engines or as ultra - low - temperature containers and tanks in manned spacecraft. 

(5) Non - Magnetic

Titanium is non - magnetic. When it is used in the hull of a submarine, it will not trigger the explosion of a mine. 

(6) Low Thermal Conductivity

See Table 2 - 2 for the comparison of the thermal conductivity of titanium and other metals. Table 2 - 2 Comparison of the thermal conductivity of titanium and other metals. The thermal conductivity of titanium is small, only 1/5 of that of steel, 1/13 of that of aluminum, and 1/25 of that of copper. Poor thermal conductivity is a disadvantage of titanium, but in some cases, this characteristic of titanium can be utilized. 

(7) Low Elastic Modulus

See Table 2 - 3 for the comparison of the elastic modulus of titanium and other metals. Table 2 - 3 Comparison of the elastic modulus of titanium and other metals. The elastic modulus of titanium is only 55% of that of steel. When it is used as a structural material, a low elastic modulus is a disadvantage. 

(8) Tensile Strength and Yield Strength Are Close

The tensile strength of Ti - 6AI - 4V titanium alloy is 960MPa, and the yield strength is 892MPa. The difference between the two is only 58MPa. (See Table 2 - 4. Comparison of the tensile strength and yield strength of titanium and other metals) 

(9) Titanium Is Easily Oxidized at High Temperatures

Titanium has a strong binding force with hydrogen and oxygen. Attention should be paid to preventing 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 in a vacuum. The heat treatment of titanium forgings should be carried out with the control of a slightly oxidizing atmosphere. 

(10) Low Damping Performance

If clocks made of titanium and other metal materials (copper, steel) with the same shape and size are struck with the same force, it will be found that the sound of the clock made of titanium lasts for a long time. That is, the energy given to the clock through knocking does not disappear easily. Therefore, we say that titanium has a low damping performance. 

Three Special Functions of Titanium 

(1) Shape - Memory Function

It refers to the Ti - 50% Ni (atomic) alloy. Under certain temperature conditions, it has the ability to recover its original shape. This kind of material is called a shape - memory alloy. 

(2) Superconducting Function

It refers to the NbTi alloy. When the temperature drops close to absolute zero, the wire made of NbTi alloy will lose its resistance. Any large current passing through the wire will not cause the wire to heat up, and there is no energy consumption. NbTi is called a superconducting material. 

(3) Hydrogen - Storage Function

It refers to the Ti - 50% Fe (atomic) alloy, which has the ability to absorb a large amount of hydrogen. By using this characteristic of TiFe, hydrogen can be stored safely. That is, storing hydrogen does not necessarily require a high - pressure steel gas cylinder. Under certain conditions, TiFe can also release hydrogen. TiFe is called an energy - storage material.