Ten properties and three special functions of titanium

Titanium is an element with an atomic number of 22 in the periodic table. It is a subgroup element of the fourth period, namely ⅣB. Besides titanium, there are zirconium and hafnium in this group, which are characterized by high melting point and Titanium Welded Tube forming stable oxide film on its surface at room temperature.

Ten properties of titanium

(1) Low density, high strength and high specific strength

Titanium has a density of 4.51g/cm3, which is 57% that of steel. Titanium is less than twice as heavy and three times as strong as aluminum. The specific strength (strength/density ratio) of titanium alloy is the largest among common industrial alloys (see Table 2-1). The specific strength of titanium alloy is 3.5 times that of stainless steel. 1.3 times of aluminum alloy; Magnesium alloy is 1.7 times, so it is an essential structural material for aerospace industry.

(2) Excellent corrosion resistance

The dullness of titanium depends on the presence of an 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 gas, chlorite and hypochlorite solution, nitric acid, chromic acid, metal chloride, sulfide and organic acid. However, in media that react with titanium to produce hydrogen (such as hydrochloric acid and sulfuric acid), titanium usually has a higher corrosion rate.

But adding a small amount of oxidant to the acid can cause a passivating film to form on the surface of the titanium. So in a mixture of strong sulfuric acid and nitric acid or hydrochloric acid, or even hydrochloric acid containing free chlorine, titanium is resistant to corrosion. Titanium's protective oxide film is often formed when the metal meets water, even in small amounts of water or steam. If titanium is exposed to a strong oxidizing environment in which there is no water at all, it will oxidize rapidly and produce violent reactions, often spontaneous combustion. Such phenomena have occurred in the reaction of titanium with fuming nitric acid containing excessive nitrogen oxide and with dry chlorine gas. So to prevent this kind of reaction, you have to have a certain amount of water.

(3) Good heat resistance

Usually aluminum at 150℃, stainless steel at 310℃ that lost the original performance, and titanium alloy at 500℃ or so still maintain 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, while titanium alloy can meet the requirements. Titanium is heat-resistant and is used in the discs and blades of aeroengine compressors and in the skin of aircraft rear fuselages.

(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 temperature, which is suitable for use at ultra-low temperature. It can be used in dry liquid hydrogen and oxygen rocket engines, or as cryogenic containers and storage tanks on manned spacecraft.

(5) No magnetic field

Titanium is non-magnetic. It is used in submarine shells and does not cause mines to explode.

(6) Low thermal conductivity

Table 2-2 shows the comparison of thermal conductivity between titanium and other metals.

Table 2-2 Compares the thermal conductivity of titanium with that of 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 drawback of titanium, but it can be exploited in some situations.

(7) Low elastic modulus

The comparison of elastic modulus between titanium and other metals is shown in Table 2-3.

Table 2-3 Comparison of elastic modulus between titanium and other metals

The elastic modulus of titanium is only 55% of that of steel, which is a disadvantage when it is used as a structural material.

(8) The tensile strength is close to the yield strength

Ti-6AI-4V titanium alloy tensile strength is 960MPa, yield strength is 892MPa, the difference between the two is only 58MPa, as shown in Table 2-4.

Table 2-4 Comparison of tensile strength and yield strength between titanium and other metals

(9) Titanium is easy to be oxidized at high temperature

Titanium has a strong binding force with hydrogen and oxygen, and attention should be paid to prevent oxidation and hydrogen absorption. Titanium welding should be carried out under the protection of argon to prevent pollution. Titanium tube and thin plate should be heat treated under vacuum, and the micro oxidizing atmosphere should be controlled during heat treatment of titanium forging.

(10) Low damping resistance

Clocks made of titanium and other metal materials (copper, steel) have exactly the same shape and size. If each bell is struck with the same force, it will be found that the sound of the clock made of titanium will last a long time when it oscillates, that is, the energy given to the clock by knocking is not easy to disappear. Therefore, we say that the damping performance of titanium is low.

Three special functions of titanium

(1) Shape memory function

Refers to Ti-50% Ni (atomic) alloy, under certain temperature conditions, the ability to restore its original shape, called this material shape memory alloy.

(2) Superconducting function

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

It refers to Ti-50% Fe (atomic) alloy, which has the ability to absorb large amounts of hydrogen. Using this feature of TiFe, hydrogen can be stored safely, that is, steel high-pressure gas cylinders are not necessary for hydrogen storage. Under certain conditions, hydrogen can be released by TiFe, which is called energy storage material.

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