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Text 12

 

Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant (including sea water, aqua regia and chlorine) transition metal with a silver color.

Titanium was discovered in Cornwall, Great Britain, by William Gregor in 1791 and named by Martin Heinrich Klaproth for the Titans of Greek mythology. The element occurs within a number of mineral deposits, principally rutile and ilmenite, which are widely distributed in the Earth's crust and lithosphere, and it is found in almost all living things, rocks, water bodies, and soils. The metal is extracted from its principal mineral ores via the Kroll process or the Hunter process. Its most common compound, titanium dioxide, is a popular photocatalyst and is used in the manufacture of white pigments. Other compounds include titanium tetrachloride (TiCl4), a component of smoke screens and catalysts; and titanium trichloride (TiCl3), which is used as a catalyst in the production of polypropylene.

Titanium can be alloyed with iron, aluminium, vanadium, molybdenum, among other elements, to produce strong lightweight alloys for aerospace (jet engines, missiles, and spacecraft), military, industrial process (chemicals and petro-chemicals, desalination plants, pulp, and paper), automotive, agri-food, medical prostheses, orthopedic implants, dental and endodontic instruments and files, dental implants, sporting goods, jewelry, mobile phones, and other applications.

The two most useful properties of the metal form are corrosion resistance and the highest strength-to-weight ratio of any metal. In its unalloyed condition, titanium is as strong as some steels, but 45% lighter. There are two allotropic forms and five naturally occurring isotopes of this element, 46Ti through 50Ti, with 48Ti being the most abundant (73.8%). Titanium's properties are chemically and physically similar to zirconium, because both of them have the same number of valence electrons and are in the same group in the periodic table.

Titanium is always bonded to other elements in nature. It is the ninth-most abundant element in the Earth's crust (0.63% by mass) and the seventh-most abundant metal. It is present in most igneous rocks and in sediments derived from them (as well as in living things and natural bodies of water). Of the 801 types of igneous rocks analyzed by the United States Geological Survey, 784 contained titanium. Its proportion in soils is approximately 0.5 to 1.5%.

It is widely distributed and occurs primarily in the minerals anatase, brookite, ilmenite, perovskite, rutile and titanite (sphene). Of these minerals, only rutile and ilmenite have economic importance, yet even they are difficult to find in high concentrations. About 6.0 and 0.7 million tonnes of these minerals have been mined in 2011, respectively. Significant titanium-bearing ilmenite deposits exist in western Australia, Canada, China, India, Mozambique, New Zealand, Norway, Ukraine and South Africa. About 186,000 tonnes of titanium metal sponge were produced in 2011, mostly in China (60,000 t), Japan (56,000 t), Russia (40,000 t), United States (32,000 t) and Kazakhstan (20,700 t). Total reserves of titanium are estimated to exceed 600 million tonnes.

Titanium is contained in meteorites and has been detected in the sun and in M-type stars; the coolest type of star with a surface temperature of 3,200 °C (5,790 °F). Rocks brought back from the moon during the Apollo 17 mission are composed of 12.1% TiO2. It is also found in coal ash, plants, and even the human body.

As an engineering material titanium has been widely applied only in the last years. Titanium is a silvery-white metal which melts at approximately 1668°C and has a specific gravity of 4.505. Commercially pure titanium possesses high strength prop­erties. The tensile strength of most titanium alloys ranges from 100 to 140 kg/mm2, in conjunction with high elongation.

The hardness, tensile strength and yield point of tita­nium are increased with the degree of cold deformation. The elongation value drops rapidly when the degree of cold deformation (reduction) exceeds 50 per cent and becomes equal to 10 per cent. Impurities found in com­mercial titanium can be divided into two groups: elements which form interstitial solid solutions with titanium ( O2, N, C and H2) and elements which form substitution solid solutions (Fe and other metallic elements). The first have a much greater effect on the mechanical properties than those in the second group.

Even very small amounts of oxygen and nitrogen in titanium alloys sharply reduce the ductility. A carbon content of more than 0.2 per cent reduces both the ductility and impact strength of a titanium alloy. It is supposed that the brittleness of titanium is a result of strain ageing and is connected with the presence of dissolved hydrogen in the beta-phase.

Titanium and its alloys are hardened either by a sur­face heat treatment followed by ageing at 400°—500° C or by producing a case which contains nitrogen, carbon and boron. Industrial titanium alloys contain vanadium, molybdenum, chromium, manganese, aluminum, tin, iron or other elements, singly or in various combinations. A combination of high mechanical properties with low specific weight and excellent corrosion resistance enables titanium to be used in building supersonic air craft.

 




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