Titanium

Titanium is a chemical element with the symbol Ti and atomic number 22. Sometimes called the "space age metal",^[2] 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, England, 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.^[3] The metal is extracted from its principal mineral ores via the Kroll process^[4] or the Hunter process. Its most common compound, titanium dioxide, is a popular photocatalyst and is used in the manufacture of white pigments.^[5] 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.^[3]

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.^[3]

The two most useful properties of the metal form are corrosion resistance and the highest strength-to-weight ratio of any metal.^[6] In its unalloyed condition, titanium is as strong as some steels, but 45% lighter.^[7] There are two allotropic forms^[8] and five naturally occurring isotopes of this element, ⁴⁶Ti through ⁵⁰Ti, with ⁴⁸Ti being the most abundant (73.8%).^[9] 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.

Characteristics

Physical properties

A metallic element, titanium is recognized for its high strength-to-weight ratio.^[8] It is a strong metal with low density that is quite ductile (especially in an oxygen-free environment),^[3] lustrous, and metallic-white in color.^[10] The relatively high melting point (more than 1,650 °C or 3,000 °F) makes it useful as a refractory metal. It is paramagnetic and has fairly low electrical and thermal conductivity.^[3]

Commercial (99.2% pure) grades of titanium have ultimate tensile strength of about 63,000 psi (434 MPa), equal to that of common, low-grade steel alloys, but are 45% lighter.^[7] Titanium is 60% more dense than aluminium, but more than twice as strong^[7] as the most commonly used 6061-T6 aluminium alloy. Certain titanium alloys (e.g., Beta C) achieve tensile strengths of over 200,000 psi (1,400 MPa).^[11] However, titanium loses strength when heated above 430 °C (806 °F).^[12]

It is fairly hard (although not as hard as some grades of heat-treated steel), non-magnetic and a poor conductor of heat and electricity. Machining requires precautions, as the material will soften and gall if sharp tools and proper cooling methods are not used. Like those made from steel, titanium structures have a fatigue limit which guarantees longevity in some applications.^[10] Titanium alloys specific stiffnesses are also usually not as good as other materials such as aluminium alloys and carbon fiber, so it is used less for structures which require high rigidity.

The metal is a dimorphic allotrope whose hexagonal alpha form changes into a body-centered cubic (lattice) β form at 882 °C (1,620 °F).^[12] The specific heat of the alpha form increases dramatically as it is heated to this transition temperature but then falls and remains fairly constant for the β form regardless of temperature.^[12] Similar to zirconium and hafnium, an additional omega phase exists, which is thermodynamically stable at high pressures, but is metastable at ambient pressures. This phase is usually hexagonal (ideal) or trigonal (distorted) and can be viewed as being due to a soft longitudinal acoustic phonon of the β phase causing collapse of (111) planes of atoms.^[13]

References

1. Andersson, N. et al. (2003). "Emission spectra of TiH and TiD near 938 nm". J. Chem. Phys. 118: 10543. doi:10.1063/1.1539848. http://bernath.uwaterloo.ca/media/257.pdf. 
2. William L. Masterton; Cecile N. Hurley (2008). Chemistry: Principles and Reactions (6th ed.). Cengage Learning. p. 18. ISBN 0495126713. http://books.google.com/?id=teubNK-b2bsC&pg=PT44&lpg=PT44&dq=titanium+%22space-age+metal%22&q=titanium%20%22space-age%20metal%22. 
3. "Titanium". Encyclopædia Britannica. 2006. http://www.britannica.com/eb/article-9072643/titanium. Retrieved 2006-12-29. 
4. Lide, D. R., ed. (2005), CRC Handbook of Chemistry and Physics (86th ed.), Boca Raton (FL): CRC Press, ISBN 0-8493-0486-5 
5. Krebs, Robert E. (2006). The History and Use of Our Earth's Chemical Elements: A Reference Guide (2nd edition). Westport, CT: Greenwood Press. ISBN 0313334382. 
6. Matthew J. Donachie, Jr. (1988). TITANIUM: A Technical Guide. Metals Park, OH: ASM International. p. 11. ISBN 0871703092. 
7. Barksdale 1968, p. 738
8. "Titanium". Columbia Encyclopedia (6th edition ed.). New York: Columbia University Press. 2000–2006. ISBN 0-7876-5015-3. http://www.answers.com/Titanium. 
9. Barbalace, Kenneth L. (2006). "Periodic Table of Elements: Ti - Titanium". http://environmentalchemistry.com/yogi/periodic/Ti-pg2.html#Nuclides. Retrieved 2006-12-26. 
10. Stwertka, Albert (1998). "Titanium". Guide to the Elements (Revised ed.). Oxford University Press. pp. 81–82. ISBN 0-19-508083-1. 
11. Matthew J. Donachie, Jr. (1988). Titanium: A Technical Guide. Metals Park, OH: ASM International. Appendix J, Table J.2. ISBN 0871703092. 
12. Barksdale 1968, p. 734
13. Sikka, S. K.; Vohra, Y. K., Chidambaram, R. (1982). "Omega phase in materials". Progress in Materials Science 27: 245–310. doi:10.1016/0079-6425(82)90002-0.