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INTRODUCTION
Titanium's light weight, excellent corrosion resistance, and high strength to weight ratio have led to its widespread application in the chemical, aerospace, marine and medical fields. Application of titanium alloys in the petrochemical industry and in the manufacture of sports equipment are among recent developments.
Titanium is on of the most common metals occuring in the earth's crust. In North America particularly, there is an abundance of titanium ores available for commercial exploitation. This availability, independent of foreign sources, assures that development of new and better titanium alloys will continue for years to come.
Pure titanium is a silvery-coloured metal that melts at approximately 3035°F (1670°C) and boils at 5900°F ((3260°C). The average coefficient of expansion is 4.67 µ-in./in./°F (8.41 µm/m°/C) at 68°F (20°C). It has a density of 0.163 lb/in.³ (4.51 g/cm³).
Titanium has a strong chemical affinity for oxygen, and it forms a tight microscopic oxide film on freshly prepared surfaces at room temperature (similar to magnesium and aluminum). The oxide film makes titanium passive to further reactivity. This accounts in part for titanium's excellent corrosion resistance in aqueous salt or oxidizing acid solutions as well as its above average corrosion resistance to mineral acids.
Titanium tends to oxidize rapidly when heated in air above 1200°F (650°C). At elevated temperatures it has the property for dissolving discrete amounts of its own oxide into solution. For these reasons the welding of titanium requires the use of protective shielding, such as an inert gas atmosphere, to prevent contamination and embrittlement from oxygen and nitrogen.
Titaniums's relatively low coefficient of thermal expansion and conductivity minimize the possibility of distortion due to welding.
Pure titanium is quite ductile (15 to 25% elongation), and it has a relatively low ultimate tensile strength [approximately 30 ksi (207 MPa) at room temperature]. Adding limited amounts of oxygen and nitrogen in solid solution will strengthen titanium markedly, but it also will embrittle the metal if present in excessive quantity. Carbon exerts a similar but less intensive effect on titanium. Hydrogen also promotes embrittlement when present above specified limits. Addition of these elements usually occurs unintentionally through contamination when the metal is processed.
Intentional additions of various alloying elements can result in tensile strengths beyond 200 ksi (1380 MPa), although ductility is sacrificed at such high levels. The combination of high strength and low density in commercially available titanium alloys provides a very desirable strength to weight ratio, along with excellent corrosion resistance up to 1200°F (1650°C).
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