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Titanium alloy is a metallic material that consists of titanium mixed with other metals, usually small quantities of palladium, vanadium, aluminum, and/or tin. These metals provide improved properties over pure titanium, such as corrosion resistance, good weldability (fabricability), stability, and strength at elevated temperatures. Pure titanium is very hard, which can make it challenging to weld and shape.
The only typical application for pure titanium is orthopedic and dental implants, while the myriad other applications of titanium, including aerospace engineering, high-temperature engines, medical and marine processing, and athletic equipment use titanium alloy. Many alloys of other metals also contain small amounts of titanium, but these they are not considered titanium alloys unless titanium makes up the majority of the substance.
Titanium is often considered a wonder metal for its high strength and lightness. About as strong as steel and twice as strong as aluminum, titanium is 45% lighter than steel and only 60% heavier than aluminum. It also has the benefit of being non-reactive with the human body, making it ideal for medical implants such as pins to hold together broken bones. However, its high cost has limited its use.
Titanium is relatively common in the Earth's crust, making up about 0.57% (the 9th most common element), but extracting it from its ores -- the minerals Rutile (TiO2), Ilmenite (FeTiO3) and Sphene (CaTiSiO5) -- can be pricey, due to the high heat input required. Elemental titanium was only first isolated in its pure form in 1910, when Matthew A. Hunter heated TiCl4 with sodium at 800° C (1472° F).
There are 38 common types of titanium alloy, but the typical mix is 90% titanium, 6% aluminum, and 4% vanadium. This mix is called Grade 5. There are titanium grades 1 through 38, with 38 being the most recently invented. The grades do not indicate strength or anything, they are just used for easy reference, though the earlier grades were the first to be commonly produced. Grade 5 titanium is also known as Titanium 6AL-4V, which is considered suitable for military use. This grade of titanium alloy is stable in applications up to 400° C (752° F), with the stereotypical application being in aircraft turbines, which rotate very quickly and get very hot.
Besides seeing widespread use in military hardware, titanium alloy can be found in the connecting rods in high-end automobiles like Porsche and Ferrari. The presence of these metals is part of what can make these cars so expensive, but also reliable and capable of high speeds. A similar high-end material found in premium products is carbon fiber.
Titanium alloy also makes a great metal for jewelry. While it's not indestructible, it is a very pretty metal (especially for men who prefer not to wear the feminine-associated gold).
It's also mostly hypoallergenic because the alloy doesn't contain metals that people are typically allergic to, such as nickel or cobalt.