Titanium alloy

Titanium alloys are metallic materials which contain a mixture of titanium and other chemical elements. Such alloys have very high tensile strength and toughness (even at extreme temperatures), light weight, extraordinary corrosion resistance, and ability to withstand extreme temperatures. However, the high cost of both raw materials and processing limit their use to military applications, aircraft, spacecraft, medical devices, connecting rods on the Acura NSX and some premium sports equipment and consumer electronics. Auto manufacturers Porsche and Ferrari have also followed Acura's lead to use titanium alloys in engine components due to its durable properties in these high stress engine environments.

Although "commercially pure" titanium has acceptable mechanical properties and has been used for orthopedic and dental implants, for most applications titanium is alloyed with small amounts of aluminium and vanadium, typically 6% and 4% respectively, by weight. This mixture has a solid solubility which varies dramatically with temperature, allowing it to undergo precipitation strengthening. This heat treatment process is carried out after the alloy has been worked into its final shape but before it is put to use, allowing much easier fabrication of a high-strength product.

Some alloying elements raise the alpha-to-beta transition temperature [In a titanium or titanium alloy, alpha-to-beta transition temperature is the temperature below which the beta phase becomes thermodynamically favorable.] (i.e. alpha stabilizers) while others lower the transition temperature (i.e. beta stabilizers). Aluminium, gallium, germanium, carbon, oxygen and nitrogen are alpha stabilizers. Molybdenum, vanadium, tantalum, niobium, manganese, iron, chromium, cobalt, nickel, copper and silicon are beta stabilizers. Titanium alloys are usually classified as alpha alloys, near alpha alloys, alpha + beta alloy or beta alloys depending on the type and amount of alloying elements.

Generally, alpha-phase Titanium is stronger yet less ductile and beta-phase Titanium is more ductile. Alpha-beta-phase Titanium has a mechanical property which is in between both.

Titanium dioxide dissolves in the metal at high temperatures, and its formation is very energetic. These two factors mean that all titanium except the most carefully purified has a significant amount of dissolved oxygen, and so may be considered a Ti-O alloy. Oxide precipitates offer some strength (as discussed above), but are not very responsive to heat treatment and can substantially decrease the alloy's toughness.

Aside from titanium-based alloys, the term may refer to "binary" alloys which consist of a nearly even mix, atom-by-atom, of titanium and another element. "Nitinol", a shape memory alloy, is a mixture of titanium and nickel, while niobium-titanium alloys are used as wires for superconducting magnets.

Many alloys also contain titanium as a minor additive, but since alloys are usually categorized according to which element forms the majority of the material, these are not usually considered to be "titanium alloys" as such. See the sub-article on titanium applications.

Titanium is a strong, light metal. It is as strong as steel and twice as strong as aluminum, but is 45% lighter than steel and only 60% heavier than aluminum. Titanium is not easily corroded by sea water and is used in propeller shafts, rigging and other parts of boats that are exposed to sea water. Titanium and titanium alloys are used in airplanes, missiles and rockets where strength, low weight and resistance to high temperatures are important. Since titanium does not react within the human body, it is used to create artificial hips, pins for setting bones and for other biological implants. Unfortunately, the high cost of titanium has limited its widespread use. Titanium is the ninth most abundant element in the earth's crust and is primarily found in the minerals Rutile (TiO2), Ilmenite (FeTiO3) and Sphene (CaTiSiO5). Titanium makes up about 0.57% of the earth's crust. The word titanium comes from the Greek word Titans the mythological "first sons of the earth". The pure elemental metal was not made until 1910 by Matthew A. Hunter, who heated TiCl4 together with sodium in a steel bomb at 700-800°C.

Grades

The ASTM defines a number of alloy standards with a numbering scheme for easy reference.

* Grade 1-4 are unalloyed and considered commercially pure or "CP". Generally the tensile and yield strength goes up with grade number for these "pure" grades. The difference in their physical properties is primarily due to the quantity of interstitial elements. They are used for corrosion resistance applications where cost and ease of fabrication and welding are important.

* Grade 5 is the most commonly used alloy. It has a chemical composition of 6% Aluminum, 4% Vanadium, remainder titanium, and is commonly known as "Ti6Al4V", "Ti-6AL-4V" or simply "Ti 6-4". Grade 5 is used extensively in Aerospace, Medical, Marine, and Chemical Processing. [ [http://www.azom.com/details.asp?ArticleID=1547 Titanium Alloys - Ti6Al4V Grade 5 ] ]

* Grade 6 contains 5% Aluminum and 2.5% Tin. It is also know as Ti-5Al-2.5Sn. This alloy is used in airframes and jet engines due to its good weldability, stability and strength at elevated temperatures.

* Grade 7 contains 0.12 to 0.25% Palladium. This grade is similar to Grade 2. The small quantity of Palladium added gives it enhanced crevice corrosion resistance at low temperatures and high pH.

* Grade 7H contains 0.12 to 0.25% Palladium. This grade has enhanced corrosion resistance.

* Grade 9 contains 3.0% Aluminum and 2.5% Vanadium. This grade is a compromise between the ease of welding and manufacturing of the "pure" grades and the high strength of Grade 5. It is commonly used in aircraft tubing for hydraulics and in athletic equipment.

* Grade 11 contains 0.12 to 0.25% Palladium. This grade has enhanced corrosion resistance.

* Grade 12 contains 0.3% Molybdenum and 0.8% Nickel.

* Grades 13, 14, and 15 all contain 0.5% Nickel and 0.05% Ruthenium.

* Grade 16 contains 0.04 to 0.08% Palladium. This grade has enhanced corrosion resistance.

* Grade 16H contains 0.04 to 0.08% Palladium.

* Grade 17 contains 0.04 to 0.08% Palladium. This grade has enhanced corrosion resistance.

* Grade 18 contains 3% Aluminum, 2.5% Vanadium and 0.04 to 0.08% Palladium. This grade is identical to Grade 9 in terms of mechanical characteristics. The added Palladium gives it increased corrosion resistance.

* Grade 19 contains 3% Aluminum, 8% Vanadium, 6% Chromium, 4% Zirconium, and 4% Molybdenum.

* Grade 20 contains 3% Aluminum, 8% Vanadium, 6% Chromium, 4% Zirconium, 4% Molybdenum and 0.04% to 0.08% Palladium.

* Grade 21 contains 15% Molybdenum, 3% Aluminum, 2.7% Niobium, and 0.25% Silicon.

* Grade 23 contains 6% Aluminum, 4% Vanadium.

* Grade 24 contains 6% Aluminum, 4% Vanadium and 0.04% to 0.08% Palladium.

* Grade 25 contains 6% Aluminum, 4% Vanadium and 0.3% to 0.8% Nickel and 0.04% to 0.08% Palladium.

* Grades 26, 26H, and 27 all contain 0.08 to 0.14% Ruthenium.

* Grade 28 contains 3% Aluminum, 2.5% Vanadium and 0.08 to 0.14% Ruthenium.

* Grade 29 contains 6% Aluminum, 4% Vanadium and 0.08 to 0.14% Ruthenium.

* Grades 30 and 31 contain 0.3% Cobalt and 0.05% Palladium.

* Grade 32 contains 5% Aluminum, 1% Tin, 1% Zirconium, 1% Vanadium, and 0.8% Molybdenum.

* Grades 33 and 34 contain 0.4% Nickel, 0.015% Palladium, 0.025% Ruthenium, and 0.15% Chromium .

* Grade 35 contains 4.5% Aluminum, 2% Molybdenum, 1.6% Vanadium, 0.5% Iron, and 0.3% Silicon.

* Grade 36 contains 45% Niobium.

* Grade 37 contains 1.5% Aluminum.

* Grade 38 contains 4% Aluminum, 2.5% Vanadium, and 1.5% Iron. This grade was developed in the '90 for use as an armor plating. The iron reduces the amount of Vanadium needed for corrosion resistance. It's mechanical properties are very similar to Grade 5. [http://www.armycorrosion.com/past_summits/summit2007/download1.cfm?fname=Patrick%20Snow.pdf]

References

[http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/B265.htm?L+mystore+wyav9845, ASTM Reference sheet]
[http://www.stainless-steel-world.net/titanium/ShowPage.aspx?pageID=176]
[http://www.tifab.com/subpages/tech_spec_grades.htm]