Titanium alloys are alloys that contain a mixture of titanium and other chemical elements. Such alloys have very high tensile strength and toughness (even at extreme temperatures). They are light in weight, have extraordinary corrosion resistance and the ability to withstand extreme temperatures. However, the high cost of processing limits their use to military applications, aircraft, spacecraft, bicycles, medical devices, jewelry, highly stressed components such as connecting rods on expensive sports cars and some premium sports equipment and consumer electronics. 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.
Categories
Titanium alloys are generally classified into four main categories, with a fifth miscellaneous catch-all.
Alpha alloys which contain neutral alloying elements (such as tin) and/ or alpha stabilisers (such as aluminium or oxygen) only. These are not heat treatable. Examples include: Ti-5Al-2Sn-ELI, Ti-8Al-1Mo-1V. Near-alpha alloys contain a small amount of ductile beta-phase. Besides alpha-phase stabilisers, near-alpha alloys are alloyed with 1–2% of beta phase stabilizers, such as molybdenum, silicon, or vanadium. Examples include Ti-6Al-2Sn-4Zr-2Mo, Ti-5Al-5Sn-2Zr-2Mo, IMI 685, and Ti 1100. Alpha-beta alloys, which are metastable and generally include some combination of both alpha and beta stabilisers, and which can be heat treated. Examples include: Ti-6Al-4V, Ti-6Al-4V-ELI, Ti-6Al-6V-2Sn, Ti-6Al-7Nb, and Ti62A. Beta and near-beta alloys, which are metastable and which contain sufficient beta stabilisers (such as molybdenum, silicon and vanadium) to allow them to maintain the beta phase when quenched, and which can also be solution treated and aged to improve strength. Examples include: Ti-10V-2Fe-3Al, Ti–29Nb–13Ta–4.6Zr, Ti-13V-11Cr-3Al, Ti-8Mo-8V-2Fe-3Al, Beta C, and Ti-15-3. Although uncommercialized in the west, binary titanium alloys with magnesium, potassium, calcium, and lithium have been produced in an arc melting pressure vessel at up to 140 atmospheres.
Alpha-titanium Pure titanium is Alpha-titanium.
Beta-titanium Beta titanium alloys exhibit the BCC allotropic form of titanium (called beta). Elements used in this alloy are one or more of the following other than titanium in varying amounts. These are molybdenum, vanadium, niobium, tantalum, zirconium, manganese, iron, chromium, cobalt, nickel, and copper. Beta titanium alloys have excellent formability and can be easily welded. Beta titanium is nowadays largely utilized in the orthodontic field and was adopted for orthodontics use in the 1980s. This type of alloy replaced stainless steel for certain uses, as stainless steel had dominated orthodontics since the 1960s. It has strength/modulus of elasticity ratios almost twice those of 18-8 austenitic stainless steel, larger elastic deflections in springs, and reduced force per unit displacement 2.2 times below those of stainless steel appliances. Some of the beta titanium alloys can convert to hard and brittle hexagonal omega-titanium at cryogenic temperatures or under influence of ionizing radiation.
Omega-titanium
Transition temperature The crystal structure of titanium at ambient temperature and pressure is close-packed hexagonal α phase with a c/a ratio of 1.587. At about 890 °C, the titanium undergoes an allotropic transformation to a body-centred cubic β phase, which remains stable up to the melting temperature. Some alloying elements, called alpha stabilizers, raise the alpha-to-beta transition temperature, while others (beta stabilizers) lower the transition temperature. 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.
Properties Generally, beta-phase titanium is the more ductile phase and alpha-phase is stronger yet less ductile, due to the larger number of slip planes in the bcc structure of the beta-phase in comparison to the hcp alpha-phase. 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. 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 alone is a strong, light metal. It is stronger than common, low-carbon steels, but 45% lighter. It is also twice as strong as weak aluminium alloys but only 60% heavier. Titanium has outstanding corrosion resistance to seawater, and thus is used in propeller shafts, rigging and other parts of boats that are exposed to seawater. Titanium and its 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 and its alloys are used in artificial joints, screws, and plates for fractures, and for other biological implants. See: Titanium orthopedic implants.
Titanium grades
The ASTM International standard on titanium and titanium alloy seamless pipe references the following alloys, requiring the following treatment:
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