Titanium alloys


Titanium alloys are alloys that contain a mixture of titanium and other chemical elements. Such alloys have very high tensile strength and toughness. 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 and/ or alpha stabilisers 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 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. 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 to the melting temperature.
Some alloying elements, called alpha stabilizers, raise the alpha-to-beta transition temperature, while others 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, 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:
"Alloys may be supplied in the following conditions: Grades 5, 23, 24, 25, 29, 35, or 36 annealed or aged; Grades 9, 18, 28, or 38 cold-worked and stress-relieved or annealed; Grades 9, 18, 23, 28, or 29 transformed-beta condition; and Grades 19, 20, or 21 solution-treated or solution-treated and aged."

"Note 1—H grade material is identical to the corresponding numeric grade except for the higher guaranteed minimum UTS, and may always be certified as meeting the requirements of its corresponding numeric grade. Grades 2H, 7H, 16H, and 26H are intended primarily for pressure vessel use."

"The H grades were added in response to a user association request based on its study of over 5200 commercial Grade 2, 7, 16, and 26 test reports, where over 99% met the 58 ksi minimum UTS."

; Grade 1: is the most ductile and softest titanium alloy. It is a good solution for cold forming and corrosive environments. ASTM/ASME SB-265 provides the standards for commercially pure titanium sheet and plate.
; Grade 2: Unalloyed titanium, standard oxygen.
; Grade 2H: Unalloyed titanium.
; Grade 3: Unalloyed titanium, medium oxygen.
; Grade 5 also known as Ti6Al4V, Ti-6Al-4V or Ti 6-4
"This alpha-beta alloy is the workhorse alloy of the titanium industry. The alloy is fully heat treatable in section sizes up to 15 mm and is used up to approximately 400 °C. Since it is the most commonly used alloy – over 70% of all alloy grades melted are a sub-grade of Ti6Al4V, its uses span many aerospace airframe and engine component uses and also major non-aerospace applications in the marine, offshore and power generation industries in particular."

"Applications: Blades, discs, rings, airframes, fasteners, components. Vessels, cases, hubs, forgings. Biomedical implants."

; Grade 6: contains 5% aluminium and 2.5% tin. It is also known 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: is identical to Grade 7.
; Grade 9: contains 3.0% aluminium 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. This alloy has excellent weldability.
; 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: is identical to Grade 16.
; Grade 17: contains 0.04 to 0.08% palladium. This grade has enhanced corrosion resistance.
; Grade 18: contains 3% aluminium, 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% aluminium, 8% vanadium, 6% chromium, 4% zirconium, and 4% molybdenum.
; Grade 20: contains 3% aluminium, 8% vanadium, 6% chromium, 4% zirconium, 4% molybdenum and 0.04% to 0.08% palladium.
; Grade 21: contains 15% molybdenum, 3% aluminium, 2.7% niobium, and 0.25% silicon.
; Grade 23 also known as Ti-6Al-4V-ELI or TAV-ELI:
contains 6% aluminium, 4% vanadium, 0.13% Oxygen. ELI stands for Extra Low Interstitial. Reduced interstitial elements oxygen and iron improve ductility and fracture toughness with some reduction in strength. TAV-ELI is the most commonly used medical implant-grade titanium alloy. Due to its excellent biocompatibility, corrosion resistance, fatigue resistance, and low modulus of elasticity, which closely matches human bone, TAV-ELI is the most commonly used medical implant-grade titanium alloy.
; Grade 24: contains 6% aluminium, 4% vanadium and 0.04% to 0.08% palladium.
; Grade 25: contains 6% aluminium, 4% vanadium and 0.3% to 0.8% nickel and 0.04% to 0.08% palladium.
; Grades 26, 26H, and 27
; Grade 28: contains 3% aluminium, 2.5% vanadium and 0.08 to 0.14% ruthenium.
; Grade 29: contains 6% aluminium, 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% aluminium, 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. Both grades are identical but for minor difference in oxygen and nitrogen content. These grades contain 6 to 25 times less palladium than Grade 7 and are thus less costly, but offer similar corrosion performance thanks to the added ruthenium.
; Grade 35: contains 4.5% aluminium, 2% molybdenum, 1.6% vanadium, 0.5% iron, and 0.3% silicon.
; Grade 36: contains 45% niobium.
; Grade 37: contains 1.5% aluminium.
; Grade 38: contains 4% aluminium, 2.5% vanadium, and 1.5% iron. This grade was developed in the 1990s for use as an armor plating. The iron reduces the amount of Vanadium needed as a beta stabilizer. Its mechanical properties are very similar to Grade 5, but has good cold workability similar to grade 9.