Titanium alloy

Titanium alloys are metals 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 both raw materials and processing limit their use to military applications, aircraft, spacecraft, medical devices, 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:[1]

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.

The titanium alloys have excellent formability and can be easily welded.[3]

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.


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 raise the alpha-to-beta transition temperature[lower-roman 1] (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.[4]

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 sea water, and thus is used in propeller shafts, rigging and other parts of boats that are exposed to sea water. Titanium and its alloys are used in airplanes, missiles and rockets where strength, low weight and resistance to high temperatures are important. Further, since titanium does not react within the human body, it and its alloys are used to create artificial hips, pins for setting bones, and for other biological implants. See Titanium#Orthopedic implants.

Grades of titanium

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."[5]
"Note 1—H grade material is identical to the corresponding numeric grade (that is, Grade 2H = Grade 2) 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."[5]
"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."[5]
Grades 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, ease of fabrication, and welding are important.
"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 (750 °F). 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."[9]
"Applications: Blades, discs, rings, airframes, fasteners, components. Vessels, cases, hubs, forgings. Biomedical implants."[7]
Generally, Ti-6Al-4V is used in applications up to 400 degrees Celsius. It has a density of roughly 4420 kg/m3, Young's modulus of 120 GPa, and tensile strength of 1000 MPa.[10] By comparison, annealed type 316 stainless steel has a density of 8000 kg/m3, modulus of 193 GPa, and tensile strength of 570 MPa.[11] Tempered 6061 aluminium alloy has a density of 2700 kg/m3, modulus of 69 GPa, and tensile strength of 310 MPa, respectively.[12]
Ti-6Al-4V standard specifications include:[13]
  • UNS: R56400,
  • AMS: 4911, 4920, 4928, 4934-4935, 4965, 4967, 6930-6931, T9046
  • ASTM: B265, B348, B381 F136
  • MIL: T9046-T9047
  • MMS: 1217, 1233
  • DMS: 1570, 1583, 1592, 2285, 2442 R-1
  • BMS: 7-348

Heat treatment[17]

Titanium alloys are heat treated for a number of reasons, the main ones being to increase strength by solution treatment and aging as well as to optimize special properties, such as fracture toughness, fatigue strength and high temperature creep strength.

Alpha and near-alpha alloys cannot be dramatically changed by heat treatment. Stress relief and annealing are the processes that can be employed for this class of titanium alloys. The heat treatment cycles for beta alloys differ significantly from those for the alpha and alpha-beta alloys. Beta alloys can not only be stress relieved or annealed, but also can be solution treated and aged. The alpha-beta alloys are two-phase alloys, comprising both alpha and beta phases at room temperature. Phase compositions, sizes, and distributions of phases in alpha-beta alloys can be manipulated within certain limits by heat treatment, thus permitting tailoring of properties.

Alpha and near-alpha alloys
The micro-structure of alpha alloys cannot be strongly manipulated by heat treatment since alpha alloys undergo no significant phase change. As a result, high strength can not be acquired for the alpha alloys by heat treatment. Yet, alpha and near-alpha titanium alloys can be stress relieved and annealed.
Alpha-beta alloys
By working as well as heat treatment of alpha-beta alloys below or above the alpha-beta transition temperature, large micro-structural changes can be achieved. This may give a substantial hardening of the material. Solution treatment plus aging is used to produce maximum strengths in alpha-beta alloys. Also, other heat treatments, including stress-relief heat treatments, are practiced for this group of titanium alloys as well.
Beta alloys
In commercial beta alloys, stress-relieving and aging treatments can be combined.

Titanium alloys used biomedically

Titanium alloys has been extensively used for the manufacturing of metal orthopedic joint replacements and bone plate surgeries. They are normally produced from wrought or cast bar stock by CNC, CAD-driven machining, or powder metallurgy production. Each of these techniques comes with inherent advantages and disadvantages. Wrought products come with an extensive material loss during machining into the final shape of the product and for cast samples the acquirement of a product in its final shape somewhat limits further processing and treatment (e.g. precipitation hardening), yet casting is more material effective. Traditional powder metallurgy methods are also more material efficient, yet acquiring fully dense products can be a common issue.[18]

With the emergence of solid freeform fabrication the possibility to produce custom-designed biomedical implants (e.g. hip joints) has been realized. While it is not applied currently on a larger scale, freeform fabrication methods offers the ability to recycle waste powder (from the manufacturing process) and makes for selectivity tailoring desirable properties and thus the performance of the implant. Electron Beam Melting (EBM) and Selective Laser Melting (SLM) are two methods applicable for freeform fabrication of Ti-alloys. Manufacturing parameters greatly influence the micro-structure of the product, where e.g. a fast cooling rate in combination with low degree of melting in SLM leads to the predominant formation of martensitic alpha-prime-phase, giving a very hard product.[18]

References

Notes
  1. ↑ In a titanium or titanium alloy, alpha-to-beta transition temperature is the temperature above which the beta phase becomes thermodynamically favorable.
Sources
  1. ↑ Characteristics of Alpha, Alpha Beta and Beta Titanium Alloys
  2. 1 2 3 4 Titanium – A Technical Guide. ASM International.
  3. ↑ An Evaluation of Beta Titanium Alloys for Use in Orthodontic Appliances
  4. ↑ Vydehi Arun Joshi. Titanium Alloys: An Atlas of Structures and Fracture Features. CRC Press, 2006.
  5. 1 2 3 ASTM B861 – 10 Standard Specification for Titanium and Titanium Alloy Seamless Pipe (Grades 1 to 38)
  6. ↑ Titanium | CP Titanium | Bars | Coils | Rods | Sheets | Plates | Tubing | Wire
  7. 1 2 3 4 "Titanium-6-4". Retrieved 2009-02-19.
  8. ↑ Compare Materials: Commercially Pure Titanium and 6Al-4V (Grade 5) Titanium
  9. ↑ Titanium Alloys – Ti6Al4V Grade 5
  10. ↑ Material Properties Data: 6Al-4V (Grade 5) Titanium Alloy
  11. ↑ Material Properties Data: Marine Grade Stainless Steel
  12. ↑ Material Properties Data: 6061-T6 Aluminum
  13. ↑ "Ti-6Al-4V Titanium". Rickard Metals.
  14. 1 2 "Archived copy". Archived from the original on 2012-04-26. Retrieved 2011-12-19.
  15. 1 2 3 Titanium Grade Overview
  16. ↑ ArmyCorrosion.com
  17. ↑ Donachie, Matthew J. (2000). Titanium : A Technical Guide (2nd Edition). USA: ASM International. pp. 55–57. ISBN 9780871706867.
  18. 1 2 Murr, L. E.; Quinones, S. A.; Gaytan, S. M.; Lopez, M. I.; Rodela, A.; Martinez, E. Y.; Hernandez, D. H.; Martinez, E.; Medina, F. (2009-01-01). "Microstructure and mechanical behavior of Ti–6Al–4V produced by rapid-layer manufacturing, for biomedical applications". Journal of the Mechanical Behavior of Biomedical Materials. 2 (1): 20–32. doi:10.1016/j.jmbbm.2008.05.004.
  19. ↑ Velasco-Ortega, E (Sep 2010). "In vitro evaluation of cytotoxicity and genotoxicity of a commercial titanium alloy for dental implantology". Mutat Res. 702: 17–23. doi:10.1016/j.mrgentox.2010.06.013. PMID 20615479.
  20. 1 2 The fatigue resistance of commercially pure titanium(grade II), titanium alloy (Ti6Al7Nb) and conventional cobalt-chromium cast clasps by Mali Palanuwech; Inaugural-Dissertation zur Erlangung des Doktorgrades der Zahnheilkunde der Medizinschen Fakultät der Eberhard-Karls-Universität zu Tübingenvorgelegt; Munich (2003). Retrieved 8 September 2012
  21. ↑ Titanium Alloys – Ti6Al7Nb Properties and Applications. Retrieved 8 September 2012

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