Ti-6Al-7Nb

Ti-6Al-7Nb (UNS designation R56700) is an alpha-beta titanium alloy first synthesized in 1977 containing 6% aluminum and 7% niobium. It features high strength and has similar properties as the cytotoxic vanadium containing alloy Ti-6Al-4V. Ti-6Al-7Nb is used as a material for hip protheses.[1] Ti―6Al―7Nb is one of the titanium alloys that built of hexagonal α phase (stabilised with aluminium) and regular body-centred phase β (stabilised with niobium). The alloy is characterized by added advantageous mechanical properties, it has higher corrosion resistance and biotolerance in relation to Ti-6Al-4V alloys.[2][3][4]

Mechanical properties

Mechanical properties of the alloy are mostly dependent on the morphology and the fractions volume of the phases presence from the parameters obtained from the manufacturing process.[5][6]

PropertyMinimum ValueMaximum ValueUnit
Density4.514.53g/cm3
Hardness27002900Mpa
Melting point18001860K
Specific heat540560J/kg*K
Elastic limit895905MPa
Energy content7501250MJ/kg
Latent heat of fusion360370kJ/kg

[7]

As shown in the above table, alloying is one of the effective methods to improve the mechanical properties and since Niobium belongs to the same group of Vanadium in the periodic table it is of course acts as α –β stabilizing elements (similar to Ti-6Al-4V alloy), however the strength of Nb alloy is little less than that of Ti-6Al-4V .The main difference between Ti-6Al-4V and Ti-6Al-7Nb is related to different factors such as solid-solution strengthening, the structure-refining strengthening provided by the refined two-phase structure and the difference in the microstructure between the two alloys.[8]

Production

Ti-6Al-7Nb is produced by powder metallurgy methods. The most common methods are hot pressing, metal injection mouldering and blending and pressing. In the production of Ti-6Al-7Nb a sintering temperature between 900-1400o C usually are used. Altering the sintering temperature gives the Ti-6Al-7Nb different properties such as different porosity and microstructure. It also gives a different composition between alpha, beta and alpha+beta phases. In the recent years Ti-6Al-7Nb alloys could also be made by different 3D-printer technique such as SLM and EBM.[9][10]

Heat treatment

Heat treatment of titanium is demonstrated to have significant influences on reducing the residual stresses, improving the mechanical properties (i.e. tensile strength or fatigue strength by solution treatment and ageing). Moreover, heat treatment provides an ideal combination of ductility, machinability and structural stability due to the differences in microstructure and cooling rates between α and β phases.[11]

The cooling rate have an impact of the morphology . When the cooling rate is reduced for example from air cool to slow cooling, the morphology of the transformed α increases in thickness and length and is contained within fewer, larger α colonies.[12] The α colony size is the most important microstructural properties due to its influences the fatigue properties and deformation mechanics of β processed α+ β alloys.[13]

Applications
  • Implant devices replacing such as : failed hard tissue, artificial hip joints, artificial knee joints, bone plates, screws for fracture fixation, cardiac valve prostheses, pacemakers, and artificial hearts.[14]
  • Dental application [15]
  • Aircraft materials

Biocompatibility

Ti-6Al-7Nb has a high biocompatibility. The oxides from Ti-6Al-7Nb is saturated in the body and are not transported in vivo or are a bioburden. The alloy will not create adverse tissue tolerance reactions and creates fewer giant cell nucleis. Ti-6Al-7Nb also shows a high compatibility to ingrowth to the human body.[16]

Specification[17]
  • UNS: R56700
  • ASTM Standard: F1295
  • ISO Standard: ISO 5832-11
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References
  1. Mamoun Fellah, Mohamed Labaïz, Omar Assala, et al., “Tribological behavior of Ti-6Al-4V and Ti-6Al-7Nb Alloys for Total Hip Prosthesis,” Advances in Tribology, vol. 2014, Article ID 451387, 13 pages, 2014. doi:10.1155/2014/451387
  2. Chlebus, Edward, et al. "Microstructure and mechanical behaviour of Ti―6Al―7Nb alloy produced by selective laser melting." Materials Characterization 62.5 (2011): 488-495.
  3. X. Liu, P.-K. Chu, C. Ding Surface modification of titanium, titanium alloys, and related materials for biomedical application. Materials Science and Engineering, R47 (2004), pp. 49–121
  4. M.-F. López, A. Gutiérrez, J.-A. Jiménez. In vitro corrosion behaviour of titanium alloys without vanadium. Electrochimica Acta, 47 (2002), pp. 1359–1364
  5. G. Lütjering. Influence of processing on microstructure and mechanical properties of (α + β) titanium alloys. Materials Science and Engineering, A243 (1998), pp. 32–45
  6. S.A. Ajeel, T.L. Alzubaydi, A.K. Swadi. Influence of heat treatment conditions on microstructure of Ti―6Al―7Nb alloy as used surgical implant materials. Engineering and Technology, 25 (2007), pp. 431–442
  7. http://www.azom.com/properties.aspx?ArticleID=2064
  8. Kobayashi, Equo, et al. "Mechanical properties and corrosion resistance of Ti-6Al-7Nb alloy dental castings." Journal of Materials Science: Materials in Medicine 9.10 (1998): 567-574.
  9. Bolzoni, Leandro, et al. "Comparison of Microstructure and Properties of Ti-6Al-7Nb Alloy Processed by Different Powder Metallurgy Routes." Key Engineering Materials. Vol. 551. 2013.
  10. http://www.scielo.br/scielo.php?pid=S0104-66321998000400002&script=sci_arttext
  11. Sercombe, Tim, et al. "Heat treatment of Ti-6Al-7Nb components produced by selective laser melting." Rapid Prototyping Journal 14.5 (2008): 300-304.
  12. 2. Tim Sercombe Noel Jones Rob Day Alan Kop, (2008),"Heat treatment of Ti-6Al-7Nb components produced by selective laser melting", Rapid Prototyping Journal, Vol. 14 Iss 5 pp. 300 - 304
  13. 1. Lütjering, G. "Influence of processing on microstructure and mechanical properties of (α+ β) titanium alloys." Materials Science and Engineering: A 243.1 (1998): 32-45.
  14. Elias, C. N., et al. "Biomedical applications of titanium and its alloys." Jom 60.3 (2008): 46-49.
  15. Kobayashi, Equo, et al. "Mechanical properties and corrosion resistance of Ti–6Al–7Nb alloy dental castings." Journal of Materials Science: Materials in Medicine 9.10 (1998): 567-574.
  16. http://www.synthes.com/sites/NA/NAContent/Docs/Product%20Support%20Materials/Materials%20Booklets/Implant%20Materials%20-%20Titanium--6_%20Aluminum--7_%20Niobium.pdf
  17. https://www.atimetals.com/Documents/ati_tja-1537_tds_en_v2.pdf
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