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Titanium alloys for total joint replacements

John Stechschulte MS&E 410. Titanium alloys for total joint replacements. Outline. Motivation Material Demands Titanium overview (Brief) History Phases Types of Ti alloys A few orthopaedic alloys Ti64 TMZF TNZT. Motivation. TJR is a common orthopaedic procedure

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Titanium alloys for total joint replacements

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  1. John Stechschulte MS&E 410 Titanium alloys for total joint replacements

  2. Outline • Motivation • Material Demands • Titanium overview • (Brief) History • Phases • Types of Ti alloys • A few orthopaedic alloys • Ti64 • TMZF • TNZT

  3. Motivation • TJR is a common orthopaedic procedure • 90% of population over 40 suffers from some degenerative joint disease • Over 500,000 knees and hips per year • Success rate of >90%

  4. Material Demands • Biocompatibility • Non-toxic • Debris-free • Similar density to bone • Corrosion and wear resistance • Harsh biological environment • Cyclical loading • Mechanical properties • Low Young’s modulus: 40 GPa • High yield stress

  5. Titanium and alloys • Phases • Aerospace alloys • Ti-6Al-4V • Biomedical alloys • Ti-Mo-Zr-Fe • Ti-Nb-Zr-Ta • others

  6. Titanium Phases • Equilibrium: • α: hexagonal close packed—below 883° C • β: body centered cubic—above 883° C • Non-equilibrium: • α’: HCP martensitic phase that precedes α formation • α”: orthorhombic martensitic phase; occurs in alloys with high refractory element content • ω: submicroscopic phase; non-martensitic diffusionless transformation

  7. Types of Alloys • cp-Ti • good corrosion resistance and tissue tolerance • density 55% that of steel • low strength • cold-worked cp-Ti used for dental implants, etc. • α alloys • alloyed with Al or Sn, for example • better creep resistance • strength and toughness are so-so

  8. Types of Alloys • α-β alloys • 10-50% β at room temperature • includes Ti-6Al-4V • much stronger, good formability • β alloys • stronger and tougher than α-β alloys • can contain elements to reduce modulus

  9. Ti-6Al-4V • Aluminum helps to control grain size • Vanadium stabilizes β phase • Micrograph shows Widmanstätten phase morphology • E: 110 GPa • Yield Strength: 900-1100 MPa

  10. But . . . • Vanadium is bad • Toxic, although there are no known cases of V poisoning from a Ti64 implant. • Aluminum isn’t too great either. • Modulus, 110 GPa, is too high • Leads to stress shielding • Intrinsic property of the alloy • Summary: it’s great for planes—that’s what it was designed for—not so good for joints.

  11. Other options? • Ti-15Mo: metastable β alloy • Ti-12Mo-6Zr-2Fe (TMZF) • All-β after homogenization • More to come . . . • Ti-13Zr-13Nb • Near-β alloy with α’ precipitates • Ti-34Nb-9Zr-8Ta (TNZT) • β with α precipitates after homogenization • Also more to come . . .

  12. TMZF • Modulus: 80 GPa • Yield Strength: 1000 MPa • Homogenized condition: • 1100°C for 7 days • β grains • α precipitates at grain boundary and within grains, plate-like morphology

  13. TMZF • Aged condition: • 600°C for 4 hours • Vicker’s hardness increases: 345 VHN to 366 VHN • Fine-scale, oriented secondary α precipitates • No significant grain growth (trust me) • But is that all?

  14. TMZF • TEM reveals ω! • [0 1 1] planes • (a) homogenized • (b) aged

  15. TNZT • Modulus: 55 GPa!!! • Yield Strength: ~600 MPa • Same homogenization: • again, β grains with α precipitates at grain boundaries and within grains • curious α phase morphologies

  16. TNZT • Same heat treatment: • Precipitate fine-scale α • Again, oriented • Vicker’s hardness decreases! (291 VHN to 258 VHN) Precipitation softening?

  17. TNZT • Look to the TEM: • (a) homogenized • (b) aged • B2 (CsCl) ordering in the homogenized state • Ordering increases Burgers vector  increased hardness • Aging  increased solute concentration in β phase  loss of ordering

  18. TMZF and TNZT • TMZF behaves as expected: • Non-equilibrium β phase can be aged to precipitate α • Leads to precipitation hardening • TNZT gets a bit creative: • Similar phase behavior, except that β phase contains B2 ordering • Result: precipitation softening

  19. Conclusion • Titanium is a good choice for TJR • Low weight • Biocompatible and corrosion-resistant • High strength • Can have low modulus • Mechanical properties of meta-stable β alloys can be easily tuned by precipitation hardening

  20. References Banerjee R, Nag S, Stechschulte J, Fraser HL. Strengthening mechanisms in Ti –Nb–Zr–Ta and Ti–Mo–Zr –Fe orthopaedic alloys. Biomaterials 25 (17) (2004) 3413. Davis, JR. Metals Handbook. ASM International (1998). Materials Park, OH. Long MJ, Rack HJ. Titanium alloys in total joint replacement—a materials science perspective. Biomaterials 19 (1998) 1621–39. Nag S, Banerjee R, Fraser HL. Microstructural evolution and strengthening mechanisms in Ti-Nb-Zr-Ta, Ti-Mo-Zr-Fe and Ti-15Mo biocompatible alloys. Mater Sci Eng C 25 (2005) 357-62. Niinomi M. Mechanical properties of biomedical titanium alloys. Mater Sci Eng A 243 (1998) 231-6. Ohmori Y, Ogo T, Nakai K, Kobayashi S. Effects of ω-phase precipitation on βα, α” transformations in metastable β titanium alloy. Mater Sci Eng A 312 (2001) 182-8. Wang K. The use of titanium for medical applications in the USA. Mater Sci Eng A 213 (1996) 134–7. Images: http://www.thehipclinic.co.uk/images/P1010019.jpg http://www.msm.cam.ac.uk/phase-trans/2004/titanium/ti64a.jpg Dmitriy Bekker

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