Titanium alloys are suitable for orthopedic applications that are subject to large loads, because they combine properties such as high mechanical strength and corrosion with good biocompatibility and a relatively low elastic modulus (closer to that of bone in comparison to others alloys); hydroxyapatite has a Young’s modulus even more similar to that of bone, but its biomechanical properties make it unusable by itself.
The most used biomedical alloys are:
- · ASTM F167 (semi-pure titanium 98.9 – 99.6% titanium);
- · ASTM F136 (Ti-6AI-4V);
The ASTM F136 (Ti-6AI-4V) has vast applications in the orthopedic field. The former is more commonly used in dental implants, or as a coating due to lower mechanical properties.
In ASTM F167 the oxygen content must be carefully checked because it has a large influence on the yield strength and fatigue strength: the yield strength varies from 170MPa for 0.18% of oxygen to 485MPa for 0.4%, while the fatigue limit ranges from 88.2 MPa (107 cycles) for 0.085% oxygen to 216 MPa (107 cycles) for 0.27% oxygen.
The addition of AI and V in F136 has the purpose of obtaining a α-β alloy thanks to the stabilizing effect of the α form by AI and the β form by V.
Microstructure and properties of alloys
Being made up almost entirely of titanium, the structure of this alloy is typically monophonic of the α type: grain diameter from 10 to 150 µm, depending on the work undergone. Typically cold worked, it has mechanical properties lower than Ti-6AI-4V. The presence of interstitial atoms (C, N, O) in the titanium lattice can produce a strengthening solution from solid solution. The presence of titanium oxide (TiO2) on the metal surface increases corrosion resistance and contributes to a better biological impact (good osseointegration).