Titanium based alloys. Microstructure and properties of alloys

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).