Part. 4: Titanium-based Alloys
Titanium-based alloys are suitable for orthopedic applications with load charges, because they combine the high mechanical resistance to the corrosion with a good bio-compatibility and a quite low elastic module ( more similar to the one of bone than other alloys); the Young module of the hydroxyapatite is more similar to the one of the bone, but its biomechanical properties make it unusable alone.
The most common alloys in biomedical field are:
- ASTM F167 (semi-pure titanium 98,9 – 99,6% titanium);
- ASTM F136 (Ti-6AI-4V);
The ASTM F136 (Ti-6AI-4V) is largely used in the orthopaedic field. The first one is commonly used in dental implants or like coating due to its lower mechanical properties.
In ASTM F167 the oxygen content has to be carefully controlled because it affects very much the yield point and the fatigue resistance; the yield point goes from 170MPa for the 0,18% of oxygen up to 485MPa for the 0,4%, whereas the fatigue limit goes from 88,2 MPa (107 cycles) for the 0,085% of oxygen up to 216 MPa (107 cycles) for the 0,27% of oxygen.
The addition of AI and V in F136 aims to obtain an alloy α-β thanks to the stabilizing effect of the shape α from AI and of the shape β from V.
Microstructure and alloys properties
Given that this alloy is almost made of titanium, the structure is monophonic of type α: drain diameter from 10 to 150 µm, according to the treatments undergone. Usually cold-treated, its mechanical properties are lower than the Ti-6AI-4V. The presence of interstitial atoms (C, N, O) in the titanium reticulum can produce an enforcement effect of the solid solution. The presence of titanium oxide (TiO2) on the metal surface increases the corrosion resistance and improves the biologic impact (good osseointegration)