Ti-6Al-4V Sam Bodmer

I've always biked a lot. When I lived in Chicago I would bike down the entire lakefront every day. I was even a bike messenger for one summer. This past summer in Chicago, as I was biking I was clipped by a car. It wasn't a bad hit, but my collarbone was directly hit. It was a bad break with a high displacement amount. They had to perform surgery. The surgery involved placing the two halves of the bone in the right position and screwing both sides into a plate that lies across both of them. After the surgery I was able to make a full recovery in roughly three months. I credit the quick recovery to the surgery and the plate they put in. The metal they used for the plate was a titanium alloy, so that's why I chose the titanium alloy Ti-6Al-4V.

Pre-surgery
Post-surgery

Why Titanium?

When repairing the bone with a plate, there are many material properties that have to be taken into consideration. Some of the most important properties to consider are the strength of the material, the density of the material, the biocompatibility of the material, and the elasticity of the material. It has to be strong as to keep your bone in place before it's able to fully grow back into place. It should have a low density, so it's able to be generally small in size and weight. Obviously it's biocompatibility is one of the most important properties. The elasticity of the material is very important for post-recovery. The modulus of elasticity should be similar to that of bone, or less, because if there is tension in the bone and the implanted plate is more rigid than the bone, there is more stress on the bone as a result of the plate, which can lead to chronic pain. Titanium and later titanium alloys were found to exhibit some of the best properties for this criteria.

Mechanical Properties of Ti-6Al-4V

Strength to Weight Ratio

Titanium is lighter and stronger than most other surgical metals making it a much better option than other surgical metals. The average density of medical stainless steel is 1.8 times larger than that of Ti6Al4V. However, the average tensile yield strength of Ti6Al4V is 1.9 times larger than the average strength of medical stainless steel. These properties are very advantageous, because they allow engineers to create smaller, lighter, and less intrusive medical devices.

Biocompatibility

Titanium has a generally low electrical conductivity, which contributes to it being relatively chemically inert in the human body. When titanium is exposed to pH levels comparable to that of the human body, a thin chemically inert oxide layer is formed over the surface of the metal which shields from corrosion. This film is a huge contributing factor to the biocompatibility of titanium.

Elasticity

Any surgical implant meant for repairing bone needs to be stiff enough to hold the bones in place, but also should still have as low of a modulus of elasticity as possible, because after healing it can put unwanted stress on the bone, if the bone is trying to naturally stretch past a threshold that the implant won't allow. Ti6Al4V has an average modulus of elasticity that is 1.7 times lower than that of the most common stainless steal used in medical implants. Although it is better when compared with stainless steel it still has a modulus of elasticity that is 4 to 6 times greater than that of cortical bone.

The Material Structure of Ti-6Al-4V

Ti-6Al-4V is categorized as an alpha/beta titanium alloy. It's chemical make up is shown in Figure 1 below. Pure titanium has an HCP microstructure below 882 C this is known as the alpha phase of titanium. When the temperature is over 882 degrees Celsius the microstructure changes to a beta-phase BCC structure. When aluminum is alloyed with titanium it stabilizes the alpha phase of titanium, however vanadium stabilizes the beta-phase of titanium. Vanadium allows the beta phase to occur under the temperature of 882 C. The beta phase can be precipitated even further in samples of Ti-6Al4V with heat treatments. This is why it is categorized as an alpha/beta titanium alloy. The multi-phasic microstructure strengthens the material by resisting dislocation motion by adding strain and grain boundaries.

Figure 1: Chemical makeup of Ti-6Al-4V (reference 3)

Other applications of Ti-6Al-4V

Ti-6Al-4V has excellent mechanical properties which make it a great choice in many applications. It is used in aerospace engineering for structural frames and connections. It is also used in engines for fan blades and areas that aren't exposed to a high amount of heat. Its anti-corrosive properties make it ideal for marine applications. It is also sometimes used in sports equipment due to its high strength-to-weight ratio.

References

1. http://pacificmetallurgical.net/albums/OpticalMicros/slides/Ti6Al4V.html

2. https://www.researchgate.net/figure/282395721_fig1_Fig-1-Optical-micrographs-of-equiaxed-Ti-6Al-4V-microstructures-at-different

3. http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MTP641

4. http://cdn.intechopen.com/pdfs/26862.pdf

5. http://www.supraalloys.com/medical-titanium.php

6. http://www.nssmc.com/en/tech/report/nssmc/pdf/106-05.pdf

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