Ural federal university: Scientists Produce a Composite With Unique Properties For Implants

Scientists from the Ural Federal University, universities in Germany, South Korea and Japan have developed a new class of materials based on magnesium and titanium. The material has an unusual combination of properties – increased strength and a very low Young’s modulus (elastic modulus). Scientists have also discovered a new mechanism for regulating the value of the elastic modulus in such materials.

The work creates the prospect of using the obtained composites and other modified materials in the production of high-quality implants.

The article describing the study was published in the journal Scripta Materialia (supplement to Acta Materialia, one of the leading international publications on materials science).

“The modulus of elasticity of typical composites is approximately the average of the modulus of elasticity of each of the two materials of which it is composed, taking into account their volume fraction in the composition of the composite. Studying the properties of the materials we obtained from magnesium and titanium, we found that in this case this rule does not work. Magnesium has an elastic modulus of 45 gigapascals, titanium – 110 gigapascals, and a composite material – 17.6 – less than that of the magnesium phase with the lowest modulus of elasticity. This led us to bewilderment, we have not seen anything like this before,” says Ilya Okulov, a member of the research group, a researcher at UrFU, Bremen University and the Leibniz Institute of Materials Science (Germany), Tohoku University (Japan).

In an effort to explain the anomaly, scientists hypothesized that such a significant decrease in elastic modulus is due to the unique state of the interface. This is the contact area of two phases, magnesium and titanium. These substances are immiscible and do not create joint phases under equilibrium conditions, that is, a slow cooling rate (the process of creating a composite proceeds at a temperature of 700–800 ℃). During mechanical testing of this composite material, local slip of one phase relative to the other is observed. Digital experiments carried out on a computer model of the composite confirmed the validity of scientists’ assumptions about the determining role of the interface.

“In the process of creating a composite, the magnesium phase is in a liquid, molten state, and the titanium phase is in a solid. When the composite is cooled, the magnesium phase solidifies. In this case, the volume of magnesium gradually decreases, which should lead to the formation of pores. In our model, we have distributed the pores – voids – along the phase boundary – the interface. The presence of pores in the interface zone weakens it, and this leads to a significant decrease in the elastic modulus, which we observed during the modeling, ”notes Ilya Okulov.

This method of obtaining a porous composite was applied for the first time. The most obvious area of application for low modulus composites is in the production of biomedical implants that provide rapid bone healing.

“The problem is that, reacting to the presence of an implant, which takes a part of the load on the regenerating bone, the body“ decides ”that in this location it does not need“ extra ”bone tissue, and with the help of special cells it begins to destroy it. This phenomenon is called the voltage shielding effect. As a result, the implant may be displaced or even deformed, and a second operation will be required. Therefore, for a complete and prompt treatment of bones, materials are required whose elastic properties are comparable to the elasticity of bones. Young’s modulus of the composite created by us is in the range of values corresponding to the characteristics of human bone, which allows us to use our material for medical purposes,” adds Ilya Okulov.

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