Ti-6Al-4V alloy is a rather obvious material choice for orthopaedic and dental implants. Therefore, it is surprising to know that there are still some long-term concerns over these implants due to the potential release of harmful ions such as Al and V. Pure Ti is not also an ideal option because of the much lower strength and hardness. One way to solve this problem is to embed bio-friendly reinforcements in titanium matrix, i.e., to make a metal matrix composite (Ti MMC) instead of a pure titanium. However, this technique vastly fails to provide the intended mechanical properties due to various material-based issues. To overcome these issues, the reinforcements in MMCs should be pushed to become as fine as possible, where ideally they should even reach nano-sizes.
With this background, we wanted to create the scientific know-how to generate a new category of titanium materials in which they can show high strength but without the use of Al and V. To this end, we needed the finest reinforcements from a biocompatible or ideally even a bioactive material such as titanium carbide. However, direct or ex situ addition of finest reinforcements to a material was extremely challenging since nano-size reinforcements are usually expensive, tedious, unsafe, and even hazardous. Therefore, we needed a shortcut in which we can produce our finest reinforcements during the actual manufacturing. The creation of reinforcements during manufacturing is called in situ reinforcing, which typically starts from a suitable and reactive powder material. In addition to the reinforcements, the presence of biocompatible elements are allowed and could be useful for various reasons. This made us think of Mo which not only imposed no bio-concern, but also it could manipulate the crystal structure and result in some exotic and interesting transformations.
According to the criteria set above, instead of Al and V, we thought of addition of molybdenum carbide (Mo2C) to pure titanium powder. Theoretically, we expected that Mo2C in situ reacts with Ti and creates more thermodynamically stable products, i.e., Mo and TiC. To trigger this reaction, we needed an advanced powder-based additive manufacturing (AM) technique to make our in situ titanium matrix composites uniformly in complex 3D shapes. For this, we had laser powder bed fusion (LPBF) or selective laser melting (SLM) available in house at KU Leuven. After setting the experiments, in a research work published in scientific reports, we showed and analysed how in situ TiC particles are created in a titanium matrix from Mo2C (Figure 1). We also demonstrated the influence of in situ alloying by Mo as it stabilised a β-titanium instead of its more observed allotrope, i.e., α-titanium. This resulted in an increase of the hardness and strength of a Ti component without any fear for long-term cytotoxicity of the material. On the down side, this material showed a low ductility, though this can be still improved by a subsequent heat treatments and/or use of less reinforcements (as will be presented in other research works). Nevertheless, we anticipate that this work opens new prospects to create novel and strong bio-grade Ti alloys without any Al and V for orthopaedic and dental implant applications.
Figure 1: Schematic transformation of Mo2C particles in Ti during laser melting, resulting in a Mo dissolved β-Ti matrix reinforced by fine but long TiC whiskers. This occurs via decomposition of secondary Mo2C particles and diffusion of the products.