Abstract
Ti-6Al-4V is the most common alloy used for orthopaedic implants. Its popularity is due to low density, superior corrosion resistance, good osseointegration and lower elastic modulus when compared to other commonly used alloys such as CoCrMo and stainless steel. In fact, the use of Ti64 has even further increased lately since recent controversy around adverse local tissue reactions and implant failure related to taper corrosion of CoCrMo alloy. However, implants made from Ti64 can fail in some cases due to fatigue fracture, sometimes related to oxide induced stress corrosion cracking or hydrogen embrittlement, or preferential corrosion of the beta phase. Studies performed with Ti-6Al-4V do often not consider that the alloy itself may have a range of characteristics that can vary and could significantly impact the implant properties. These variations are related to the material microstructure which depends not only on chemical composition, but also the manufacturing process and subsequent heat treatments. Different microstructures can occur in implants made form wrought alloys, cast alloys, and more recently, additive manufactured (AM) alloys. Implant alloy microstructure drives mechanical and electrochemical properties. Therefore, this study aims to analyse the microstructure of Ti-6Al-4V alloy of additive manufactured and conventional retrieved orthopaedic implants such as acetabular cups, tibial trays, femoral stem and modular neck by means of electron backscatter diffraction (EBSD). Microstructural features of interest include grains shape and size, phase content and distribution, preferred grain orientation (texture), alloying elements distribution (homogenization) and presence of impurities. Additionally, we demonstrate the direct impact of different microstructural features on hardness. We analysed 17 conventional devices from 6 different manufacturers, 3 additive manufactured devices from 2 different manufactures and 1 control alloy (bar stock). The preliminary results showed that even though all implants have the same chemical composition, their microstructural characteristics vary broadly. Ti64 microstructure of conventional alloys could be categorized in 3 groups: equiaxed grains alloys (Fine and Coarse), bimodal alloys and dendritic alloys. The additive manufactured implants were classified in an additional group on its own which consists of a needle-like microstructures - similar to Widmanstätten patterns, Fig. 1, with a network of β phase along α phase grains. Furthermore, AM alloys exhibited residual grain boundaries from the original β grains from the early stage of the solidification process, Fig. 2. These characteristics may have implication on the fatigue and corrosion behaviour. In addition, it we observed inhomogeneous alloying element distribution in some cases, Fig. 3, especially for the additive manufactured alloys, which also may have consequences on corrosion behaviour. Finally, the hardness testing revealed that the implants with large grain size, such as AM alloys, exhibit low hardness values, as expected, but also the amount of beta phase correlated positively with lower hardness. Grain aspect ratio and beta phase grain size correlated positively with higher hardness. In summary, we found that common Ti64 implants can exhibit a broad variety of different alloy microstructures and the advent of AM alloys introduces an entirely new category. It is imperative to determine the ideal microstructure for specific applications.
