| dc.description.abstract | Stress shielding is a potential problem facing the long-term reliability of structural medical implants such as knee and hip replacements. Because the large elastic modulus mismatch between the implant and the human bone, the load on the bone surrounding the implant is greatly reduced, causing a reduction in bone mass over time. This bone resorption causes difficulties in revision surgeries, and if left unattended, may eventually lead to the loosening of the implant. Thus, it is desirable to introduce an implant material that possess an elastic modulus comparable to that of the bone, while still retaining the other desired characteristics for biomedical alloys, such as strength, biocompatibility, corrosion resistance, and fatigue resistance. However, the lowest elastic modulus achieved so far in a biomedical alloy has been 55 GPa, more than double that of bones.
In this study, we achieve an effective elastic modulus as low as 25 GPa in a Ti74Nb26 shape memory alloy without sacrificing other mechanical properties. Furthermore, the alloy is able to automatically adjust its effective modulus based on the condition of the surrounding bone: when the implant carries a larger-than-desired portion of the load, its modulus will be reduced to transfer more load to the surrounding bone. This low effective modulus is brought about by the simultaneous activation of elastic deformation and stress-induced phase transformation enabled by superelastic cycling.
We utilize a combination of equal channel angular extrusion and post-extrusion heat treatments to optimize the microstructure and crystallographic texture of the Ti74Nb26 shape memory alloy in order to achieve the best shape memory and superelastic response. We also investigate the effect of ternary zirconium alloy on these properties and further study the low-cycle functional fatigue behavior of the alloy to determine the microstructure that leads to the most effective modulus reduction. We find that microstructure plays a large role in the stability of the reduced effective modulus. Finally, we confirm the excellent biocompatibility and corrosion resistance of the Ti-Nb shape memory alloys. Since they also possess comparable fatigue resistance to other biomedical alloys, we believe that they have the potential to bring significant improvements to medical implants. | en |
