Ni-free Shape Memory Alloys (SMAs) for Biomedical Applications

Abstract

Biomedical materials are used to construct medical devices aimed at treating or replacing underperforming natural body tissues. As more and more countries are stepping into aging societies, an increasing demand for the development of better biomaterials and medical devices is foreseeable. Shape memory alloys (SMAs) are considered to be a promising implant material in certain biomedical applications such as treating bone fracture, thanks to their two main unique properties: Shape Memory Effect (SME) and Superelasticity (SE). In this project, the effects of heat-treatment on the crystal structure, transformation temperatures, mechanical and superelastic behaviour of Ti-19Nb-3Zr (at.%) alloy were systematically investigated. Ingots with nominal composition of Ti-19Nb-3Zr (at.%) alloy were produced by arc melting under Ar atmosphere. The ingots were cold rolled in the same direction at room temperature with a 90% thickness reduction. The produced Ti-19Nb-3Zr (at.%) thin plate were then heat-treated in the temperature range of 823-1223K. Some of the specimens were also aged after solution-treatment at 1223K. The X-ray diffraction measurement showed that the as-rolled Ti-19Nb-3Zr (at.%) alloy contained both α phase and β phase at room temperature. Martensite α" phase was found in the specimens heat-treated at 923K to 1223K. Aging at 673K promoted the formation of α precipitates, while no ω phase could be detected in all the specimens. The Ti-19Nb-3Zr (at.%) alloy’s martensitic transformation start temperature (Ms) and austenite finish temperature (Af) were determined to be around 295°C (≈570K) and 480°C (≈750K), respectively. The heat-treatment did not affect the transformation temperatures much. The temperature hysteresis was about 200K. All the Ti-19Nb-3Zr (at.%) specimens have a Young's modulus between 20-30 GPa, which is comparable to that of human cortical bone (≈30-35GPa). Solution treatment at 1223K did not have a significant effect on the Young’s modulus of the alloy, while it was increased slightly after low temperature aging. Both the solution treatment at 1223K and low temperature aging significantly improved the yield stress of the alloy. For the specimen solution-treated at 1223K, almost complete superelastic behaviour was observed in the first four cycles with a total applied strain up to 2%. However, plastic deformation began to accumulate afterwards. Maximum transformation strain of about 2% and maximum total recoverable strain of about 3% are observed after the 7th cycle. A strain hardening effect was observed. It is concluded that the Ti-19Nb-3Zr (at.%) alloy (solution-treated at 1223K followed by water quenching) may not be suitable for biomedical applications due to its unsatisfying superelastic behaviour at room temperature. It is proposed to increase the content of Nb or Zr to lower the transformation temperatures. Nevertheless, this alloy could be a good candidate for high-temperature SMA applications.