Ni-free shape memory alloys (SMA) for biomedical application

Abstract

Shape memory alloy (SMA) is an adaptive material with intelligent function. Due to its high reliability, biocompatibility and functionality in shape memory effect and superelasticity, it is a promising material for biomedical application. Shape memory effect (SME) and superelasticity (SE) are the keys for shape memory alloy to recovery their shape. For shape memory effect, the deformed material can recover its shape by being heated up to a particular temperature. For superelasticity, the strain recovery is obtained by releasing the applied load to the materials. And reverse martensitic transformation is involved in both of the processes. SMA is an excellent material in the biomedical application, such as the bone implant, orthodontic archwires, etc. Ni-Ti is the most widely used SMA in biomedical field. However, due to the detrimental of Ni-ion in human body, future focus is moved to the Ni-free SMA. The quaternary alloying Titanium-Niobium-Zirconium-Sn (Ti-Nb-Zr-Sn) SMA with different Sn content was studied. Ti-19Nb-9Zr-0.5Sn (at%) and Ti-19Nb-9Zr-1Sn (at%) were fabricated by arc melting and used for investigation. This project focuses on determining the phase transformation by using the Dynamic Mechanical Analysis (DMA). The mechanical properties due to different heat-treatment were also interested. Several thermal-mechanical treatments were carried out on various samples. Cold-rolling and heat-treatments at 600°C-900°C for 30 minutes were conducted and followed with iced-water quenching. Re-crystallization temperature was found below 700°C and the grain size is increased with increasing heat-treatment temperature. α" and β phases are found in most of the cold-rolled and heat-treated alloys, except the Ti-19Nb-9Zr-1Sn with 900°C heat-treatment. By analyzing the lattice parameters, a suppressing effect of the Sn content on the lattice parameter ratio is observed, which can further affect the reverse martensitc transformation. By using the DMA with a static stress around 2MPa, the reverse martensitic transformation was found at approximately 0-100°C and -20-60°C for the Ti-19Nb-9Zr-0.5Sn and Ti-19Nb-9Zr-1Sn respectively. The martensitic transformation was finished below -100°C for both type of alloys. No martensitic transformation could be found once the alloys were heated up from 100°C to 600°C and cooled back to -100°C with a temperature changing rate of 5°C/min. By conducting 3 thermal cycles from -100 to 100°C, the martensitic transformation starting temperature is noticed for a maximum 20°C shift. The Young’s modulus is the lowest for the 900°C heat-treated Ti-19Nb-9Zr-0.5Sn and Ti-19Nb-9Zr-1Sn samples, where the corresponding values are 273MPa and 290MPa. High performance in recovery strain is reported for the Ti-19Nb-9Zr-1Sn alloy with 900°C heat-treatment, it has a strain recovery of 2.93% under 3.5% loading strain. Double yielding is founded in the 600°C heat-treated samples. The Ti-19Nb-9Zr-1Sn with 900°C heat-treatment has the potential for biomedical application due to its low Young’s modulus and high performance in strain recovery. But further investigation should be carried out to decrease the reverse transformation temperature and to increase the strain recovery. Thus, addition of Sn content and various heat-treatments are recommended as the investigation directions.