Microstructures and properties of Ti-51at.%Ni, Ti-23at.%Nb and Ti-30at.%Ta shape memory alloys fabricated by microwave sintering for biomedical applications

Titanium-nickel (Ti-Ni) shape memory alloys (SMAs) have been widely used for biomedical applications. However, Ni is recently known as a toxic element that can cause hypersensitivity on human body. Therefore, the development of Ni-free SMAs for biomedical applications is crucial. The best candidate to substitute Ti-Ni alloy is β type Ti alloys composed of nontoxic elements. The purpose of this research is to investigate the possibility of Ni-free Ti alloys, namely, titanium-niobium (Ti-Nb) and titanium-tantalum (Ti-Ta) to replace Ti-Ni. In the research, Ti-51at.%Ni, Ti-23at.%Nb and Ti-30at.%Ta SMAs were produced from elemental powders and fabricated by microwave sintering with addition of ternary elements namely, cerium, silver and tin. The microstructures of the sintered alloys were characterised by using differential scanning calorimetry (DSC) equipment, optical microscope, scanning electron microscope (SEM) and X-ray diffractometer (XRD). The mechanical and shape memory properties were determined using compressive test and specially designed equipment respectively, whereas corrosion and antibacterial behaviour were determined by using electrochemical test in simulated body fluid and agar disc diffusion technique with E.coli bacteria, respectively. Based on the experimental work on varying the sintering temperatures and times, it was found that 700˚C for 15 min gave the least porosity of 20% for Ti-51at.%Ni alloy, whereas 900˚C for 30 min gave the lowest porosity of 23% and 28.72% for Ti-23at.%Nb and Ti-30at.%Ta, respectively. It was observed that the microstructures of Ti-51at.%Ni alloys feature B2 austenite and B19´ martensite phases, while Ti-23at.%Nb alloys exhibit β austenite, α" martensite and α phases. The binary Ti-51at.%Ni alloy gives the highest hardness of 152 Hv, whereas the ternary Ti-Ta-3Ag gives the lowest hardness of 43 Hv. It was also found that Ti-Ni-1Ce alloy has the lowest elastic modulus of 5.2 GPa indicating good biocompatibility. The addition of Ce, Ag and Sn elements to Ti-23at.%Nb and Ti-30at.%Ta SMAs improved the total strain recovery (ԐT). The highest and lowest ԐT of 51.68% and 30.17% are shown by Ti-Ni-0.5Sn and Ti-Ni-3Ce alloys, respectively. The corrosion resistance was enhanced for all the ternary alloys due to the formation of passive layer on the surface and various phases within the material. The lowest corrosion rates observed in each type of the SMAs are 0.4076, 0.0155 and 0.0059 mm/year for Ti-Ni-0.5wt.%Sn, Ti-Nb-3wt.%Sn and Ti-Ta-0.5wt.%Sn alloys, respectively. Antibacterial property was improved after the addition of the alloying elements for all the ternary alloys indicated by the size of the inhibition zones against E. coli bacteria. It was found that Ti-Ta-3wt.%Ce alloy has the best anti-bacterial property with the largest inhibition zone of 7.75 mm compared with other SMAs. It can be concluded that the Ni-free SMAs, namely Ti-Nb and Ti-Ta alloys with the addition of alloying elements show promising candidates to be used as biomaterials due to their enhanced biocompatibility properties.