Ni-free Shape Memory Alloys (SMA) for biomedical application

Abstract

Button-shaped ingots of Ti-22Nb-10Zr (at.%) were prepared by arc melting method in an argon atmosphere. Cylindrical rod specimens, used for cyclic compression test and thermomechanical analysis (TMA), were prepared by applying vacuum suction in the copper mould casting. The ingots were cold rolled for mechanical treatment with 93% thickness reduction. Solution treatment was applied on all cold rolled thin plates. Heat treatment at 300℃, 450℃ and 550℃ for 30min and 120min with subsequent oil quench were carried out. Effects of thermo-mechanical treatments on the crystal structure, transformation temperatures, hardness and elastic deformation behaviour were investigated in this project. Austenite phase was found in the X-ray diffraction (XRD) of all specimens. Changes in grain orientation after thermo-mechanical treatments were observed. This was attributed to the oriented nucleation and oriented growth of the deformed microstructure in the recrystallization process. Martensitic transformation temperatures were investigated by differential scanning calorimetry (DSC), thermomechanical analysis (TMA) and dynamic mechanical analysis (DMA). Both DSC and TMA failed to reveal the transition temperatures. The peaks at 20℃ and 45℃ in the heating curve of DMA were related to the reverse martensitic transformation. The Vickers hardness was increased by mechanical treatment. Thermal treatment also played a role in the hardening, although it was not as effective as mechanical treatment. Increasing the heat treatment temperature from 300℃ to 450℃ and 550℃ caused softening. However, increasing the heat treatment time from 30min to 120min showed little effect on the hardness. Tensile test was carried out on rectangular specimen with 20mm gage length. The Young’s modulus determined was 18GPa, which is much lower than previous reports of similar alloys. The critical stress for slip deformation also showed a dramatic increase compared with others’ published work. This was attributed to the difference in grain size and rolling and recrystallization texture. Elastic and plastic deformation behaviour in strain increment cyclic loading was investigated. The first cycles of all specimens at imposed strains greater than 2.5% deformed plastically. On the contrary, the second to fifth cycles superimposed themselves and deformed elastically. A maximum recoverable strain of 5% was achieved in the cyclic tensile test of the specimen with 93% CR and underwent solution treatment.