Preparation Of Nano-SiO₂ Particle Reinforced Tini-based Composites And Their Properties

TiNi shape memory alloys(SMAs) have been widely used as biological implants due to its excellent mechanical properties, good corrosion resistance, high biocompatibility, special shape memory effect and pseudoelasticity. By adding ceramics particles into the TiNi alloys, one can make the particle-reinforced TiNi composites with excellent mechanical properties. In this thesis, the nano-SiO₂ particles reinforced TiNi matrix composites were fabricated by Spark Plasma Sintering (SPS) and Vacuum-Sintering(VAS) methods, and the effects of the contents of nano-SiO₂ particles on the microstructure, phase transformation points, mechanical properties and superelasticity were investigated. The fracture morphology were observed and the fracture feature, deformation mechanism were discussed. The main conclusions are as follows:1. The microstructure of the composites fabricated by SPS and VAS mainly consist of the phases of TiNi,Ni3Ti,Ti2Ni and SiO₂, and the SiO₂ particles are evenly dispersed in TiNi matrix.2. The addition of SiO₂ reduced the phase transformation point of martensite finish temperature(Mf), reverse transformation start temperature(As) and reverse transformation finish temperature(Af), but it has no strong influence on the martensite start temperature(Ms) for SPS specimens. By contrast, the addition of SiO₂ has no strong influence on the phase transformation point of the VAS specimens.3. The compression test at room temperature showed that the SiO₂ particles can obviously improve the strength of TiNi alloys. However, with the increase of SiO₂ content, the plasticity of TiNi alloys decreases and the fracture mode of the alloys changes from ductile fracture to brittle fracture. Meanwhile, compared with the VAS specimens, the SPS specimens has higher strength but poorer plasticity.4. The cyclic loading-unloading test indicated that in range of low deformatiom, the strain recovery ratio of the SPS specimens increases with the increase of the SiO₂ content, while for VAS specimens, it exhibits the reverse trend. In comparison with SPS specimens, the VAS specimens exhibit higher strain recovery ratio when the content of SiO₂ was less than 1%, but they have lower strain recovery ratio when the content of SiO₂ was more than 1%, due to the increase of the porosity.