Effect Of Hydrogen On NiTi Alloy And Preparation Of NiTi Alloy Spring

NiTi alloys are one of the most important functional materials in the current applications because of their unique superelastic property, good fatigue and corrosion resistance, and biocompatibility. It can be used as the actuators in the engineering and implants in biomaterials. However, the effects of hydrogen couldn't be ignored for some certain applications. Thus the effects of hydrogen on the microstructure and properties of NiTi shape memory alloy are studied in the thesis.In order to study the effect of H element on the microstructure and mechanical properties of NiTi alloy, the specimens of Ni-rich NiTi alloy is electrolytically hydrogen charged. The NiTi specimens are cathode and graphite is used as the anode. The investigations indicate that the hydrides, which form during the electrolytically hydrogen charge, are dispersed along the grain boundaries, and the hydride shape is worm-like. The amount of hydrides increases with the augment of current densities or the charging time. The specimens that are solution treated for 1h and then ages at 600℃for 3h to 5h have the least amount of hydrides. The H element has a significantly negative effect on the mechanical performances of the Ni-rich NiTi alloy. When the charging time or the current density is prolonged, the deformed-bending angle, which characterizes the superelasticity of NiTi alloy, increases, thus the superelasticity of NiTi alloy deteriorates. As the charging time increases, however, the deformed-bending angle reaches to a certain value. The hydrides form at the outer part of the NiTi alloy wire. As the charging time increases the width of the hydrides layer increases slowly. The shape memory effect recovery ratio decreases with the increase of the charging time, which duo to increase of the hydrides. XRD spectra indicate that after electrolytically hydrogen charge the crystal structure of the NiTi alloy is changed.Because of the shape memory effect, especially the two-way shape memory effect (TWSME), NiTi alloy has been used as a promising material device as micro-sensors andmicro-actuators. The TWSME, however, is not a natural behavior of NiTi alloy; proper thermal-mechanical training is required to develop this effect. In this thesis, the effects of heat treatment, thermal-mechanical training and heating-cooling working circle on the TWSME of Ti-50.3at%Ni are studied. The results have been found that the TWSME springs, which can contract upon heating and extend upon cooling, are obtained through constrained aging and thermo-mechanical training, and the recovery ratio could reach to 45.5%; The optimum heat treatment process is ageing at 500℃for 5h, higher or lower aging temperature would impair the TWSME; There is a decrease of the TWSME recovery ratio with the increase of working cycles. At the beginning, the recovery ratio decreases sharply and then reaches to a certain value; TWSME springs are also investigated with alternating the current density. It is found that the time response is greatly depended on the magnitude of the current density.