| Titanium system shape memory alloys were widely used in the field of biological medicine, aerospace for its good deformation and corrosion resistance, biomechanical stability and good biological compatibility, etc. The shape memory effect of Ti-Ni system alloy and the superelasticity of Ti-Nb system alloy is a hotspot of current research. It is not easy to obtain high performance of Ti system shape memory alloy. Adjusting the fabrication process to obtain excellent mechanical property is as important as focusing on shape memory property in synthesis the alloys. It is know to all that the spark plasma sintering(SPS) has the advantage of high speed of heating up, low sintering temperature, short sintering time, high production efficiency, etc. It is a kind of fabrication process to synthesis the ultra-fine crystal/fine crystal alloy material.While the fabrication process and mechanical properties of titanium system shape memory alloys were rarely reported. In this study, the Ti-Ni and Ti-Nb system alloys were fabricated by spark plasma sintering(SPS) and casting method. A variety of analysis methods, such as DSC, XRD, SEM, and TEM, were used to analyse phase composition, microstructure and mechanical property of the fabricated alloys to obtain shape memory alloys with excellent comprehensive property. The results obtained in this paper would provide a theoretical guidance and experimental basis to fabrication design of shape memory alloys.Firstly, Ti49.8Ni50.2-xCux(x=0,2.5,5,7.5) amorphous powders were prepared after 60 h ball milling, and the resulting particle size are all roughly 20 mm. The x=5 and x=7.5 alloys were selected to study crystallization kinetics and crystallization mechanism. It is found that the activation energies required for nucleation and grain growth for the x=7.5 alloy powder were higher than those of the x=5 alloy powder. This suggests that the x=7.5 alloy powder has better thermal stability. Two kinds of alloy powders have different Avrami exponent n, the average n 2.13 for the x=7.5 alloy powder and 2.46 for the x=5 alloy powder. Both of the values n have the decreased trend, indicating the decrease in the nucleation rate due to nucleation saturation.Ti49.8Ni50.2-xCux(x=0,2.5,5,7.5) bulk alloys were fabricated by SPS. Microstructure and mechanical property studies indicate that Ti49.8Ni45.2Cu5 bulk alloy has excellent mechanical properties with highest fracture strength of 2372 MPa, the fracture strain of 17%, while the other alloys is brittle fracture.The sintered Ti49.8Ni50.2 alloy has been heat-treated at 900℃ for 1 h. It is found that the content of Ni Ti2 phase decreased significantly and a lot of Ni4Ti3 precipitated in the alloy. A shift peak appeared in the DSC curve with the thermal hysteresis of 12℃ in the as-heat treated alloy. Howerver, no shifts appeared in the sinterierd alloy.For the casted alloy with the same composition heat-treated at 450℃and 550℃ for 1h, the phase transition temperature has changed during the DSC test. The Ms decreased gradually with the increasing heat treatment temperature while the As basically remain unchanged. It is some precipitated phase that changed the composition of the matrix phase, and results in the changed temperature of phase transformation.Among the as-sintered Ti-24Nb-x Zr(x =0,2,6) bulk alloys, the Ti-24Nb-x Zr bulk alloy has the excellent mechanical property with a yield strength of 1733 MPa, a fracture strength of 2680 MPa, and a fracture strain of 43%. Heat treatment was used to improve the super elasticity of the sintered alloys. It is found that the sintered alloys could almost fully recover in the process of the previous cyclic loading and unloading. Especially, the Ti-24Nb-6Zr bulk alloy can obtain the biggest super elastic strain of 2.56%, and the recoverable strain of 6.89%. By comparing the alloys treated at 600℃ and 700℃, the former alloy has good elasticity than the laters. However, the sintered Ti-24Nb-6Zr bulk alloy has the super elastictity of 1.85% and the recoverable strain of 5.58%. |
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