Effect of on-Line Annealing Temperature on Mechanical Properties, Microstructure, Fracture Structure and Superelasticity of Ti_49.1Ni_50.9 Alloy

Ti-50.9Ni (at%) shape memory alloy (SMA) ingot was prepared by the vacuum induction furnace, and was produced into Φ2.0 mm wire successively by forging, rolling and hot drawing. After complete annealing of 800 °C×5 min and cold drawing with 30% deformation, on-line annealing with different temperatures (500 °C×5 min, 600 °C×5 min, 700 °C×5 min) were employed to TiNi wires. Both tensile tests and fivecycles loading-unloading tensile tests with 7% strain were applied to study the mechanical properties, and superelasticity (SE) of TiNi wires. In addition, microstructure and fracture characteristics were investigated by OM, SEM and EDS. The results showed that the tensile strength of 500 °C×5 min annealing reached 1431 MPa, the elongation of 700 °C×5 min annealing reached 129%. The superelasticity behavior of 500 °C×5 min showed excellent performance in five-cycles loading-unloading tensile tests, and the transformation platform peak stress with >7% strain showed a small attenuation (about 4 MPa ), and cumulative residual strain was only 0.2%. In addition, fracture characteristics for all TiNi wires were all plastic fractures, there were a large number of dimples in fractures.