The Effects of Cold Work and Heat Treatment on the Properties of Nitinol Wire

Shape memory and superelastic capabilities coupled with good biocompatibility give Nitinol the ability to provide functionality seldom possible with traditional engineering alloys. In this study the effect of heat treatments of 300 ∼ 550°C for 2 ∼ 180 minutes on Ti-50.8%Ni (at.%) wire of 30% and 50% cold work was investigated. Transformational and mechanical properties were characterized through the bend and free recovery (BFR) method and tensile testing. Thermally activated precipitation and annealing processes occurred. Annealing processes tended to increase the slope and the total strain recovery of the BFR plots. Two TTT diagrams were constructed illustrating the trends in the Austenite Finish Temperature (Af) of the wires. A maximum precipitation rate occurred at approximately 450°C. Precipitation strengthening was evident in both 30% and 50% cold-worked wires. However, only in the former did an increase in UTS occur. Recrystallization began at approximately 450°C for both wires.