The design and production of biomedical devices from Nitinol requires precise understanding and control of the transformational and mechanical properties of the material.  Heat treatments are often utilized to tailor properties and optimize performance.  During heat treatment the formation of Ni-rich precipitates can be utilized to deplete Ni from the surrounding matrix, resulting in an increase in the transformation temperature.  Precipitation can also induce stress in the lattice, decreasing dislocation mobility and strengthening the material.  If Nitinol has been sufficiently cold-worked, heat treatments will activate the processes of annealing - recovery, recrystallization, and grain growth.  The goal of this study was to determine the results of such complex microstructural evolution on the properties of cold-worked Nitinol wires.
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 reduced the curvature and increased the slope of BFR plots.  The R-phase was present with greater frequency and prominence in the 50% cold-worked wire than the 30% cold-worked wire.  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.  The trends in Ultimate Tensile Strength (UTS), Upper Plateau Stress (UP), and Lower Plateau Stress (LP) were outlined for all heat treatment conditions.  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.