Nitinol is an equiatomic alloy of nickel and titanium with special smart properties that allow it to perform a variety of functions. Its superelasticity and stress hysteresis, along with its kink resistance make it ideal for use in medical devices such as self-expanding stents that hold open diseased or weakened circulatory vessels (see figure 1).
A shape memory effect makes this material unique; it can be deformed, stretched and then reshaped and returned to its original, flat state without losing any of its mechanical properties. These features enable the design of medical devices with complex geometries that would be impossible to manufacture in other materials.
While the nitinol industry has focused on the stent market, this material is used in many other medical devices including catheters, staples, biopsies needles and surgical instruments. During manufacturing, Nitinol is heated to high temperatures and then rapidly cooled in order to create a variety of intermediate forms such as rods, coils and plates. Careful melting and recent advances in melt chemistry have allowed for the production of nitinol with low inclusion sizes which contribute to superior fatigue behaviour.
These materials are then subjected to a range of thermomechanical processing operations to produce the finished device. During this process, it is essential that the material maintains its good tensile properties and transformation temperatures. It is therefore important to understand how the material responds to the welding and forming process in terms of ultimate tensile strength and plate thickness, especially when it is exposed to cyclic loading conditions.