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Nitinol is a highly complex nickel-titanium alloy that has been used for decades to make medical devices. It is popular for its shape memory effect, superelasticity and fatigue and kink resistance. It is used to manufacture catheter tubes, guide wires, stone retrieval baskets, filters, needles and other surgical instruments. It is also the basis for self-expanding stents that are implanted in the coronary, peripheral and other arteries to help keep them open.
Generally speaking, nitinol is made by melting a mixture of nickel and titanium and then drawing the hot melt into rods, sheet or shaped components. The process is done through combinations of vacuum induction melting (VIM) and vacuum arc remelting (VAR). The exact ratio of these two constituents is important, as subtle changes can dramatically affect transformation temperatures.
After the nitinol is melted and drawn into desired shapes, it undergoes heat treatment to set the shape. This is often called’shape setting’. A good example is a straight Ni-Ti wire constrained in a hook shape fixture as shown in Figure 7. The material is then annealed at temperatures above the active martensite temperature, thereby setting the part in its final shape.
During this phase, the nitinol undergoes a significant amount of work hardening, so its strength increases dramatically. It must be reheated and reshaped in order to avoid damage, so it is often annealed several times. This is to prevent the formation of cracks. The heat treatment process also sets the shape memory and other properties of the Ni-Ti components and enables them to handle high mechanical stress loads.