Nitinol is a special alloy that can automatically recover its plastic deformation to its original shape through the Shape Memory effect, when it is heated. It also has excellent expansion rates, fatigue life, corrosion resistance and high damping characteristics.
NiTi alloys can work at a wide range of temperatures. The temperature at which the alloy works can be controlled by adjusting its components, with higher nickel content giving lower transition temperatures and lower titanium content allowing for higher transition temperatures. Nitinol can be made reversibly martensitic by cooling it quickly below its transformation temperature (Af). This can be done without damaging the material or changing its properties in any way, as the transformation is reversible.
Using this property, nitinol is used in a variety of medical applications such as stents and superelastic needles. It is also a popular choice for less-invasive mechanical devices such as cell phone antenna springs. These flexible springs are able to change shape according to the temperature of the environment, and can be activated by a switch when needed, enabling them to act as a thermal sensor and actuator in a single device.
Until recently, however, it was difficult to weld nitinol, either to itself or to other materials. This has changed with the development of a new process that allows for the production of strong joints between nitinol and stainless steel, using a combination of laser welding and tungsten inert gas (TIG). More research is underway into the use of other processes to weld different metals to nitinol.