Nickel Titanium (also known as Nitinol or NiTi) is in the unique class of shape memory alloys. 

A thermoelastic martensitic phase transformation in the material is responsible for its extraordinary properties. Nitinol alloys are typically made of 55%-56% Nickel and 44%-45% Titanium. Small changes in composition can significantly impact the properties of the material. 

There are two primary categories of Nitinol. 

The first, known as "SuperElastic", is characterized by extraordinary recoverable strains and kink resistance. 

The second category, "Shape Memory" alloys, is valued for the Nitinol's capacity to recover a pre-set shape when heated above its transformation temprature. The first category is often used for orthodontics (braces, wires, etc) and eyeglasses.  makes shape memory alloys, which are primarily useful for actuators, used in many different mechanical devices.