Nickeltitanium

Nickel titanium, commonly known as NiTi or Nitinol, is a nickel-titanium alloy that combines nickel and titanium in near-equal proportions. It is notable for two unique properties: shape memory and superelasticity. The alloy undergoes reversible transformations between a high-symmetry austenite phase and a low-symmetry martensite phase. By adjusting the nickel content and thermal history, transformation temperatures can be tuned, allowing the material to recover its original shape after deformation or to sustain large strains with little permanent deformation at a given temperature.

In its memory effect, a NiTi component deformed in the martensitic state will revert to its pre-deformed shape when heated above its transformation temperature. In the superelastic regime, at temperatures above the austenite finish temperature, the material can endure sizable, recoverable strains, with stress-induced martensite forming under load.

Composition and processing: Typically around 50-60% nickel by weight; the precise transformation temperatures are set by alloying and heat treatment. Surface passivation and polishing improve corrosion resistance; the material is generally resistant to corrosion in physiological environments and exhibits good fatigue properties, though performance depends on composition, processing, and environment.

Applications: Widely used in medicine and dentistry, including guidewires, stents, orthodontic wires, and endodontic files; also used in actuators, robotic components, and some consumer products such as spectacle frames.

Safety and limitations: NiTi can release small amounts of nickel, which can cause allergic reactions in nickel-sensitive individuals; however, many NiTi devices are designed to minimize nickel release through surface oxidation and coatings. It is not used in all applications due to cost and the need for specialized processing.