nitinol

Nitinol is an alloy of nickel and titanium with a near equal composition, typically around 50 percent nickel and 50 percent titanium. It is known for two distinctive properties: shape memory and superelasticity, both arising from reversible solid–state phase transformations between martensite and austenite. The temperatures at which these transformations occur, and thus the material’s behavior, can be tuned through precise composition and heat treatment.

Shape memory refers to the ability of nitinol to recover a memorized shape when heated above a certain temperature, after being deformed at a lower temperature while in the martensitic phase. Superelasticity, or pseudoelasticity, allows nitinol to sustain large deformations at temperatures above the austenite finish temperature and return to its original shape upon unloading, without a phase change being driven by heat. The transformation temperatures are commonly described by terms such as Ms, Mf, As, and Af.

Nitinol’s properties stem from its highly stable titanium oxide surface that provides corrosion resistance and good biocompatibility. Although nickel can cause allergic reactions in some individuals, medical-grade nitinol is processed to minimize nickel release and to maximize biocompatibility. The material is also relatively fatigue resistant and can be precisely shaped through thermomechanical processing, making it suitable for demanding applications.

Applications include medical devices such as stents, guidewires, and orthodontic wires, as well as actuators in robotics and flexible eyeglass frames. Nitinol was discovered in the 1960s at the Naval Ordnance Laboratory, and the name derives from nickel titanium and the laboratory’s acronym NiTi NOL.