nitinolin

Nitinol is an alloy of nickel and titanium known for its shape memory and superelastic properties. The name is an acronym from Nickel Titanium Naval Ordnance Laboratory, where it was discovered in the 1960s. The alloy is typically near equiatomic NiTi, with roughly equal parts nickel and titanium by atom and is processed through melting and thermomechanical treatment to achieve a stable austenite phase at the desired operating temperature. The microstructure comprises austenite (B2) at higher temperatures and martensite (B19') at lower temperatures, with possible intermediate R-phase in some compositions.

Phase transformation is central to its behavior. Nitinol exhibits reversible martensitic transformations between martensite and austenite phases. Transformation temperatures Ms, Mf, As, and Af determine the temperature range over which memory and superelastic effects occur. In the shape memory regime, deforming the material in the martensite phase and heating above Af causes it to revert to the original shape. In the superelastic regime, loading above the austenite finish temperature allows a large, recoverable strain upon unloading.

Properties and applications include a combination of ductility, recoverable strains, and good corrosion resistance due to a surface titanium oxide layer. Nitinol is widely used in medical devices such as self-expanding stents and guidewires, orthodontic wires, and various actuators in robotics and aerospace, as well as in consumer items like eyeglass frames. Limitations include nickel content posing allergy risks for some individuals, as well as cost and the need for careful surface treatment and heat treatment to control transformation temperatures and prevent nickel release.