NiTi

NiTi, commonly called Nitinol, is an alloy of nickel and titanium with near equiatomic composition. It was discovered in the 1960s at the Naval Ordnance Laboratory; the name Nitinol derives from Nickel Titanium Naval Ordinance Laboratory. NiTi is renowned for shape memory and superelasticity. Shape memory behavior arises from a reversible martensitic transformation between a low-temperature martensite and a high-temperature austenite. Deforming NiTi in the martensitic state and heating above Af returns it to its original shape. Superelasticity occurs at temperatures above Af, where stress-induced martensitic transformation allows large recoverable strains, typically several percent, on unloading.

Transformation temperatures are described by Ms, Mf, As, and Af, and can be tailored by composition and thermomechanical processing. The high-temperature phase is austenite (often described as B2 structure); martensite can be monoclinic (B19′) or related variants. NiTi is produced and shaped by thermomechanical processing, including controlled cooling and heating cycles, to set transformation temperatures and shape-memory behavior.

NiTi alloys have good corrosion resistance due to a stable titanium oxide surface and generally high biocompatibility, though nickel release may pose risks for nickel-allergic individuals. The alloy’s relatively low density and modulus, combined with biocompatibility, underlie widespread medical uses.

Common applications include medical devices such as stents, guidewires, orthodontic wires, and endodontic files, as well as actuators and sensors in aerospace, robotics, and consumer devices. In these uses, tailoring transformation temperatures and processing improves performance under physiological or service conditions.