Nitinol wire exhibits shape memory primarily because of its ability to undergo reversible phase transformations. It retains its memorized shape in the Martensite form, which is a low-temperature, easily deformable phase. When heated, it transforms to the Austenite form, causing it to return to its original shape. This characteristic is crucial for surgical applications like stents and orthodontic wires, where precise shape recovery under body temperature is essential. The Martensite phase allows Nitinol to be manipulated and deformed without permanent damage, enabling minimally invasive procedures and improved patient outcomes.

Reference: Biomaterials Science: An Introduction to Materials in Medicine, 3rd Edition.