History and Properties of Nickel Titanium (Nitinol)
- Nitinol was discovered by William J. Buehler and Frederick Wang in 1959 at the Naval Ordnance Laboratory.
- It was originally developed for missile nose cones and commercialization efforts began in the 1980s.
- Nitinol is an alloy of nickel and titanium.
- It exhibits a shape memory effect, allowing deformation at one temperature and recovery of the original shape upon heating.
- Nitinol also has superelasticity, enabling large deformations and immediate return to the undeformed shape.
- The alloy can deform 10-30 times more than ordinary metals.
- The properties of Nitinol depend on the specific alloy's composition and processing.

Mechanism and Thermal Hysteresis of Nitinol
- Nitinol undergoes a reversible solid-state phase transformation called martensitic transformation between austenite (parent phase) and martensite (daughter phase).
- There are four transition temperatures associated with the transformations.
- The martensite structure allows limited deformation through twinning.
- Nitinol exhibits thermal hysteresis during the phase transformation, with the width depending on composition and processing.
- The transformation is reversible and instantaneous in both directions.
- Alloying and processing can amplify or reduce the hysteresis.

Manufacturing and Challenges
- Nitinol is difficult to make due to tight compositional control and titanium's reactivity.
- Primary melting methods used are vacuum arc remelting (VAR) and vacuum induction melting (VIM).
- VIM melted material has smaller inclusions and higher fatigue resistance compared to VAR.
- Other boutique scale methods include plasma arc melting, induction skull melting, and e-beam melting.
- Heat treating Nitinol is critical for fine-tuning transformation temperatures and controlling properties.
- Challenges include fatigue failures, concerns about nickel release, proper treatment to form a stable protective TiO layer, inclusions in the alloy, and difficulties in welding.

Applications of Nickel Titanium (Nitinol)
- Nitinol can undergo free recovery, constrained recovery, work production, and superelasticity.
- It acts as a super spring through the superelastic effect.
- Nitinol wires exhibit the elastocaloric effect, which is stress-triggered heating/cooling.
- It is used in various biomedical applications such as orthopedic implants, catheters, stents, and surgical instruments.
- Nitinol is used in thermal valves, autofocus actuators, pneumatic valves, and damping systems in structural engineering.
- Other applications include heat engines, resilient glasses frames, aerospace applications, temperature control systems, and retractable antennas.

Other Considerations and Prototypes
- Nitinol has been used in vascular self-expandable metallic stents without evidence of corrosion or nickel release.
- Ongoing research explores other welding processes and metals for Nitinol.
- Nitinol releases nickel at a slower pace than stainless steel and corrosion was observed in early medical devices made without proper treatment.
- Nitinol is used in prototypes like the Banks Engine, a commercial engine, and demonstration model heat engines.
- It is also used in civil structures, dentistry, and neurovascular interventions.