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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.
Nickel titanium, also known as nitinol, is a metal alloy of nickel and titanium, where the two elements are present in roughly equal atomic percentages. Different alloys are named according to the weight percentage of nickel; e.g., nitinol 55 and nitinol 60.
 Nitinol wires | |
| Material properties | |
|---|---|
| Melting point | 1,310 °C (2,390 °F) |
| Density | 6.45 g/cm3 (0.233 lb/cu in) |
| Electrical resistivity (austenite) | 82×10−6 Ω·cm |
| (martensite) | 76×10−6 Ω·cm |
| Thermal conductivity (austenite) | 0.18 W/cm·K |
| (martensite) | 0.086 W/cm·K |
| Coefficient of thermal expansion (austenite) | 11×10−6/°C |
| (martensite) | 6.6×10−6/°C |
| Magnetic permeability | < 1.002 |
| Magnetic susceptibility (austenite) | 3.7×10−6 emu/g |
| (martensite) | 2.4×10−6 emu/g |
| Elastic modulus (austenite) | 75–83 GPa |
| (martensite) | 28–40 GPa |
| Yield strength (austenite) | 195–690 MPa |
| (martensite) | 70–140 MPa |
| Poisson's ratio | 0.33 |
| Nitinol properties are particular to the precise composition of the alloy and its processing. These specifications are typical for commercially available shape memory nitinol alloys | |
Nitinol alloys exhibit two closely related and unique properties: the shape memory effect and superelasticity (also called pseudoelasticity). Shape memory is the ability of nitinol to undergo deformation at one temperature, stay in its deformed shape when the external force is removed, then recover its original, undeformed shape upon heating above its "transformation temperature". Superelasticity is the ability for the metal to undergo large deformations and immediately return to its undeformed shape upon removal of the external load. Nitinol can deform 10–30 times as much as ordinary metals and return to its original shape. Whether nitinol behaves with the shape memory effect or superelasticity depends on whether it is above the transformation temperature of the specific alloy. Below the transformation temperature it exhibits the shape memory effect, and above that temperature it behaves superelastically.
