The intermetallic equiatomic binary alloy NiTi forms the basis of the nickel-titanium shape memory alloy (SMA) family. It has a moderate solubility range enabling changes in composition and alloying with other elements to modify both shape memory and mechanical properties.
It displays common shape memory behaviour, returning to its original shape after being heated to above its phase transformation temperature. Heating to this temperature sees the alloy change from its low temperature monoclinic martensitic structure to the high temperature cubic austenitic structure.
While the first shape memory alloy was discovered in 1932, it wasn’t until the early 1960’s that the nickel-titanium SMA’s were first discovered by Buehler. He was working at the Naval Ordinance Laboratory at the time, hence the name Nitinol.
His discovery formed the basis of the first commercial shape memory alloy.
The equiatomic composition forms the basis of many Nitinol alloys.
Adding an additional nickel up to an extra 1% is the most common modification. This increases the yield strength of the austenitic phase while at the same time depressing the transformation temperature.
Other common additions are made to alter the phase transformation temperature, such as iron and chromium which lower the temperature. Copper can also be added to lower the stress required to deform the martensitic phase and decreases hysteresis.
Oxygen and carbon are contaminants which should be excluded form the system. They can affect the transformation temperature and reduce mechanical properties.
Work Hardening and Two Way Shape Memory Effect
Deformation stress can be reduced by work hardening and suitable heat treatment. This process can also improve the strength of the austenitic phase as well as giving the alloy two way shape memory effect.
The properties of Nitinol alloys are dictated by their composition. Other factors such as heat treatments can also play a part in affecting properties of Nitinol alloys. However, for most Nitonol alloys, their density is approximately 6.5 g/cm3 and will have a melting temperature in the range 1240 to 1310°C. The transformation temperature can be modified from less than –100°C to over 100°C.
Key properties of Nitinol alloys include:
- Large forces that can be generated due to the shape memory effect
- Excellent damping properties below the transition temperature
- Excellent corrosion resistance
- High fatigue strength
- Moderate impact resistance
- Moderate heat resistance
- Aerospace and naval applications - Nitonol fluid fittings or coupling have are being used in military aircraft and naval craft.
- Medical Applications - Tweezers for removing foreign objects via small incisions, anchors for tendon fixation and stents for cardiovascular applications
- Dentistry - Orthodontic wires, which no not need to be retightened and adjusted
- Safety devices - Safety valves/actuators to control water temperature and fire sprinklers
Other uses include:
- Spectacle frames
- Household appliances and deep fryers
- Clothing including underwire brassieres
- Vibration control in the form of engine mounts and actuators for buildings
- Fasteners, seals, connectors and clamps
- Mobile telephone antennaes