Ni-Ti alloy was developed in 1962 by Naval Ordnance Laboratory, White Oak, Maryland, USA and is commercialized under the name nitinol (nickel-titanium Naval Ordance Laboratory). Ni-Ti alloy shows shape memory effect when temperature or stress is applied.
The Ni-Ti has proven to be the most flexible and beneficial in engineering applications. The Ni-Ti alloy have greater shape memory strain (up to 8% versus 4% – 5% for the copper – base alloys), tend to be much more thermally stable, and have excellent corrosion resistance compared to the copper-base alloys, which have higher ductility, more stable transformation temperatures, better biocompatibility and the ability to be electrically heated for shape recovery.
Some physical properties of Ni-Ti are shown below:
| 1. Density : | 6.45 g/cm³ |
| 2. Melting temperature : | 1240°C – 1310°C |
| 3. Resistivity : | 82-76 μΩcm |
| 4. Thermal conductivity : | 0.1 W/cm°C |
| 5. Heat capacity : | 0.007 cal / g°C |
| 6. Latent heat : | 5.78 cal/g;24.2 J/g |
| 7. Magnetic susceptibility : | (3.8-2.5) emu / g |
The mechanical properties of nitinol are as follows:
| 1. Ultimate tensile strength : | 754-960 MPa |
| 2. Typical elongation to fracture : | 15.5% |
| 3. Typical yield strength : | 560-100 MPa |
| 4. Approximate elastic modules : | 75-28 GPa |
| 5. Approximate Poisson’s ratio : | 0.3 |
Applications of SMAs
Shape memory alloys (SMA) in future will replace the highly expensive hydraulic systems in the aircraft flaps and wings.
SMA will help to fabricate and surgical bone plates which will increase the healing process and reduce recovery time.
Antenna wires: SMA will facilitate production of antenna, with superior durability.
SMA can be deployed to open valves to release hot or cold water at proper temperatures.
Stents based on SMA principle can be deployed by heart surgeons to open the heart blocks and reinstate blood flow.
Fire safety valve: SMA application in fire safety valve can shut off toxic or flammable gas flow when fire occurs.
SMA component in Circuit- board edge connector will force to open a spring when the connector is heated.
Ni-Ti is used as the eye glass frame to protect from excess deformation.
The life time of helicopter blades depends on vibrations and their return to its original shape. Hence shape memory alloys are used in helicopter blades.
The flexibility of super elastic Ni-Ti wire makes it ideal for use as retractable antennas.
Medical Applications Ni-Ti alloys have been employed in
- Artificial hip joints.
- For marrow pins.
- For connecting broken bones.
- For dental arch wires.
- Micro surgical instruments.
Advantages of shape memory alloys
- Bio-compatibility
- Diverse fields of application, clean, silent, spark free working condition.
- Good mechanical properties.
- Strong, corrosion resistant
- High power/weight ratios.
- Simplicity, compactness and safety mechanism
Disadvantages of shape memory alloys
- Expensive
- Low anergy efficiency
- Complex control
- Limited bandwidth due to heating and cooling restrictions.
- Degradation of poor fatigue properties.
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