What is Nitinol?
Nitinol – pronounced night-in-all – is a nickel-titanium alloy that demonstrates a unique ability, the capacity to change its shape, dependent on the surrounding environment. This unusual property derives from a reversible solid to solid-state transformation and makes it one of the most utilized shape memory alloys.
Its name is derived from its composition and place of discovery; NIckel TItanium, and the Naval Ordinance Laboratory in America. In 1959, William J Buehler was attempting to design better missile cones to resist fatigue, heat, and force of impact. His 1:1 alloy of nickel to titanium was first demonstrated in 1961 when its potential was quickly realized, although not commercialized until a decade later.
The shape memory alloy can be deformed when cold but returns to its original shape when heated. It exhibits a shape memory effect and superelasticity – an elastic or reversible response to applied stress, caused by a phase transformation. It also displays low stiffness, biocompatibility, and corrosion resistance.
It is a functional material for many engineering applications – sensors, actuators, smart structures, biomedical implants, and aerospace components.
Shape memory materials
Nanotechnology employs the unique physical properties and interactions of nanoscale materials to create novel structures, devices, and systems. Shape memory materials are a class of nanotechnology materials responding to changes in the environment; they can be designed to respond dramatically upon exposure to energy input, such as heat, light or electricity. They are capable of remembering their original shape and have many applications, including heart stents, thermostat control wires and couplings that close with heat.
Most materials undergo phase changes – from solid to liquid to gas – at specific transition temperatures. Nitinol, however, experiences a solid to solid change. Rather than undergoing a phase change when it reaches its transition temperature, nitinol uses the energy to move atoms into a different arrangement – it changes shape but remains solid.
Like all alloys, Nitinol has a crystalline structure with atoms arranged to minimize energy; this repeated pattern is a lattice of points, either atoms, ions or molecules. When thermal or mechanical input is applied, it undergoes a crystal structure phase transformation. This high-temperature phase is known as austenite and has a very ordered cubic structure. When cooled below a set temperature, it transforms to martensite and has a plastic, deformed shape. The alloy will hold its deformed shape until it is heated back to its transition temperature, where atoms move back to their original location and the metal returns to its original, rigid shape.
It is difficult to make Nitinol due to the exceptionally tight compositional control required and the high reactivity of titanium. Every atom of titanium that combines with oxygen or carbon, is an atom taken from the Nitinol lattice, thus shifting the transformation temperature lower.
There are two primary methods, both of which have their advantages:
- Vacuum arc re-melting (VAR); a casting process where a consumable electrode is melted under vacuum at a carefully controlled rate using heat generated by an electric arc struck between the electrode and the ingot.
- Vacuum induction melting (VIM); an alternating magnetic field heats the raw material in a crucible, under a high vacuum.
Nitinol is easier to work with when hot, rather than cold. Machining of the alloy is also extremely difficult, although it is easy to perform Grinding, laser cutting and Electrical Discharge Machining (EDM) on this metal. The material may also undergo casting and powder metallurgy processes, and additive manufacturing, which is effective in producing highly complex geometries with pre-designed porosity, composition, and properties.
Laser-based additive manufacturing techniques are increasingly being used to produce Nitinol parts, with selective laser melting emerging as an effective means of producing the alloy with desirable functional properties. Powder quality and material composition can influence microstructure and phase transformation. Equi-atomic Nitinol has the most uniform and finest grains, which are necessary from high-density fabrication. The particle size also affects the flowability and packing density of the feedstock during fabrication.
References and further reading
Advances in Selective Laser Melting of Nitinol Shape Memory Alloy Part Production
Nanotechnology Invention and Design: Phase Changes, Energy, and Crystals
The Properties and Applications of Nitinol Wire