NITINOL Shape Memory Alloy
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 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, and then recover its original, undeformed shape upon heating above its "transformation temperature."
NiTi compound.
Nitinol's unusual properties are derived from a reversible solid-state phase transformation known as a martensitic transformation between two different martensite crystal phases, which requires 69–138 MPa (10,000–20,000 psi) of mechanical stress.
At high temperatures, nitinol assumes an interpenetrating simple cubic structure referred to as austenite (also known as the parent phase). At low temperatures, nitinol spontaneously transforms into a more complicated monoclinic crystal structure known as martensite (daughter phase).[8] There are four transition temperatures associated with the austenite-to-martensite and martensite-to-austenite transformations. Starting from full austenite, martensite begins to form as the alloy is cooled to the so-called martensite start temperature or Ms, and the temperature at which the transformation is complete is called the martensite finish temperature, or Mf. When the alloy is fully martensite and is subjected to heating, austenite starts to form at the austenite start temperature, As, and finishes at the austenite finish temperature, Af.[9]
Thermal hysteresis of nitinol's phase transformation
The cooling/heating cycle shows thermal hysteresis. The hysteresis width depends on the precise nitinol composition and processing. Its typical value is a temperature range spanning about 20–50 °C (36–90 °F) but it can be reduced or amplified by alloying[10] and processing.[11]
Crucial to nitinol properties are two key aspects of this phase transformation. First is that the transformation is "reversible", meaning that heating above the transformation temperature will revert the crystal structure to the simpler austenite phase. The second key point is that the transformation in both directions is instantaneous.
Martensite's crystal structure (known as a monoclinic, or B19' structure) has the unique ability to undergo limited deformation in some ways without breaking atomic bonds. This type of deformation is known as twinning, which consists of the rearrangement of atomic planes without causing slip, or permanent deformation. It is able to undergo about 6–8% strain in this manner. When martensite is reverted to austenite by heating, the original austenitic structure is restored, regardless of whether the martensite phase was deformed. Thus the shape of the high-temperature austenite phase is "remembered," even though the alloy is severely deformed at a lower temperature.[12]
2D view of nitinol's crystalline structure during the cooling/heating cycle
A great deal of pressure can be produced by preventing the reversion of deformed martensite to austenite-from 240 MPa (35,000 psi) to, in many cases, more than 690 MPa (100,000 psi). One of the reasons that nitinol works so hard to return to its original shape is that it is not just an ordinary metal alloy, but what is known as an intermetallic compound. In an ordinary alloy, the constituents are randomly positioned in the crystal lattice; in an ordered intermetallic compound, the atoms (in this case, nickel and titanium) have very specific locations in the lattice.[13] The fact that nitinol is an intermetallic is largely responsible for the complexity of fabricating devices made from the alloy.
Applications
A nitinol paperclip bent and recovered after being placed in hot water
There are four commonly used types of applications for nitinol:
Free recovery
Nitinol is deformed at a low temperature, remains deformed, and then is heated to recover its original shape through the shape memory effect.
Constrained recovery
Similar to free recovery, except that recovery is rigidly prevented and thus stress is generated.
Work production
The alloy is allowed to recover, but to do so it must act against a force (thus doing work).
Superelasticity
Nitinol acts as a super spring through the superelastic effect.
Superelastic materials undergo a stress-induced transformation and are commonly recognized for their "shape-memory" property. Due to its superelasticity, NiTi wires exhibit an "elastocaloric" effect, which is stress-triggered heating/cooling. NiTi wires are currently under research as the most promising material for the technology. The process begins with tensile loading on the wire, which causes fluid (within the wire) to flow to HHEX (hot heat exchanger). Simultaneously, heat will be expelled, which can be used to heat the surroundings. In the reverse process, tensile unloading of the wire leads to fluid flowing to CHEX (cold heat exchanger), causing the NiTi wire to absorb heat from the surroundings. Therefore, the temperature of the surroundings can be decreased (cooled).
Elastocaloric devices are often compared with magnetocaloric devices as new methods of efficient heating/cooling. Elastocaloric device made with NiTi wires has an advantage over magnetocaloric devices made with gadolinium due to its specific cooling power (at 2 Hz), which is 70X better (7 kWh/kg vs. 0.1 kWh/kg). However, electrocaloric devices made with NiTi wires also have limitations, such as its short fatigue life and dependency on large tensile forces (energy consuming).
