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SHAPE MEMORY METALS: NİTİNOL

Memory Shape Metals are materials that once they have learned something, they remember it. Although in principle, this statement may seem like science fiction, the truth is that they have a property that allows them to recover their original form after being subjected to deformation. They are materials with shape memory, capable of undergoing significant transformations at an atomic scale before external stimuli, such as, for example, temperature changes or the application of a magnetic field. This article aims to explore Memory Shape Metals, specifically Nitinol.

The effect of shape memory and super-elasticity on SMA (Shape Memory Alloy) alloys was already observed by Büehler and collaborators in 1963 when they discovered Nitinol, an alloy of Nickel and Titanium that at low temperatures can be easily deformed, but when taken high-temperature changes to a harder form, exerting a stable force. Shape Memory Alloys (SMA) have gained great commercial interest in recent years due to the wide range of functions they can perform in the area of medicine, dentistry and electronic applications.

Nitinol

Shape Memory Wire Spring

The Nitinol is an alloy of nickel and titanium, was developed by William Beuhler, in the laboratories of the US Navy in the 1960s, however, the ability to create alloys with shape memory is we have known since 1932. The name NiTiNOL is an acronym for Nickel, Titanium and Naval Ordnance Laboratory, the place where it was developed. The novelty of this alloy, compared to previous SMAs, is that it is low-cost, resistant to corrosion and has no toxic effects. The exact ratio between nickel and titanium is 55% and 45% respectively and, although they seem very similar proportions, a minimal variation has a dramatic effect on the transition temperature.

Nitinol is the best-known example of so-called shape memory alloys. It is a nickel and titanium alloy. The team of researchers who discovered it baptized the new material with the name of NiTiNOL. It is an alloy of nickel and titanium in almost equimolar proportions and has spectacular memory properties. Shape memory is manifested when, after plastic deformation, the material regains its shape after gentle heating. Phase transitions in solids can be produced by two very different mechanisms. The most common is the displacement of atoms from their equilibrium positions, through a process known as diffusion, to adopt a more stable new structure in the conditions of pressure and temperature at which the material is found.

This type of transition generally occurs slowly. However, in the AMF, the atoms undergo very small displacements of their equilibrium positions and there is no diffusion, the transformations being rapid. This type of transformation is called martensitic because they were first described for the transformation of steel between its austenite (ductile and malleable) and martensite (fragile and hard) phases. Martensite (low temperature) is a less symmetrical phase than austenite. Once the martensite phase has been generated by cooling, it can be easily deformed and in a plastic form, but the transformation by heating recovers the austenite type structure. This effect, on a macroscopic scale, manifests itself in the recovery of the initial form. The applications that have been developed so far derive from its two fundamental properties: super-elasticity and the recovery of the form by heating. Due to their properties of a super-elastic material, medical application devices have been developed, such as self-expanding cylinder-meshes to maintain the permeability of blood vessels (stents), or devices for occlusion of heart defects.

How Does The Transformation Work?

In this type of transformation, we have a high-temperature phase called austenite, also known as the generating phase and cubic structure. If we cool the material, its structure changes and passes to a lamella structure, extremely interwoven and arranged in alternate cuts, called martensite. The structure cut alternately, that is, in consecutive opposite cuts, retains the general shape of the crystal.

When this material is cold or below its transformation temperature, it has a very low elastic limit and can be deformed quite easily in any new form, which is then maintained. However, when the material is heated above its transformation temperature, it experiences a change in the crystal structure that causes it to return to its original shape. If the alloy meets any resistance during this transformation, it can generate very large forces. This phenomenon offers a unique mechanism for remote operation.

In the state of martensite, an SMA is very easy to deform by applying stresses, by virtue of the spread of the outline of the twin. If at this stage the load is eliminated, the deformation of the martensite persists, which gives it the appearance of plastic deformation. However, after being deformed in the martensitic state, the heating causes a transformation of the martensite into austenite, whereby the component recovers its original form.

These properties make it a material capable of recovering a predetermined shape after having suffered a macroscopic deformation and can also be elastically deformed up to 8-10%. Together with these unique properties, good corrosion behavior, good biocompatibility have been demonstrated. It has good cytotoxicity that make NiTi an excellent candidate for biomedical applications.

 

Properties of Nitinol Memory Metal

In the previous section, we have seen how atoms are weakly formed in the martensite state, go through a transformation process, and when they receive heat they return to their original form and we see that in the austenite state, the atoms are perfectly aligned.

This process of transformation of the material is the basis of the two fundamental properties of this alloy: remembering its shape and superelasticity. The property of superelasticity implies that in both states the material is highly malleable.

Nitinol has all the typical properties of shape memory alloys. The most prominent properties are stated below:

1.Memory simply

2.Thermoelastic martensitic transformation

3.Memory double

4.Pseudoelasticity

5.Superplasticity

6.Damping capacity

7.Density: 6.45 gr / cm³

8.Transformation temperature: -200 to 100ºC

9.Melting point: 1,300ºC or 2,370ºF

10.Corrosion Performance: Excellent

The state by which these materials recover their form is the result of the solid-solid phase transformation between two material structures, that is, austenite and martensite.

 

Applications of Nitinol

A great diversity of applications has been developed for this metal with memory and other intelligent materials. The properties of remembering the original form and the superelasticity make this material, and in general of all SMA’s, an excellent resource to innovate in increasingly sophisticated applications.

1.The properties of nitinol have been useful in many fields, it has been used in military applications, security, and robotics. But the most innovative and relevant use exists is in the medical field. From more efficient tweezers and scissors for orthodontic surgeries and wires to guides for vascular probes.

2.Nitinol is one of the most commonly used alloys because it is biocompatible and inexpensive, but we can also find other alloys with shape memory like copper-aluminum-nickel; copper-zinc-aluminum or iron-manganese-silicon.

3.Its applications also include lens frames, golf clubs, coffee container thermostats, electrical connectors, solar screens, clamps and structural vibratory elements to reduce the effect of earthquakes. For fun, a spoon has been invented that twists when it comes in contact with hot water.

4.Similarly, memory alloys are used to deploy solar panels and satellite antennas and to control the balance in helicopter blade rotors.

The following will describe in more detail some applications:

Medical applications

The most important characteristic for medical applications is that Nitinol has compatibility and does not generate rejection in the human body. For example, Nitinol wires have been used to build micropumps that can replace functions of the heart or kidneys, also to decongest clogged arteries (stents) and are also widely used in orthodontics.

Tight Coating

The best example of application in this field is undoubtedly the Cryofit hydraulic coupling. These anchors are manufactured as sleeves just smaller than the metal pipe to join. Its diameter is expanded during martensite and, after heating to austenite, the tubes shrink and hold tightly. The tubes mean that the coupling cannot recover the diameter with which it was made, and the tension that is generated is in many cases is higher than that of a weld.

Action force

In some applications, the memory component is designed to exert force in a wide range of movements, always for several cycles. In these electrical connector systems, memory alloys are used to force the opening of spring when the connector is hot. This allows the insertion or force-free removal of a circuit card in the connector. After cooling, the NiTi becomes weak and the spring easily deforms it while the board is tightly closed and the connections are formed.

Based on the same principle, CuZnAl alloys have found important applications in this area. An example is a safety valve that incorporates CuZnAl designed to close the flow of toxic gas when a fire occurs.

Super elastic applications

A number of products have been launched into commerce with pseudo-elastic (or super-elastic) properties. Super-elastic NiTi eyeglass frames have been developed to absorb large deformations without breaking. Others are the wires used in orthodontics which must withstand a great deal of readjustment.

Proportional Control

It is possible to use only a part of the recovery of the form since the change occurs in a wide range of temperatures and not a single one. A valve has been developed that controls the flow of a substance, heating a component with enough memory to close the valve, the desired amount. It is possible to use this technique to position between 0.25 mm

Conclusion

Memory Shape Metals could be defined as those materials capable of “remembering” their form and capable of returning to that form after having been deformed. This memory effect can be produced by thermal or magnetic change. They are also able to repeat this process countless times without deteriorating. Materials with “memory” can be produced in many shapes and sizes, and developed for various uses. Although they are not as strong as steel, they are much more elastic and their properties allow them to take the necessary shape when exposed to high temperatures. In the U.S. Navy labs, an alloy of nickel (Ni) and titanium (Ti) was discovered that had unique properties in a research program aimed at obtaining an alloy with high corrosion resistance. Nitinol is an important memory shape metal due to its enormous applications.

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