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Meet Shape Memory Alloy

What is Shape Memory Alloy?

Nickel-Titanium alloys are intermetallic compounds in which Nickel and Titanium elements are combined in equal or almost equal atomic ratios. The discovery of the Nickel-Titanium alloy took place in 1962. William James Buehler, a Metallurgical Engineer at the U.S. Naval Warfare Equipment Laboratory, and his colleagues discovered the Nickel-Titanium alloy for the first time. They named this alloy they discovered NITINOL. While giving the name NITINOL, "Ni" refers to the element Nickel, "Ti" refers to the element Titanium, while "NOL" refers to the "Naval Ordnance Laboratory (Naval Warfare Equipment Laboratory).

It is popularly known as "Metal with Memory". Although it has a very hard structure at high temperatures, the increase in flexibility as the temperature decreases is an important feature that makes the alloy special. The reason for this is that the atoms in NiTi are different in hot and cold environments.

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​Shape Memory Materials

Shape Memory Alloys (SFAs) belong to a class of shape memory materials (CHMs) that have the ability to "memorize" or retain their previous form when exposed to certain stimuli such as applied hot mechanical (thermomechanical) or magnetic variables. SBAs are material groups that can return to their original form (shape or size) when memorized between two transformation phases that depend on temperature or magnetic field. This transformation is known as the shape memory effect (SHA). The basic implementation of SBA is quite simple. It can be easily deformed by an external force applied to these materials and returns to its original shape by remembering its shape in memory when a certain temperature is exceeded by external or internal heating (such as above the Transformation temperature - Öf) or magnetic stimuli (such as electric current).

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What is Shape Memory Effect?

There are three known different crystal structures of SBA groups (These are Twined Martensite, Separated Twined Martensite and Austenite), these alloys can exhibit six possible transformations (Figure 1). When examined in these phases, the Austenite phase structure is stable (stable state) above the transformation temperature, and the martensite structure becomes stable at low temperatures. As a result of the heat applied to the SBAs, it completes its transformation from the Martensite phase to the Austenite phase by increasing it to a sufficient temperature range (it is important in sufficient time). For the transformation, the Austenite initial temperature (As) is the temperature at which the transformation begins, and the Austenite-end temperature (Af) is the temperature at which the transformation is completed (for sufficient temperature and time). When materials with SBA group are heated on Ös, they begin to deform and transform into an Austenite structure, and at the Austenite finish temperature (Öf), the structure completely transitions to the Austenite phase. The transformation that takes place is also possible with excessive applied loads. During the cooling process to the SBA groups, the transformation starts to turn to martensite when the martensite-starting temperature is lowered (Ms), and the transformation is completed when the martensite-finish temperature is reached (Mf). As seen in Figure 1, Md can be said to be the highest temperature at which the possible residual stress will not occur in the martensite transformation. Above the Md temperature, SBA behaves like the results expected from the properties of a normal metal material, and they are subject to permanent deformation with the applied deformation. In order to understand these changes (Shape Memory Effects) in these alloys, a brief description is given below.

1. One-Way Shape Memory Effect
Unidirectional SBA groups undergo deformation due to the applied external force load, and thus the structure transitions to the martensite phase (the transformation here  diffusion is a transformation) and retains its shape even when the load is removed. Afterwards, when the SBA reaches the Austenite temperature and sufficient temperature and time are expected, it remembers its original shape.

2. Bidirectional Shape Memory Effect
Bidirectional Shape Memory Alloys have two different shapes that can be remembered at high temperatures and low temperatures, unlike Unidirectional Shape Memory Alloys. In this way, opening or closing (whichever shape is reminded) when heated and opening or closing when cooled (whichever shape is reminded) can be obtained from a single material. However, the shape memory training that can be imparted to NiTinol materials showing Bidirectional Shape Memory Effect is almost half of the Unidirectional Shape Memory NiTinol materials, limiting their commercial use. Despite this, "Actuators" produced with NiTinol materials with Bi-Directional Shape Memory Training  Thanks to the unique contributions it provides to the user by replacing many devices and devices that need to be used at once, its use is increasing. Thanks to the widespread use of these materials today, they are among the unique material groups of the future.

3. Superelasticity
SBA groups can immediately return to their original shape when the applied mechanical loads are removed at a certain temperature range (usually at room temperature, superelastic materials are common). These materials are much more resistant than normal springs and materials, and they can exhibit 10 times higher fatigue life than high-strength materials such as steel. Thanks to these features they have, they provide contributions in shock absorbers, load damping, and resistance-requiring positions.

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