NiTiNOL austenitic

For Nitinol - at the transition Ms, atoms begins to shear uniformly throughout the crystal. As the temperature is lowered the atomic shear continues to increase. At temperature, Mf, the atoms shear to their maximum point and assume a new structure. Thus, between Ms and Mr temperature interval the crystal structure of Nitinol is undefined and belongs neither to austenite nor to martensite . Therefore, thermodynamically, it should be classified as the second-order transformation. This is illustrated in Fig. 3. Conventionally - above Ms temperature, the whole crystal assume a crystal structure identified as austenite . At Ms temperature, a new crystal structure of martensite begins to form through two-dimensional (planar) atomic shear. The two crystal structures of austenite and martensite therefore share an identical plane known as Invariant Plane. As the temperature is lowered, the two dimensional shear (or more correctly, shift ) continue to take place one plane at a time such that the Invariant Plane moves in the direction as to increase the volume of martensite at the expense of austenite . Ultimately, at Mt temperature the whole crystal becomes martensite . Since between Ms and Mf any given micro-volume of the crystal must belong to either the austenite or the martensite , the transformation is of the first-order thermodynamically. This case is pictorially illustrated in Fig. 4. 

The mechanics of the TiNi transition is, as shown by the X-ray, quite complex particularly when it comes to formation of twin or antiphase boundary. Although, personally I consider them as secondary importance toward the understanding of Nitinol transition itself, perhaps they should be mentioned to complete the picture. It has been known that the martensite (low-temperature phase) always has a crystal structure with lower symmetry than the austenite (high-temperature phase). In order to lower the free-... 

Nitinol is an alloy of nickel and titanimn that has the austenite phase structure. Both the nickel and titanimn atoms are arranged in cubes. As you can see above, each nickel atom is at the center of a cube of titanium atoms, and a titanium atom is at the center of each cube of nickel atoms. If niti-... 

One interesting alloy of titanium and nickel, called Nitinol, exhibits shape-memory properties. Below a particular temperature (the transformation temperature), the crystal structure of the alloy is such that it can be plastically deformed (martensitic). As the alloy is heated, the crystal structure alters to one that is more ordered and rigid (austenitic), and the deformed metal reverts to its original shape. This effect has been exploited in a number of devices, including a stent (a device used to hold open passageways such as arteries). The stent is placed inside a small-diameter catheter for insertion into the body, where it expands on being warmed to bod y temperature. 

A NiTiNOL shape memory metal alloy can exist in two different temperature-dependent crystal structures or phases called martensite (i.e., lower-temperature phase) and austenite (i.e., higher-temperature or parent phase). Several properties of the austenite and martensite phases are notably different. When martensite is heated, it begins to change into austenite. The temperature at which this phenomenon starts is called the austenite start temperature A). The temperature at which the phenomenon is complete is called the austenite finish temperature (A). When austenite is cooled, it begins to change into martensite. The temperature at which this phenomenon starts is called the martensite start temperature (M ). The temperature at which martensite is again completely reverted is called the martensite finish temperature (Mj). Composition and metallurgical treatments have dramatic impacts on the above transition temperatures. From the point of view of practical applications, NiTiNOL can have three different forms ... 

When the material is in its martensite form, it is soft and ductile and can be easily deformed like tin pewter. Superelastic NiTiNOL is highly elastic, while austenitic NiTiNOL is quite strong and hard, similar in that way to titanium metal. The NiTi material has all these properties, their specific expression depending on the temperature at which it is used. 

The unique behavior of NiTiNOL is based on the temperature-dependent austenite-to-martensite phase transformation on an atomic scale, which is also called thermoelastic martensitic transformation. The thermoelastic martensitic transformation causing the shape... 

The intermetallic Ni-Ti system has the imusual property of after being distorted, returning to its original shape when heated. This was the first of the shape memory alloys (SMAs) and was discovered by accident at the Naval Ordnance Laboratory, hence its name Nitinol. Other SMAs include Cu-Al-Ni, Cu-Zn-Al, and Fe-Mn-Si alloys. The shape memory mechanism depends on a martensitic solid-state phase transition that takes place at a modest temperature (50°C—150°C), depending on the alloy. The high temperature phase is referred to as austenite and the low temperature phase is called martensite (following the terminology of the Fe-FeCa system). 

The original shape (the one that is to be remembered) is created by heating to well above the Af temperatiue (such that the transformation to austenite is complete) and then restraining the material to the desired memory shape for a sufficient time period. For example, for Nitinol alloys, a 1-h treatment at 500 C is necessary. 

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