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Principles and properties of morphological memory alloys

At present, memory alloys have attracted the attention of scientists and the general public because of their special shape memory properties. In the 20th century, in a regular meeting of the United States Naval Ordnance Research Institute, the shape memory effect of nickel-titanium alloy was accidentally discovered. Through the in-depth research of scientists, dozens of alloy materials with this effect were found. Since then, memory alloy has been applied in various fields such as aerospace, biomedical, civil engineering and so on. It is also known as the "living material" due to its great development potential.

Shape memory effect refers to the plastic deformation of an alloy caused by external force at low temperature. After heating to a certain temperature (As), the alloy will return to its original shape at high temperature. This effect is called shape memory effect. The shape memory effect is due to the fact that the alloy underwent two phase transitions at high and low temperature, first from austenite (parent phase) to martensite phase, and then from martensite phase to austenite phase.

Shape memory alloy can be hot working and cold working, its hot working forming is good and itself is easy to weld. It also has corrosion resistance, fatigue resistance, high specific strength, wear resistance and many other excellent characteristics.

The properties of shape memory alloys fall into four main categories

One-way memory effects:
(1) the austenite to martensite transformation temperature phase change into martensite, when Ms martensite to austenite transformation temperature As phase change into austenite, the transformation of martensite to austenite in the process of the inverter will restore to the original shape of austenitic phase, the effect is only the shape of the high temperature phase memory.

(2) two-way memory effect: two-way memory effect is due to the martensite phase, exerting force deformation in the alloy produced dislocation and some irreversible plastic deformation, when heated, martensite phase, and the reversible part of the inverter for austenitic phase back to the shape of a high temperature phase, cooling, in under the influence of fault, the formation of martensite, This effect is called the two-way memory effect, which can remember the shape of both high temperature phase and low temperature phase.

(3) Whole memory effect: The whole memory effect is similar to one-way memory when heated, but the restored shape is the same as the original shape when cooled, but the orientation is opposite.

(4) Superelastic effect: for shape memory alloy, both temperature and stress can cause the transformation of the alloy. At a lower temperature, the alloy will undergo deformation after loading. After unloading, the alloy will revert from martensite to austenite, and the original shape will be restored.
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