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Classification of shape memory alloys

wallpapers News 2020-05-07
The reason why the shape memory alloy has the ability to recover from deformation is due to the thermoelastic martensite transformation that occurs inside the material during the deformation process. There are two phases in the shape memory alloy: high temperature phase austenite phase and low temperature phase martensite phase. According to different thermal load conditions, shape memory alloys exhibit two properties.

Shape memory effect:

One-way memory effect. The shape memory alloy is deformed at a lower temperature, and after heating it can restore the shape before deformation. This shape memory phenomenon that exists only during the heating process is called the one-way memory effect.

Two-way memory effect. Some alloys restore the shape of the high-temperature phase when heated, and can restore the shape of the low-temperature phase when cooled, which is called the double pass memory effect.
Full memory effect. The shape of the high-temperature phase is restored when heated, and it becomes the shape of the low-temperature phase with the same shape and the opposite orientation when cooled, which is called the global memory effect.
The shape memory effect of SMA originates from the transformation of thermoelastic martensite. Once this martensite is formed, it will continue to grow as the temperature decreases, and if the temperature increases, it will decrease and disappear in a completely opposite process. The difference between the two free energies serves as the driving force for the phase change. The temperature T0 at which the two free energies are equal is called the equilibrium temperature. The martensitic transformation will occur only when the temperature is lower than the equilibrium temperature T0. Conversely, the reverse transformation will only occur when the temperature is higher than the equilibrium temperature T0.

In SMA, martensitic transformation is not only caused by temperature, but also by stress. This stress-induced martensitic transformation is called stress-induced martensitic transformation, and the transformation temperature is linear with stress.

Memory alloy systems discovered so far: Au-Cd, Ag-Cd, Cu-Zn, Cu-Zn-Al, Cu-Zn-Sn, Cu-Zn-Si, Cu-Sn, Cu-Zn-Ga, In- Ti, Au-Cu-Zn, NiAl, Fe-Pt, NiTi, Ti-Ni-Pd, Ti-Nb, U-Nb, Fe-Mn-Si, etc.

Nitinol is a shape memory alloy. Shape memory alloy is a special alloy that can automatically restore its plastic deformation to its original shape at a certain temperature and has good plasticity.

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Tag: shape memory   nitinol