Shape memory alloy classification
志联
2025-04-18
Shape memory alloys have the ability to recover their shape after deformation because of the thermoelastic martensitic phase transformation that occurs within the material during deformation. Shape memory alloys have two phases: a high-temperature austenite phase and a low-temperature martensite phase. Depending on the different thermal and load conditions, shape memory alloys exhibit two types of properties.
Shape Memory Effect
One-way memory effect. When a shape memory alloy is deformed at a lower temperature, it can recover its shape before deformation after heating. This shape memory phenomenon that only exists during heating is called the one-way memory effect.
Two-way memory effect. Some alloys recover their high-temperature phase shape when heated and can recover their low-temperature phase shape when cooled, which is called the two-way memory effect.
Full-way memory effect. When heated, it recovers the high-temperature phase shape, and when cooled, it becomes a low-temperature phase shape with the same shape but opposite orientation, which is called the full-way memory effect.
The shape memory effect of SMA originates from the thermoelastic martensitic phase transformation. Once this martensite is formed, it will continue to grow as the temperature decreases. If the temperature rises, 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 transformation. The temperature T0 where the two free energies are equal is called the equilibrium temperature. Martensite phase transformation will only occur when the temperature is below the equilibrium temperature T0, and conversely, the reverse phase transformation will only occur when the temperature is above 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 has a linear relationship with stress.
Shape 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, Ti-Ni, Ti-Ni-Pd, Ti-Nb, U-Nb, and Fe-Mn-Si, etc.
Pseudoelasticity
When a shape memory alloy in the high-temperature austenite phase undergoes a large deformation under external force, the large deformation is completely recovered after the external force is removed. However, the stress-strain curve is not linear during the deformation process, and dissipative energy will be generated.
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