Shape memory alloy properties
Stable phases of shape memory alloy
SMA has two stable phases : These two phases correspond to two different crystal structures: one is cubic and the other is monoclinic.
Martensite phase ( $\alpha$ phase)
The SMA is in its martensite phase at low temperature(when cooled). In this phase, it is weak, and can be easily deformed. It is only stable below a given temperature, $M_s$ the martensite starting temperature, and again is the only stable phase at temperature below the martensite finish temperature $M_f$.
Austenite phase ( $\beta$ phase) or parent phase
When heated at high temperature, the SMA contracts forcefully and goes into its austenite phase and returns to its original shape which was before deformation. It is only stable above a given temperature, $A_s$ known as reverse transformation starting temperature, and again is the only stable phase at temperature above the reverse transformation finishing temperature $A_f$.
This curve presents a wide hysteresis loop. Its width varies with the alloy system. For NiTi alloys, the temperature hysteresis is usually between 30 and 50 degree C.
Figure: Shape memory alloy one way memory effect
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SMA transformation temp.
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