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Fig. 101 Ms beginning martensite - Mf end martensite. As beginning austenite Af end austenite Transition from martensite to
austenite grid [186]
When the alloy is formed, it consists of a 100% austenite grid until it is cooled down and becomes
100% martensite once the alloy falls below the Mf temperature. During this cooling process, the shape
of the material has not changed. When arrived in the martensite phase, the material can easily be
transformed, thanks to the variability of the parameters a, b, c and γ. When the material is released
after this transformation, it will not return to its original shape because the temperature is below Mf.
However, if one heats up the material, it will regain its original shape once the temperature As is
attained. This recovery cycle comes to an end once Af is reached.
If the memory alloy is faced with counteraction during this cycle, it can generate great forces. This
phenomenon can be used to make actuators.
Fig. 102 Hysteresis curve by displaying length versus temperature
Above the transition temperature, in the austensitic stage, the SMA shows a superelastic behaviour.
The elasticity can be 20 times the elasticity of conventional materials. The impressive amount of
‘elastic’ strain observed in SMAs forces the material to spring back immediately to its original shape if
an applied stress is removed [187].
A big variety of metal alloys present the properties of shape memory alloys. However, there is only a
limited number commercially important. The most frequent shape memory alloy is a NiTi-alloy, known
110