Shape memory coil spring

Stretching a shape memory coil spring when it is in a martensitic (i.e. cold) state will de-twin the differently oriented martensite. The result is an almost homogenously oriented martensitic structure. Similar to a common plastic deformation the SM coil spring will stay in the stretched shape when unloaded (Fig. 6.51). 

As an example. Fig. 6.54 shows the principle of a control valve with a shape memory coil spring in the automatic transmission of a limousine [75]. Depending on the oil temperature, the SM spring lowers the pressure of the transmission oil, resulting in a smoother shifting of gears. 

If the shape memory alloy spring is heated and the temperature surpasses the As temperature, the shape memory material starts transforming to austenite and the coil spring returns to the unstretched form. On reaching the Af temperature, the transformation is completed. It is characteristic for the one-way effect that a shape recovery occurs only when the SM element is heated. There is no shape change when the element is cooled. The cold SM element must be deformed by an external force in order to achieve a movement when heated again. 

Figure 8. An example using coil springs to compare the differences between the one-way and two-way shape memories...

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