Mechanical shape memory effect: superelasticity
Shape memory alloys are able to show an obviously elastic deformation behavior which is called mechanical shape memory effect or superelasticity. This deformation can be as high as 20x of the elastic strain of steel. Superelasticity is caused by the stress-induced phase transformation from the high temperature phase austenite into the low temperature phase martensite. The strain related to this phase transformation is fully reversible after removing the stress.
The commercial nitinol alloys show as much as 8% of superelastic strain. Temperature changes are not necessary for the superelasticity.
Thermal shape memory effect
A shape memory alloy is capable of remembering a previously memorized shape. It has to be deformed in its low temperature phase martensite and subsequently heated to the high temperature phase austenite, e.g., in hot water or with an electrical current. The alloy generates a high force during the phase transformation. Thus, it can be used as an actuator in a multitude of different applications. The shape change is not restricted to just pure bending. The most suitable actuation mode has proved to be the linear contraction of a straight wire actuator.
In contradiction to the mechanical shape memory effect, the thermal shape memory effect is related to a heat stimulus, with which the nitinol or nitinol-based alloy is capable of delivering a high amount of work output per material volume.