Shape memory alloys are characterised by their ability to return to their original shape after heating to their transformation temperature after having been deformed. This is known as the shape memory effect and is caused by a change in the crystalline structure during the transition from the martensitic phase to the austenitic phase. It gives these materials attractive actuation capabilities.
Shape memory alloys have a high power to weight ratio (up to ten times that of conventional actuation systems) and in the martensitic phase they can withstand large amounts of recoverable strain (up to 8%). When heated to above their transition temperature they can exert high recovery stresses of up to 700MPa which can be used to perform work. On the downside, they are relatively inefficient (less than 10%), having a slow response speed (predominantly dictated by the need for cooling) and are relatively complex to control due to inherent non-linearities and hysteresis in the shape memory effect.
The most common shape memory alloy is Nitinol and alloy of nickel and titanium.