Charles: It is a change in shape when the alloy undergoes a temperature change; they are called "SMA's" or Shape Memory Alloys for that. A very concise and short explanation can be realized if you understand that the alloy change from say, austenite to martensite and viceversa. Both are the same metal, but its microcristalline structure is different. Most known and common SMA's are based on Nickel-Titanium alloys or "Nitinols". Visualize a change from Body-Centered Cubic cristal structure to Face-Centered Cubic structure, and this is the reason for the change in geometry. For more info look at "SMA" and "Cristal Structures" at Wikipedia for a more complete explanation.
Several years ago I became very interested in Shape Memory Alloys, because I have a young relative that was born without his right arm and hand, so that I thought to mimic muscle action in a very direct way by using some SMA artificial "muscles". But the still high cost of these alloys, and the very high electrical current consumption to heat them quickly, made me aware of the practical difficulties and limitations. Later on, I read a report about a systems conference where an engineer from an aerospace firm revealed that an F-16 Fighter jet needed several times the energy to actuate the control surfaces based on SMA actuators than with the conventional ones...
Used on short distances, SMA actuators can develop quite a force, but most uses tend to employ them as a pilot to control another larger conventional actuator, like a small pilot valve controlling a larger pneumatic or hydraulic valve. Amclaussen.
Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.