Certain alloys of two or more metallic elements exhibit a “shape memory effect.” These “shape memory alloys” (SMAs) have a crystalline structure that undergoes a transformation upon change in temperature in either direction. The temperature at which the change begins is referred to as the transition temperature, Tt. The most common of these alloys is nickel-titanium (referred to as Nitinol), an alloy that is approximately equal parts of each element. The transition temperature for warmup is about 72 C below which the alloy is in the martensite phase. Heating of the alloy through its transition temperature results in a martensite-to-austenite phase transition and a significant contraction of the alloy. Strains as large as 8% can be realized in this process. Upon cooling below the temperature at which the process reverses (around 38 C) the alloy expands to its original shape under the action of an externally applied load.

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