We bend this inconspicuous-looking wire. Next, we heat it with a hair-drier or over a candle flame. As you can see, the wire straightens. It definitely is not a scene from 'Terminator'.
This grey wire is a special alloy of nickel and titanium, discovered in the USA in the 60's and kept a military secret for long. It is used in noiseless yawing of steering parts of wings in Stealth planes. The alloy adopts the shape it has at the temperature of 500oC. Then it can be bent into any shape in room temperature. When we bring the alloy to 100oC, the alloy 'remembers' the shape it adopted at the high temperature and comes back to its original form.
Or maybe this is a new method of straightening wires? Try this with a copper or steel wire. This time it will not work so easy.
Nitinol is an alloy made of 55% nickel and 45% titanium. During heating it shows changes in the crystalline phase, between the low temperature form (the martensite, similar to long needles, which forms also in the steel during the temper process, i.e. fast cooling from red-bright temperature) and the high-temperature form (austenite - like spherical beads). Such changes are called phase transformations of the second kind (the first kind are transformations like freezing, sublimation or evaporation). In nitinol the phase transformation between martensite and austenite takes place at 70-130oC (the exact temperature depends on the exact composition). In the martensite phase nitinol is very plastic so it can be deformed easily. When transformed back to austenite, the latter phase "remembers" the configuration in which it was originally formed, i.e. at temperatures above 500oC (the annealing phase).
This TEM picture shows titanium and nickel alloy structure. There are two phases visible: martensite (long needles) and austenite (pathes). (www.fz-juelich.de/iwv/iwv1/datapool/page/9/fgl2.jpg, Forschungszentrum Jülich).
