Intelligent Metal

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 70-130 ^oC (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 500 ^oC (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).