Crystallization and phase transformations in amorphous NiTi thin films for microelectromechanical systems

Amorphous sputtered nickel-titanium thin films were deposited onto micromachined silicon-nitride membranes and subjected to heating and cooling conditions. Their associated microstructure was monitored directly and simultaneously with in situ transmission electron microscopy. These electron-transparent membranes constrained the NiTi films and rendered it possible for observation of the complete transformation cycle, which includes: the crystallization of the amorphous phase to austenite phase (cubic B2 structure) with heating; and the conversion of austenite (B2) to martensite (monoclinic B19(') structure) with cooling. Electron micrographs show the nucleation and growth of grains occurs at a temperature of 470degreesC and at a rate that indicates a polymorphic transformation. The onset of martensitic transformation occurs between 25 and 35degreesC. Calorimetric measurements are consistent with the observed crystallization. (C) 2004 American Institute of Physics.