X. Wang, Harvard University, Boston, MA; Y. Bellouard, Technische Universiteit Eindhoven, Eindhoven, Netherlands; Z. Xue, J. J. Vlassak, Harvard University, Cambridge, MA
We present the results of a crystallization study on NiTi shape memory thin films in which amorphous films are annealed by a scanning laser. Laser annealing makes it possible to spatially distributed shape memory properties in thin-film devices, e.g., amorphous or passive elements can be created in parallel with crystalline or active elements to effect in-plane actuation. A kinetics study shows that nucleation of the crystalline phase occurs homogenously in the films. Consequently, the laser annealing process produces polycrystalline films with a random crystallographic texture. The crystallized films have a uniform microstructure across the annealed areas. The material in the crystalline regions transforms reversibly to martensite on cooling from elevated temperature and stress measurements show that a significant recovery stress is achieved in the films upon transformation. Temperature profiles induced in the film by the scanning laser were calculated using a full three-dimensional finite element model. The model takes into account oxidation of the NiTi surface and allows prediction of the size of the crystallized regions as a function of laser annealing parameters.
Summary: The presentation entitled "Laser Annealing of Amorphous Ni-Ti Shape Memory Alloy Thin Films" reports on a novel study we recently concluded on the crystallization of amorphous Ni-Ti thin films using a laser annealing process. We have demonstrated that using laser annealing it is possible to locally crystallize the amorphous films and that crystalline regions indeed show the thermoelastic martensitic transformation responsible for the shape memory effect. We relate the microstructure and crystallographic texture of the annealed films to the processing parameters and to the nucleation mechanism of the crystalline phase. Furthermore, the FEM simulation of temperature profile by scanning laser allows us to predict the size of the crystallization region.
