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Size effects in shape memory alloy thin films

TEM micrograph of a single pancake-like grain in an 0.8 µm amorphous NiTi film. Stress response of an array of parallel active and passive NiTi lines. Mechanical Behavior of Metal Thin Films and Shape Memory Alloy Coatings Joost J. Vlassak, Harvard University, MET DMR-0133559.

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Size effects in shape memory alloy thin films

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  1. TEM micrograph of a single pancake-like grain in an 0.8 µm amorphous NiTi film Stress response of an array of parallel active and passive NiTi lines Mechanical Behavior of Metal Thin Films and Shape Memory Alloy CoatingsJoost J. Vlassak, Harvard University, MET DMR-0133559 Size effects in shape memory alloy thin films Motivation - Better understanding of the shape memory effect in very thin films and coatings. - Enable efficient use of these materials as actuators in MEMS. • Results • We have analyzed the crystallization kinetics of the of amorphous NiTi thin films. This has made it possible to grow NiTi films with a very wide range of microstructures (see TEM micrographs). • We have used laser annealing to selectively crystallize amorphous NiTi films to locally impart shape memory properties. The figure to the right shows the stress response of an array of parallel active and passive NiTi lines. • We are currently investigating the martensitic transformation in very small crystallites to characterize shape memory size effects. TEM micrograph of NiTi crystal-lites embedded in an amorphous matrix. Crystallites show different transformation behavior depending on size

  2. Mechanical Behavior of Metal Thin Films and Shape Memory Alloy CoatingsJoost Vlassak, Harvard University, MET DMR-0133559 • Education • Two graduate and several undergraduate students contributed to this project: • Graduate students: • Yong Xiang has graduated and has joined Intel as a senior process engineer, Xi Wang is investigating the response of NiTi shape memory coatings and is slated to graduate early next year. • Undergraduate students: • Over the last several years, more than ten undergraduate students have worked on research projects related to this grant. • Most recently, Monica Allen investigated martensitic trans-formations in ternary TiNiZr shape memory alloys using a novel combinatorial approach. Michael Rein is investigating nucleation and growth of crystallites in amorphous NiTi films. • Undergraduates were in part supported by the REU Program at Harvard University (2005-2006). • Work was performed in collaboration with Profs. Bellouard (TU/Eindhoven, the Netherlands), Needleman (Brown), Van der Giessen (University of Groningen, the Netherlands) • Broader Impact • Publications: • This grant has resulted in twenty publications in various journals and conference proceedings. • Graduate students Yong Xiang and Xi Wang have presented their work at several conferences. • Awards • Graduate students have won several poster awards at Gordon Research Conferences and MRS Meetings. • Yong Xiang received a Graduate Student Silver Award at the 2004 MRS fall meeting.

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