Crystallization and Microstructural Control of NiTi Films (Xi Wang)

NiTi thin films sputter-deposited at room temperature are usually amorphous in their as-deposited state. This observation provides an opportunity to control the microstructure by adjusting the crystallization conditions. The crystallization of NiTi has been characterized as polymorphic with continuous nucleation and growth throughout the crystallization process. From classic transformation theory, it is well known that those two quantities are essential to the definition of the final microstructure, in that they determine the grain size of the material. The temperature dependence of the crystallite nucleation and growth rates is measured for amorphous Ni_49.5Ti_50.5 thin films. Using TEM, crystallites are shown to nucleate homogeneously in the film and to grow in a channeling mode. A mechanism that suppresses heterogeneous nucleation is proposed. By manipulating nucleation and growth rates, grains as large as 60 mm can be obtained in submicron films.

Publications:

X. Wang, M. Rein, J. J. Vlassak, "Crystallization kinetics of amorphous equiatomic NiTi thin films: effect of film thickness", J. Appl. Phys. 103, 023501 (2008). (Download)

X. Wang and J. J. Vlassak, "Crystallization kinetics of amorphous NiTi shape memory alloy thin films", Scr. Mater. 54, 925-930 (2005). (Download)

X. Wang, A. Lai, J. J. Vlassak and Y. Bellouard, "Microstructure evolution of on-substrate NiTi shape memory alloy thin films", Mater. Res. Soc. Symp. Proc. 795, 275-280 (2004). (Download)

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Laser Annealing of Amorphous NiTi Films (Xi Wang)

When SMA films are used as actuators in MEMS devices, biasing springs are generally needed to restore the initial state in order to achieve a two-way shape memory effect. As a result, use of SMA actuator in MEMS has been limited mainly to bimorph-like mechanisms. Recently, laser annealing of shape memory alloys (LASMA) emerged as a promising approach for the fabrication of planar mechanisms. This technique has the advantage that shape memory properties can be spatially distributed: material crystallized by laser irradiation has shape memory properties and can be used as an actuator, while untransformed material is passive and provides a restoring force. We conduct an experimental study of the laser annealing process for NiTi thin films as a function of laser power density and scanning speed. A computational model which takes into account the crystallization kinetics of amorphous NiTi is developed to allow us predict the size of the crystallized regions as a function of laser annealing parameters if the reflectivity of the NiTi surface is known.

 

Publications:

X. Wang, Y. Bellouard, Z. Y. Xue, J. J. Vlassak, "Thermal modeling of laser-annealing-induced crystallization of amorphous NiTi thin films", Appl. Phys. A 90, 689-694 (2007). (Download)

X. Wang, Z.Y. Xue, Y. Bellouard and J.J. Vlassak, "Laser annealing of amorphous NiTi shape memory alloy thin films", Shape Memory and Superelastic Technologies (SMST) Proceedings, California (2006). (Download)

X. Wang, Y. Bellouard and J. J. Vlassak, "Laser annealing of amorphous NiTi shape memory alloy thin films to locally induce shape memory properties", Acta Mater. 53(18), 4955-4961 (2005). (Download)

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Size Effects in NiTi Shape Memory Alloys (Xi Wang)

The martensitic transformation behavior of a shape memory alloy depends sensitively on its microstructure and its dimensional constraint. The effect of film thickness on phase transformation of substrate-constrained NiTi films is investigated with wafer curvature technique. The transformation is suppressed when the film thickness approaches 200nm. The martensitic transformation in constrained NiTi particles embedded in an amorphous NiTi matrix is studied with in-situ TEM.