Crystallization and martensitic transformation behavior of NiTi shape memory alloy thin films
By: a thesis presented by Xi Wang
Published: 2007
Place of publication: 2007
Language: eng
Number of pages: 113
Description: The microstructure evolution and shape memory properties of near-equiatomic Ni-Ti thin films were investigated. Ni-Ti 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 temperature dependence of the crystallite nucleation and growth rates is measured for amorphous Ni-Ti thin films sandwiched between two SiN x layers. Crystallites are shown to nucleate homogeneously in the film and to grow with an interface-controlled mechanism. The reaction between Ni-Ti and surrounding layers results in a small composition shift at these interfaces and suppresses heterogeneous nucleation at these interfaces. The crystal growth rate shows a film thickness dependence and is much slower in thinner films. We propose that hydrogen present in surrounding SiN x layers is responsible for this decrease of the crystal growth velocity. By manipulating nucleation and growth rates, unprecedented control over the microstructure of the films is possible. Martensitic transformation behavior of Ni-Ti thin films of submicron thicknesses was investigated using the substrate-curvature technique. The appropriate annealing condition was chosen such that the grain size is much larger than the film thickness. Consequently, the effect of film thickness is independent of the grain size. The transformation temperature starts to decrease when the film thickness is below 400 nm. This decrease is associated with an increasing energy barrier to transformation in thinner films. A crystallization study in which amorphous films are annealed by a scanning laser was performed experimentally and numerically.

The nucleation and growth mechanisms in the laser annealing process were found to be the same as for furnace annealing. Uniform microstructure and shape memory properties were locally introduced in the films by the laser. A 3-D thermal model was developed to simulate the crystallization behavior of the laser annealing process of amorphous Ni-Ti thin films. The crystallization kinetics parameters determined in the furnace annealing study were included in the model to allow us predict the size of the crystallized region as a function of laser annealing parameters.
Notes: Thesis (Ph.D., School of Engineering and Appled Sciences)--Harvard University, 2007.