Compositional effects on the crystallization kinetics of nickel titanium thin films

The crystallization and phase transformations of amorphous NiTi thin films were studied using in situ transmission electron microscopy (TEM). These films were sputter-deposited onto micromachined silicon-nitride membranes and subjected to heating and cooling conditions. The microstructural evolution was monitored and recorded. Kinetic parameters such as the nucleation rate, growth rate, and area-fraction transformed were independently determined by noting the number of grains per frame and their change in size. Using the Johnson-Mehl-Avrami-Kolmogorov analysis, fitted kinetic parameters were determined and found to be consistent with TEM observations. To explore the compositional sensitivity of crystallization, samples near-equiatomic and slightly Ti-rich were studied with these methods. TEM micrographs show that equiatomic films exhibit polymorphic crystallization while samples that are slightly off-stoichiometry showed more complicated behavior.