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Video-captured TEM bright-field images taken during in situ heating show crystallization in amorphous nickel-titanium films at . The first nuclei formed after at which the clock was started. Each image was taken after the onset of nucleation at (a) ; (b) ; (c) ; (d) ; (e) ; (f) ; (g) ; (h) ; and (i) . The illustration in (d) describes how the kinetic details such as the number of nuclei, the crystal size change, and transformed fraction were estimated.
(a) The nucleation rate, , as a function of time, exhibits a sigmoidal (or S-shaped) behavior and reaches its steady-state within (within approximately 15% of the total crystallization time of ). (b) A plot of crystal size (which is proportional to the growth rate) taken at different times exhibits a linear behavior indicating the growth rate is constant over time. The dashed line is a guide for the eye.
JMAK plots for the crystallization of amorphous NiTi thin films. (a) A typical isothermal transformation curve based on the Avrami model taken by measuring the amount of area transformed as a function of time. (b) A plot of versus , which has a slope equal to the Avrami constant, . The diamonds correspond to the JMAK data converted from the plot in (a); the solid line is based on the nucleation and growth rates obtained from in situ TEM observation.
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