Compositional effects on the crystallization kinetics of nickel titanium thin films
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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.
I. INTRODUCTION
Crystallization studies of amorphous materials have been based on the determination of the transformed fraction as a function of temperature and time because these values are tracked by isothermal transformation theory.1–5 Researchers have been limited by experimental techniques that indirectly infer the extent of a reaction by correlating physical property changes to the amount transformed.6–9 These methods have some limitations, particularly for thin films, where the magnitude of physical changes is significantly reduced.10 The use of in situ transmission electron microscopy (TEM) methods, however, can directly measure and determine the full description of crystallization (i.e., nucleation and growth rates) in addition to the transformed fraction and is not limited by these signal constraints. Such studies provide a unique opportunity to showcase the utility of this exciting and powerful technique and will directly impact the development of engineering materials. In this study, we used in situ TEM techniques to uncover kinetic information on the crystallization of NiTi thin film shape memory alloys, which are potential candidates for actuators in microelectromechanical systems (MEMS) applications.11,12 The crystallization step is particularly of importance for NiTi thin films, since unlike their bulk counterparts, the as-deposited state of NiTi
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0215 1728
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 7, Jul 2005 Downloaded: 07 Apr 2015
sputtered thin films is amorphous. As a result, an annealing step (of 500 °C or higher) is required to create a crystalline (actuating) form.13,14 It is widely known that the martensitic transformation is directly linked to the microstructures that emerge from the crystallization process.15–17 For this reason, understanding the interplay between nucleation and growth processes during the crystallization step is important for they determine the final microstructure. This study utilizes in situ TEM techni
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