An Investigation on Phase Formations and Microstructures of Ni-rich Ni-Ti Shape Memory Alloy Thin Films
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THE Ni-Ti based alloys are the most important practical shape memory alloys (SMA) with excellent mechanical properties. Ni-Ti-based thin films are gaining technological interest as microelectro-mechanical system-based microactuators because their work output per volume exceeds that of other microactuation mechanisms.[1–4] Other advantages of Ni-Ti-based thin film includes high-power density, large displacement, and an actuation force resulting from the high surface-area-tovolume ratio with an enhanced cooling rate leading to a faster response speed in the micron size actuators.[4,5] Because of the difference in the coefficient of thermal expansion between the substrate and the SMA thin film, Ni-Ti-based films can act as bimorphs that are sensitive to the working temperature.[6] From the past few decades, various physical vapor deposition (PVD) techniques have been employed to fabricate the SMA NiTi thin films for microactuators. The magnetron sputtering technique is the most popular PVD process used to fabricate NiTi films of higher deposition rates with precise composition control desired for a choice of application.[2,3] In addition to controlling the working temperature, composition adjustment becomes a prime
B. GEETHA PRIYADARSHINI, Doctoral Student, and S. AICH, Assistant Professor, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721 302, India. Contact e-mail: [email protected] M. CHAKRABORTY, Director, is with the School of Mechanical Sciences, Indian Institute of Technology, Bhubaneswar 751 013, Orissa, India. Manuscript submitted February 11, 2010. Article published online October 19, 2010 3284—VOLUME 42A, NOVEMBER 2011
requisite for the fabrication of Ni-Ti thin film microactuator.[4] The shape memory characteristics and transformation behavior of Ni-Ti microactuators are sensitive to metallurgical factors and sputtering variables.[5] Several works were carried out to combat the composition inflexibility either by cosputtering the Ni-Ti alloy target with Ti targets[7–9] or by incorporating the Ti plates that required several adjustments for size, shape, geometry, number, and position of the plates.[10–13] Additional works were developed that employed sputtering from elemental targets (Ni and Ti) by adjusting the individual target power.[14–16] This method of sputtering was remarkably flexible and simple to control the composition of the Ni-Ti thin film. The transformation and shape memory characteristics of Ni-Ti thin films were shown to depend strongly on alloy composition,[17,18] annealing temperature,[19,20] aging temperature and time,[17,21] Ar pressure, sputtering power, substrate temperature, etc.[22] Extreme fine grain sizes of amorphous Ni-Ti alloys are obtained by processing in the form of thin film with superior mechanical properties by sputter deposition. Crystallization of the films by heat treatment often leads to the formation of reaction products at the interface that had an adverse effect on the shape memory behavior of the films in
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