The Influence of Irradiation Parameters on the Behavior of Martensitic Titanium Nickel Thin Films
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The Influence of Irradiation Parameters on the Behavior of Martensitic Titanium Nickel Thin Films Thomas B. LaGrange1, David S. Grummon2 and Rolf Gotthardt1 1 Swiss Federal Technical Institute of Lausanne (EPFL), Institute of Physics of Complex Matter, Ecublens, CH-1015, Switzerland 2 Michigan State University, Department of Chemical Engineering and Materials Science, E. Lansing, MI 48824, U.S.A. ABSTRACT
In view of a planar processing technique to develop a micro-actuator, irradiation experiments on plastically deformed martensitic TiNi thin films were performed using 5 MeV Ni++, with various fluences and temperatures, to study the dependence of microstructure and transformation characteristics on irradiation parameters. TEM observation and X-ray diffraction spectra show that irradiation resulted in a sharply defined continuous amorphous matrix, extending to the projected ion range, that contained both monoclinic and body centered cubic nanocrystals. The irradiation mechanisms and films’ temperature-displacement are discussed in terms of the observed microstructure. INTRODUCTION
Ion irradiation has been used as a surface-modification technique to make a novel thin film SMA actuator that exhibits reversible out-of-plane bending. The actuator design approach has been to irradiate one side of a 6 µm pre-deformed martensitic TiNi SMA thin film with 5 MeV Ni2+ ions, whose projected range is on order of 1/3 the film’s thickness. This design is attractive as micro-electro-mechanical systems (MEMS) device, since it uses a simple planar processing technique and an active mechanism that is contained within a single element. In principle, the martensitic transformation in TiNi alloys can be suppressed in an ion beammodified layer with sufficient high irradiation dose. When this modified film is heated and undergoes a reserve martensitic transformation to the austenite phase, only the un-modified martensitic layer contracts to recover the pre-deformation. This creates a sharp differential strain between damaged and un-modified layer that causes the film to curl away from the irradiated surface. Upon cooling, the stored elastic energy in the damage layer from the prior transformation, acting as an elastic spring, is then available to re-deform the martensite in the unmodified layer. Thereby in theory, a reversible two-way actuation is developed. This prediction is made on the assumption that the beam-damaged layer exhibits no athermal transformational behavior and cannot be plastically deformed. However, ion irradiation is a complex processing technique, and it must be studied in detail in order to determine the proper irradiation parameters required to obtain the desired modification. Only a few studies have been conducted about the effects of ion irradiation on the modification of the TiNi microstructure. Grummon and Gotthardt observed exotic and unpredictable motions of 5 MeV Ni ion irradiated TiNi films and noted that complexities in the motion were related to the irradiation dose [1]. LaGrange et al. also observ
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