The Effect of Cold-Working on the Crystallization Temperature of Thin-Film Shape Memory Effect TiNi.
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THE EFFECT OF COLD-WORKING ON THE CRYSTALLIZATION TEMPERATURE OF THIN-FILM SHAPE MEMORY EFFECT TiNi. John S.Madsen and A.Peter Jardine, Dept.of Materials Science and Engineering, S.U.N.Y. at Stony Brook, Stony Brook, NY 11794-2275 Abstract The minimum temperature for the crystallization of amorphous TiNi on substrates is of interest in developing thin-film SME material while minimizing chemical interactions with the substrate. Using 20 micron thick free standing TiNi material annealed in a vacuum furnace, X-Ray diffraction of the thin-films indicated that the crystallization occured within 20 minutes at 5100C, 4900C and 480'C. At 4500C, crystallization kinetics were significantly slower, and the foils were fulling crystallized after annealing for 7.5 hrs. To further lower the crystallization temperature, cold working of the foil by rolling was introduced and full crystallization was observed after 7.5 hours annealing at 4000C in a cold-worked foil. Cold working and annealing at 3000C and 350'C for 7.5 hrs did not observably promote lower crystallization temperatures.
Introduction In the intermetallic alloy NiTi, the low temperature B19 phase (a face-centered tetragonal structure) is easily twinned. The high temperature phase is the body centered cubic phase, B2. NiTi is a well known Shape Memory Effect (SME) alloy, and is of some technical importance not only for its room temperature transformation temperatures, long cyclical lifetime and strong recovery forces on transformation; but also possesses typical intermetallic properties such as hardness, corrosion resistance and high melting temperatures, with concommitant low vapor pressures. There are two competing interactions involved in the successful deposition of SME TiNi 'displaying the SME from sputtering deposition. The first are the vapor pressures of 02, $20, and other reactive gases in the deposition chamber. The extreme reactivity of the Ti, expolited technically in Titanium Sublimation Pumps, necessarily involves minimal base pressures of 10- torr to ensure a successful deposition. As the material is amorphous when initially deposited onto nominally room-temperature substrates, the second criteria for deposition is the annealing temperature for successful crystallization of the material. This second criteria in turn is determined by both kinetic and chemical factors. For example, in Micro-Electro-Mechanical Systems (MEMS), the potentially most useful substrate for the deposition of NiTi is Si(100), not only for its relatively low cost but also for its integrability into MEMS type devices. Thermodynamically, TiNi is unstable at any temperature with respect to Si, with the formation of Ni and Ti silicides being preferred [1]. However, if the kinetics of Si diffusing into NiTi can be made slow by lowering the annealing temperature, then the amorphous NiTi can be successfully crystallized into SME B2 or B19 crystal structures, with minimal adverse chemical interaction with Si. Similarily, so-called "Smart" materials will likely be composites of a variety of m
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