Phase Transformations in Sputtered Ni-Ti Film: Effects of Heat Treatment and Precipitates
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PHASE TRANSFORMATIONS IN SPUTTERED Ni-Ti FILM: EFFECTS OF HEAT TREATMENT AND PRECIPITATES J.D. BUSCH*, MITCHELL H. BERKSON**, AND A.D. JOHNSON* * TiNi Alloy Company, 1144 65th Street, Unit A, Oakland CA 94608 ** Department of Materials Science, University of California at Berkeley, CA 94720 ABSTRACT This research investigated the influence of heat treatment and precipitation on the phase transformation temperatures of sputter-deposited nickel-titanium films. Films 5 to 10 microns thick were subjected to isochronal and isothermal heat treatments in vacuum. Four-point resistance measutements were made and the T , TR, M , and M_ temperatures identified. The correlation between transformation temperatures and the film's microstructure was studied using transmission electron microscopy. Existence of Ti2Ni and Ti 1 Ni 14 precipitates was seen to adversely influence the phase transformation temperatures. It was concluded that precipitate-free film is preferable for mechanical actuator applications. INTRODUCTION Actuation of mechanisms in the size range 10 microns to 1 mm poses operational requirements that are not well-matched by electrostatic, electromagnetic, or piezoelectric technologies. In contrast, shape-memory alloy film, deposited by processes compatible with microelectronic manufacture, offers an opportunity for high density energy conversion using TTL voltages. One of the applications now being developed is a shape-memory alloy microvalve [1,2]. The nickel-titanium shape-memory alloy (SMA)*is a unique material which can be plastically deformed by as much as 8% at one temperature, yet will return to its original shape when heated to a slightly higher temperature. The mechanism behind this phenomenon is a diffusionless phase transformation from the low temperature monoclinic B19' crystal structure to the parent BCC B2 structure, respectively referred to as martensite and austenite. The ideal shape-memory film for mechanical actuation would be arbitrarily thin, reveal no fatigue after millions of cycles, exhibit 0large strain recovery, and have a phase transformation near 100 C. The purpose of this work is to determine the effects of heat treatment on the most sensitive of these shape-memory properties: transformation temperature. Most micro-actuator applications which utilize thin film shape-memory alloy will require transformation temperatures well above room temperature. EXPERIMENTAL PROCEDURE Sputtered Ni-Ti film used in this work was produced by TiNi Alloy Company. Films 5 to 10 microns thick were sputtered on [100] silicon in an inert argon atmosphere by a d.c. magnetron source. The films were deposited in an amorphous state by Mat. Res. Soc. Symp. Proc. Vol. 230. c 1992 Materials Research Society
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