Shape Memory Thin Films of the System Ti- (Ni-Pd-Cu)
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E. QUANDT, H. HOLLECK Forschungszentrum Karlsruhe GmbH, Institut for Materialforschung I, D-76021 Karlsruhe, Germany
ABSTRACT Free-standing shape memory thin films of the system Ti-Ni-Pd-Cu exhibiting the twoway-effect have been fabricated by d.c. magnetron sputtering onto unheated substrates followed by annealing and training processes. Their transformation temperatures were investigated by differential scanning calorimetry and electrical resistivity measurements. By the Ni-Pd substitution the transformation temperatures (austenite/martinsite finish temperature; A/Mf) could be varied between 32°C/-380 C for the binary TiNi films to 570°C/498°C for the binary TiPd films. By the Ni-Cu substitution the transformation hysteresis (A^/Mf) could be reduced from 70'C for the binary film to 20'C for films with 10 at % Cu. A similar behavior was observed for TiNiPdCu films. INTRODUCTION Shape memory thin films present a promising approach to realize new actuators that are in particular important for the advanced development of improved microsystems. As the traditional concept of down-scaling of successful macroscopic actuators is limited in size-reduction as their physical properties do not scale linearly with the dimension, smart materials like shape memory, piezoelectric, and magnetostrictive materials, which directly transduce electrical into mechanical energy, are an attractive alternative. Realizing actuators based on these smart materials, approaches using thin film techniques [1] are of special importance due to their high compatibility to microsystem process technology. Comparing shape memory with piezoelectric and magnetostrictive thin films, shape memory alloys show the highest work output resulting in high output forces combined with large motions, but due to their thermal actuation their applications are limited to the low frequency range. Using the binary TiNi films a further limitation of the maximum ambient temperature is given by the low maximum transformation temperature lower 60°C/140°C (MfAf, respectively) [2] being obtained for Ti-rich films. Therefore the development of new TiNi-based shape memory films is concentrated on an increase of the transformation temperatures and a decrease of the hysteresis as both features effect the cooling time which is responsible for the maximum frequency of these actuators. Within this work the influence of the Ni/Pd- and Ni/Cu-substitution on the transformation temperatures and their hysteresis was investigated. EXPERIMENTAL The shape memory thin films were d.c. magnetron sputtered (target diameter: 75 mm, sputtering power: 300..500 W, Ar sputtering pressure: 0,4 Pa) onto unheated SiO 2 substrates using hot pressed TiNiPdCu targets with a Ti content of 54 or 55 at % while the Ni and Pd contents were varied between 0 and 46 at % and the Cu content between 0 and 25 at %. The 465
Mat. Res. Soc. Symp. Proc. Vol. 459 ©1997 Materials Research Society
amorphous films were peeled-off the substrates, then crystallized at 7501C for I h and trained for the two-way-effect at 45
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