Microstructure in Cu-Al-Ni shape memory alloy thin films

CARLOS A. ESPINOZA TORRES; ADRIANA M. CONDÓ; FRANCISCO C. LOVEY; JULIO GUIMPEL

Cusco, Perú

Congreso; IX Interamerican Congress of Electron Microscopy; 2007

The development of thin films of shape memory alloys (SMA) has been motivated by the actual demand for fabricating microactuators. Among many actuator materials, SMA have the greatest capacity of work per unit volume. Because of this reason, a lot of work has been done in the study of Ni-Ti SMA thin films. An alternative to the Ni-Ti system for thin films are the Cu-base SMA which have a wider range of transformation temperature, are cheaper and their properties in bulk alloys were studied in extent. To obtain the same properties in the thin films as in bulk is not obvious because the fabrication methods are different. In this work we present a detailed TEM study of the microstructure of Cu-Al-Ni thin films after annealing. They were grown by dc magnetron sputtering from a Cu-26.9at% Al-5.5at% Ni target on Si (100) at room temperature. The thickness of the films was 5.3 mm and annealing was performed at 1023 K for 3600s. After quenching in water at room temperature, the shape memory effect was observed to be present. Specimens for TEM were prepared and observed in a Philips CM200UT microscope operated at 200 kV. As a result of the annealing, the grain size has largely increased attaining a medium size of about 300 nm. Mainly, the closed packed structures 2H and 18R of the martensite were identified by the diffraction patterns. The presence of superlattice spots in some diffraction patterns indicates that the martensite has long range order. The presence of both martensitic phases, 2H and 18R, suggests that inhomogeneities may be present in the composition of the films. This is supported by the metastable phase diagram of the martensitic phases. Moreover, these inhomogeneities may also affect the transformation temperatures which are strongly dependent on composition.