Shape memory effect in thin films of a Cu–Al–Ni alloy
Introduction
The capability of shape memory alloys (SMAs) to generate large force and displacement, even for a small sample volume, makes them suitable for their use as micro- and even nano-scale actuators. An increasing number of studies have been done on the shape memory behavior of TiNi-based thin films, towards applications for micro-actuators [1], [2], [3], [4], [5], [6], [7], [8]. Several aspects have been considered, such as fabrication procedures [1], [2], [4], [6], [7], characterization of the microstructure and shape memory properties [1], [3], [4], [5], [7] or modification by irradiation [8].
In addition, a few works were devoted to others shape memory alloys. Thus, for example, thin films of ferromagnetic SMAs like Fe–Pd [9] or Ni–Mn–Ga [10], [11] were investigated. Thin film fabrication from a Cu–Al–Ni SMA has also been subject of study [12]. Compared to NiTi, Cu-based alloys offer the advantage that a narrower hysteresis in the transformation–retransformation process is observed [13]. The Cu–Al–Ni thin films in Ref. [12] were produced by physical vapor deposition (magnetron-sputtering) on glass substrate and the optimal sputter parameters were obtained. The structure and microstructure of the as grown films were not investigated. Instead, the authors found that an annealing at 1023 K for 60 min was necessary in order to obtain the martensitic phase and the shape memory effect.
In this work Cu–Al–Ni thin films grown by dc magnetron-sputtering are investigated. Particular attention is paid to the study of the microstructure of the as grown films.
Section snippets
Experimental
The target was prepared from a high purity alloy of Cu–26.9 at.% Al–5.5 at.% Ni melted in an induction furnace in Ar atmosphere. The martensitic transformation temperature was MS = 281 K, the austenite start temperature, AS, was about 14 K higher. The films were grown on Si(1 0 0) by dc magnetron sputtering from the target. The sputtering gas was Ar at a total pressure of 0.92 Pa. Deposition rate was calibrated by surface profile measurements, giving a rate of 50 nm/min. Films with a thickness of about 3
Characteristics of the as grown film
Similar results were obtained for the films grown on both substrates: glass or silicon single crystal. A TEM image of the microstructure and the corresponding electron diffraction pattern are shown in Fig. 1, Fig. 2, respectively. In Fig. 1, small grains with the size of a few nanometers can be observed. In Fig. 2, a thin section of the rings of the electron diffraction pattern was reproduced several times to have straight intensity lines. This diffraction pattern suggests that more than one
Discussion and conclusions
Nanometric regions with disordered BCC and 2H structures were observed in the as grown film. According to Ref. [14] the hexagonal structure is the most stable among the disordered close packed structures in Cu-based alloys for an average number of conduction electrons per atom higher than 1.44. On the other hand the BCC phase is only stable at high temperatures. The electron concentration of the present alloy is about 1.5 for the nominal composition. Thus, if no composition change is permitted,
Acknowledgements
This work has been supported by Agencia Nacional de Promoción Científica y Tecnológica (PICT 2003 Red 301) and Consejo Nacional de Investigaciones Científicas y Técnicas (PIP 5657), Argentina.
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