Ion implantation inducing two-way shape memory effect in Cu-Al-Ni thin films
Introduction
Shape memory alloys (SMA) display the capacity to restore their original dimensional integrity (pre-deformed shape and size) after undergoing substantial deformation when heated to a certain temperature. The effect is related to a structural phase transition (martensitic transformation, MT) from high-temperature austenite to low-temperature martensite [1]. SMA are attractive materials for applications in the micro-electro-mechanical system (MEMS) due to the so-called two-way shape memory effect (TWSME) [2], [3]. The latter refers to a reproducible change into two defined shapes during thermal cycling. It is not intrinsic of SMA but may be induced by thermomechanical training (bulk) and surface treatments (thin films) [2], [3], [4].
Cu–Al–Ni is an SMA which have the capability to be used both as sensors and actuators at temperatures going from tens of Kelvins to 473 K [5]. The MT is affected by the chemical composition [5], the order degree [6] and the microstructure [7], [8]. Cu-Al-Ni display two possible kinds of martensite structures: γ’3 (orthorhombic, 2H) and β’3 (monoclinic, 18R) [9]. During the quenching treatment usually employed to avoid precipitation of equilibrium phases, the metastable disordered phase (β) of Cu-Ni-Al undergoes two successive order–disorder transitions: B2 (Pm3m) and L21 (Fm3m) ordering, respectively. Moreover, the order degree depends on the cooling rate of the quenching treatment and evolves during thermal aging [10]. In addition, irradiation with high energy particles can alter the phase stability [4], [11]. Regarding applications in MEMS, Cu-Al-Ni nanopillars display super-elasticity [12] and thin films exhibit MT [13], [14], [15]. As in bulk, the hysteresis in the MT of thin films is strongly affected by grain boundaries. Thin films with micrometric grain size display hysteresis Δh ≈ 30 K. Moreover, the MT is not influenced by superficial passivation layers [13]. The latter implies that it is possible to add a biocompatible material at the surface. Unlike NiTi [2], [3], [4], the TWSME in Cu-Al-Ni thin films has not been reported.
This letter reports TWSME induced by Al ion implantation in 6 μm thick Cu-Al-Ni thin films with micrometric grain size [13]. The irradiation was performed with 2 MeV Al ions and fluence 6 × 1015 ion.cm−2. The energy was selected to generate a gradient of defects and implantation of Al at a typical distance of ≈ 1.1 μm from the surface. We analyze the presence of TWSME and the influence of the irradiation on the microstructure.
Section snippets
Experimental methods
The films were grown by DC sputtering on Highly Ordered Pyrolytic Graphite HOPG (0001) at 870 K [13]. The deposition parameters were: an atmosphere of 10 mtorr of Ar, an applied power of 50 W and a distance target/substrate of 7 cm. The chemical composition of the target was Cu–27.35 at.% Al–5.45 at.% Ni (MS = 240 K and Δh ≈ 10 K). After growth, the films were cooled down in vacuum by turning off the heater. Under this procedure, the microstructure displays coexistence of austenite L21, and α
Results and discussion
The microstructure of the Cu-Al-Ni films has been reported in ref. [15]. The films display grain size average D ≈ 3.7 μm with an MT to an 18R structure. The Ms depends on chemical composition and order degree. The hysteresis is ≈ 27–30 K. Fig. 1a shows the damage profile created in the film by 2 MeV Al ions. Most of the damage occurs ≈ 1.1 µm below the surface, in a limited range of depth at the end of the ion’s flight path (Bragg peak). The reduction in Ms due to the change in the chemical
Conclusions
In summary, we report Al ion irradiation/implantation inducing TWSME in Cu-Al-Ni thin films with micrometric grain size. The effect is reproducible in several thermal cycling. The radius of curvature in the buckle is ≈ 1 mm. The effect can be related with changes in the degree of order in the austenite modifying the twin structure. The possibility to tune Ms over a wide range of temperatures and TWSME makes Cu-Al-Ni thin films promising for application in microactuators such as microwrappers.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
We thank C. Olivares for technical assistance. M. M. and N. H. are members of the Instituto de Nanociencia y Nanotecnología, CNEA-CONICET (Argentina).
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