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Thermomechanical response of Cu-based shape memory alloys suitable for micro-electro-mechanical systems (MEMS) applications: interplay between grain size and sample size effects

Final Report Summary - SMAMEMS (Thermomechanical response of Cu-based shape memory alloys suitable for micro-electro-mechanical systems (MEMS) applications: interplay between grain size and sample size effects)

The main goal of this project was to study the thermomechanical effects of superelasticity and shape memory on Cu-based alloys at the scales relevant to MEMS and NEMS applications. The objectives of the project were divided into two main groups: i) Fabrication of Cu-based shape memory alloy (SMA) thin films and ii) Fabrication of micro- and nano-sized Cu-based SMA structures. To achieve the first goal, shape memory thin films were produced by sputtering Al and co-sputtering Cu-Ni multilayers on Si substrates. After deposition, the thin films were peeled off from the substrate. The multilayer thin films were annealed at 900ºC (within the beta phase range) for 40 minutes under Ar atmosphere and quenched into iced water to obtain shape memory or superelastic thin films depending on the desired nominal composition. Three families of shape memory alloys were produced: at room temperature, two of them were in the martensitic state; as a result, they exhibit shape memory effect. The difference in their Al content (11-13 wt.%) was reflected in the distinct transformation temperatures. The other alloy, with larger Al content (14 wt.%), was austenitic at room temperature, so superelasticity was observed under the application of stresses. Nanoindentation tests with a blunt-spherical punch at low applied loads (10 mN) revealed that shape memory effect and superelasticity was not precluded at the microscale. So far, a systematic study on the mechanical response of shape memory thin films had only been carried out in NiTi systems.
To achieve the second goal of the project, i.e. fabrication of micro- and nano-sized Cu-based SMA structures, two different approaches were followed. In the first one, carried out during the outgoing period, pillars between 200 nm and 5 μm in diameter were machined in targeted grain orientations with a Focused Ion Beam (FIB) milling system. In this work, two critical issues were analysed: i) the orientation-dependence on shape memory and superelasticity behavior and ii) the minimum size required to trigger the transformation as well as the corresponding stress to induce such transformation. As in bulk Cu-Al-Ni and Cu-Zn-Al single crystals, high elastic anisotropy and transformation anisotropy was observed. In micropillars oriented close to the (001) pole, good agreement between the experimental and calculated transformation stresses and Young’s modulus was observed; however, experimentally observed transformation strains were always lower than the calculated ones. In orientations closer to (111) and (101) poles, in addition to the lower transformation strains, lower Young’s modulus and considerably larger transformation stresses were observed when compared to the theoretically calculated ones. These results were supported by a test on a bulk specimen of the sample material from which the pillars were machined. Concerning size effects (i.e. the minimum size required to trigger the transformation and the corresponding stress to induce such transformation), the experimental results reveal a slight increase in the critical stress to induce the transformation and a clear increase in the transformation strain with a decrease in the pillar size. An effect of size on the shape and hysteresis of the stress-strain superelastic curve was also noticed: higher damping was observed for pillars smaller than 1 mm in diameter while much narrower hysteresis was observed at larger diameters. The size effects observed were attributed to several factors (i.e. machining and testing procedure, pillar bending, dislocation activity, martensite type, etc). Nevertheless, it is interesting and technologically relevant that superelasticity is achieved in these alloys at the nanoscale.
In the second approach, carried out during the returning period, micro- and nano-sized Cu-Al-Ni SMA structures were fabricated by e-beam lithography and subsequent sputtering. To obtain these microstructures, sapphire substrates spun with polymethylmethacrylate (PMMA) were used. These PMMA films were patterned (using electron beam lithography technique) to produce holes with diameters ranging from 500 nm to 20 μm. After patterning the PMMA films, Cu-Al-Ni multilayers were sputtered into the template structures using the same sputtering procedure and conditions used to produce the thin films. Subsequently, the PMMA was removed and isolated micro-/nano-structres were obtained. To get beta phase the micro-/nano structures were annealed at 900ºC for 30 min and quenched into iced water. The mechanical properties of these structures are still under study but preliminary results confirmed the potential use of these materials in micro-nano/systems. It is worth to mention that this is a novel approach, free from the gallium contamination that typically accompanies FIB, to machine isolated Cu-based SMA nano-/micro-structures where a large amount of micro-nano/motives can be produced at the same time. The project will provide the European community with a new procedure to produce low-cost Cu-based SMA relevant to MEMS/NEMS applications. Up to now, numerous industrial sectors, from automotive, aerospace and telecommunications to emerging biomedical technologies, have already found usage of MEMS/NEMS as sensors and actuators.