Browsing > By author > San Juan Jose M.

Shape memory alloys (SMA) are functional materials exhibiting the shape memory and superelastic effects. These phenomena were also observed at mico/nano scale, particularly in Cu-Al-based SMA [1-3], where ultrahigh mechanical damping was discovered during nano-compression tests in micro and nano pillars. The reproducibility of the superelastic behaviour along thousands of nano-compression cycles was also tested in micro and nano pillars, which undergo a long-term stable behaviour during years [4,5].

In the present work we approach a study of the stress-induced martensitic transformation in a series of micro and nano features, for potential damping applications in MEMS technology. Different kind of micro/nano pillars, specific dog-bone nano-samples and micro-bridges were milled by focused ion beam in a FEI Helios Nanolab 650, on [001] oriented single crystal slides of Cu-Al-Ni SMA. Then, the samples were tested in compression and bending at the Hysitron 950 nanoindenter, as well as in tension during in-situ experiments with the Hysitron Picoindenter PI-85 mounted inside the JEOL 7000F field emission scanning electron microscope, and using a flat diamond indenter of 20 micrometres in diameter. An experimental difficulty was to evaluate the required load to induce the stress-induced transformation avoiding an overload that could scratch the sample in the first test. This is a particularly critical aspect because of the size effect on the critical stress for superelasticity observed in these SMA [6,7], but the predicted model in such works allowed a successful estimation of the stress required for each sample.

In this work a complete study of the superelastic behaviour at the nanoscale in compression and tension, which was approached in several Cu-Al-based shape memory alloys, will be presented. Exceptional superelastic strain above 8%, with a complete recovery when withdrawing the stress, will be reported. These results exhibit a relevant potential interest for its application in Micro-Electro-Mechanical Systems (MEMS). In addition, during superelastic tests, the Cu-Al-based SMA exhibit an exceptionally high mechanical damping [2, 5, 8, 9] that can be used to damp noisy vibrations in MEMS technology. Then, we also report a comparative study of the superelastic damping at the nanoscale in several Cu-Al-based SMAs and in different sollicitation modes, compression, tension and bending.

These results pave the way for designing micro/nano actuators and micro-dampers of Cu-Al-based SMA, which can work in different mechanical modes of compression, bending, and torsion, opening the way for many applications on micro/nano mechanics at the nanoscale.

[1] J. San Juan, M.L. Nó, C.A. Schuh, Advanced Materials 20 (2008) 272-278; [2] J. San Juan, M.L. Nó, C.A. Schuh, Nature Nanotechnology 4 (2009) 415-419; [3] J. San Juan, M.L. Nó, C.A. Schuh, J. Materials Research 26 (2011) 2461-2469; [4] J. San Juan, J.F. Gómez-Cortés, G.A. López, C. Jiao, M.L. Nó, App. Phys. Lett. 104 (2014) 011901; [5] J.F. Gómez-Cortés, et al., Acta Materialia 166 (2019) 346-356; [6] J.F. Gómez-Cortés, et al., Nature Nanotechnology 12 (2017) 790-796; [7] V. Fuster, J.F. Gómez-Cortés, M.L. Nó, J.M. San Juan, Adv. Electron. Mater. 6 (2020) 1900741; [8] J.F. Gómez-Cortés et al, J. Alloys & Compounds 883 (2021) 160865; [9] J.M. San Juan, J.F. Gómez-Cortés, M.L. Nó, J. Alloys & Compounds 929 (2022) 167307.