Extremely high recoverable strain at the nanoscale in Cu-Al-Be shape memory alloys during in-situ nano-tensile superelastic tests

Shape memory alloys (SMAs) exhibit superelasticity due to stress-induced martensitic transformation (MT), in which significant strain, up to about 10%, is recovered upon unloading. Nowadays, the search for materials with large-recoverable superelastic strains is ongoing because of their potential applications in multidisciplinary cutting-edge fields such as mechanical metamaterials, architected nanostructures, and flexible and wearable electronics. Then, it is worthy of study whether size effects on stress-induced MT could enable higher superelastic strains. Such study is approached through nano-tensile tests using a pico-indenter inside the scanning electron microscope, and specially designed small samples milled by the focused ion beam technique. Here, we demonstrate that micro/nano features of Cu-Al-Be alloy exhibit a superelastic behaviour with an extremely high recoverable strain of up to 27%, which largely exceeds present micro/nano scale levels. Samples with less than 0.2-μm 2 cross-section exhibit a reproducible two-stage MT during in situ nanotensile tests, associated with two successive stress-induced MT, β 1 → β 1 ’ → α 1 ’, which persists beyond one hundred superelastic cycles. The occurrence of size effects renders the samples resistant to stresses as high as 1.3 GPa during superelastic cycling, and their astonishing reproducibility makes them reliable for practical applications. In addition, this two-stage superelastic behaviour offers a remarkably high work-output density of 64 MJ/m 3 for actuation and offers an ultra-high mechanical damping of η =0.20. The discovery of these two-stage MT at the micro- and nanoscales, along with their reported robustness, enables the design of unanticipated flexible and stretchable microsystems and nanodevices.