Reproducible giant superelasticity at the nanoscale in Cu–Al–Ni shape memory alloys

Superelasticity in shape memory alloys arises from stress-induced martensitic transformation and is typically limited to recoverable strains of about 10% in most practical systems. Larger strains have been reported in bulk Cu–Al–Ni single crystals through successive stress-induced transformations, but this response is mechanically fragile and poorly reproducible under cyclic loading. Thus, exploring whether size effects on stress-induced martensitic transformation can enable large recoverable strains while improving cyclic reliability remains highly relevant for micro- and nanoscale functional systems. Here, we demonstrate that [001]-oriented Cu–Al–Ni single-crystal dog-bone specimens, prepared by focused ion beam milling and tested by in situ SEM nanotensile experiments using push-to-pull devices, exhibit reproducible giant superelasticity at the nanoscale. The specimens undergo a two-stage stress-induced martensitic transformation, β 3 → β 3 ′ → α 3 ′, yielding a fully recoverable tensile strain of up to 24%, far exceeding the strain levels usually reported in micro/nanoscale superelastic systems. This response remains reproducible over more than 100 loading–unloading cycles, with the specimens sustaining maximum tensile stresses of approximately 1 GPa and exhibiting large recoverable deformation. This behaviour is consistent with nanoscale size effects that enhance stress tolerance and cyclic reproducibility. In addition, the two-stage superelastic response provides a work-output density of 78 MJ m -3 and a mechanical loss factor of 0.14. These findings may contribute to the development of high-strain flexible and stretchable microsystems, micro/nanoactuators, damping elements, and nanostructurally engineered active mechanical metamaterials.