Extremely stable stress-induced martensitic transformation at the nanoscale during superelastic cycling of Ni51Mn28Ga21 shape memory alloy

Superelasticity from the stress-induced martensitic transformation in shape memory alloys (SMA) and its study at the nanoscale has a fundamental and technological relevance. However, strong size effects at the micro and nanoscale have been reported in several shape memory alloys, showing, in some cases, higher critical stress and damping, in comparison with the same alloys at the macroscopic scale. In the present work, the stress-induced martensitic transformation in Ni51Mn28Ga21 (at%) SMA is studied, at zero magnetic field, running superelastic nano-compression tests on micropillars of 1 & mu;m in diameter, milled by focused ion beam on [001] oriented single crystals. Here, we demonstrate that such micropillars exhibit a fully reversible stress-induced martensitic transformation during a nano-compression cycle, with an extremely stable hysteretic superelastic behavior. In addition, long-term superelastic nano-compression cycling, above five thousand tests, was performed on these micropillars, revealing outstanding stability of the superelastic behavior at the nanoscale. Finally, these Ni51Mn28Ga21 micropillars showed ultra-high strength, up to 1.4 GPa, while exhibiting a good reproducible superelastic behavior, and evidencing a noticeable size effect on the stress-induced martensite strength at the micro/nanoscale. This size-effect should mitigate the degradation during martensite nucleation and growing, being at the origin of the good superelastic cycling behavior. The present results open the door to further studies and new potential applications of this family of magnetic SMA in small-scale devices.