The Department of Physics of the UPV/EHU-University of the Basque Country researchers have explored superelasticity properties on a nanometric scale based on shearing an alloy’s pillars down to nanometric size. The researchers have found that below one micron in diameter the material behaves differently and requires much higher stress for it to be deformed. This superelastic behavior is opening up new channels in the application of microsystems involving flexible electronics and microsystems that can be implanted into the human body.
By using a piece of equipment known as a Focused Ion Beam, “an ion cannon that acts as a kind of atomic knife that shears the material”, explained San Juan, they built micropillars and nanopillars of this alloy with diameters ranging between 2 µm and 260 nm —a micrometer is one millionth of a meter and a nanometre one thousand-millionth of a meter—. And to them they applied a stress using a sophisticated instrument known as a nanoindenter, which “allows extremely small forces to be applied,” and then they measured their behavior.
The researchers have for the first time confirmed and quantified that in diameters of less than a micrometer there is a considerable change in the properties relating to the critical stress for superelasticity.
The materials could be used for flexible electronics, new wearable electronics and implantable electronics.
Superelastic behavior in microscale pillars.
Shape-memory alloys capable of a superelastic stress-induced phase transformation and a high displacement actuation have promise for applications in micro-electromechanical systems for wearable healthcare and flexible electronic technologies. However, some of the fundamental aspects of their nanoscale behaviour remain unclear, including the question of whether the critical stress for the stress-induced martensitic transformation exhibits a size effect similar to that observed in confined plasticity. Here we provide evidence of a strong size effect on the critical stress that induces such a transformation with a threefold increase in the trigger stress in pillars milled on  L21 single crystals from a Cu–Al–Ni shape-memory alloy from 2 μm to 260 nm in diameter. A power-law size dependence of n = −2 is observed for the nanoscale superelasticity. Our observation is supported by the atomic lattice shearing and an elastic model for homogeneous martensite nucleation.
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