Nonhysteretic superelasticity of shape memory alloys at the nanoscale

Shape memory alloys (SMAs) exhibit two closely related and unique properties: shape memory effect (SME) and superelasticity (SE). Hysteresis in martensitic transformations (MT) limits the usefulness of SMAs that require high sensitivity, high durability and high energy efficiency. Recent studies based on atomic simulations and experiments of nanosized SMAs have indicated promising solutions to slim the MT hysteresis that is associated with superelasiticity. It is summarized that SMAs at the nanoscale demonstrate a decreasing hysteretic superelasticity with reduced feature size. In particular, it exhibits nonhysteretic superelasticity below the critical size. Atomic level investigations show that the decreasing hysteresis is due to weaker spontaneous lattice distortion and spatial heterogeneity, leading to a more continuous phase transformation from the parent phase to martensite under external stress. The theoretical studies are also supported by the latest nanosized SMAs experiments. These findings suggest potential methods to achieve slim hysteresis in conventional bulk SMAs.