Giant tensile superelasticity originating from two-step phase transformation in a Ni-Mn-Sn-Fe magnetic microwire.

Large recoverable strain of more than several percent arising from superelasticity in shape memory alloys is important for actuators, sensors, and solid-state refrigeration. Here, we report a Ni_50.0Mn_31.4Sn_9.6Fe_9.0 magnetic microwire showing a giant tensile recoverable strain of about 20.0% along the ⟨001⟩ direction of austenite at 263 K. The recoverable strain represents the largest value reported heretofore in Ni-Mn-based shape memory alloys and is also larger than that of the Ni-Ti wire available for practical applications at present. This giant tensile superelasticity is associated with the stress-induced two-step transformation, and the transformation sequence could be L2₁ (austenite) → 6M (six-layered modulated martensite) → NM (non-modulated martensite), as suggested by the temperature-dependent in-situ synchrotron high-energy X-ray diffraction experiments and the transformation strain calculation based on the crystallographic theory of martensitic transformation. In addition, this Ni_50.0Mn_31.4Sn_9.6Fe_9.0 microwire shows a transformation entropy change ΔS_tr of 22.9 J kg⁻¹ K⁻¹ and has the advantages of easy fabrication and low cost, promising for miniature sensor, actuator, and solid-state refrigeration applications. [ABSTRACT FROM AUTHOR]