Thermomechanical properties and the effect of heat treatment on the phase transformation of a semi-finished Ni$_{49.9}$Ti$_{50.1}$ alloy microwires with shape memory effect for elastocaloric applications

Recently shape memory microwires have attracted the attention as a promising solid-state functional material for elastocaloric heat pumps and refrigerators. In this work we study the martensitic transition in NiTi microwires versus degree of annealing. The optimal combination of the amorphous and nano crystalline phases in the structure of the shape memory microwires provides the maximal strength and reliability together with pronounced elastocaloric properties, which is crucial for a long haul elastocaloric heat pump application. The 100 $\mu$ thick microwire was subjected to long-term tests and have demonstrated a reversible deformation 2–3 % under load up to 675 MPa. The wire retained its mechanical stability up to 900 MPa.