Towards High-Temperature Actuators Based on Ni-Ti-Hf-Nb Shape Memory Alloy Thin Wires

The addition of Nb to Ni-Ti-Hf high-temperature shape memory alloys (HTSMAs) has emerged as a promising strategy for developing thin-wire actuators capable of operating in the 100–300 °C range. Recent studies have shown that wires with diameters below 100 µm can be produced by cold drawing, exhibiting excellent shape recovery, high cycling stability, and significant work output. With appropriate exploitation of the unique properties of the Ni-Ti-Hf-Nb system, new actuation concepts may be enabled. Consequently, Ni-Ti-Hf-Nb wires have the potential not only to serve in applications requiring elevated-temperature actuation but also to compete with conventional binary NiTi in existing actuator technologies.

In this work, Ni-Ti-Hf-Nb alloys were produced via vacuum induction melting (VIM), heat treated, subjected to hot and cold deformation, and subsequently shape-annealed under various conditions and trained. Samples were collected at different processing stages and characterized using differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), and mechanical testing. Functional behavior was evaluated by Joule heating under applied stresses up to 500 MPa to assess transformation characteristics and cyclic stability. Finally, novel actuation strategies are proposed to leverage the capabilities of the Ni-Ti-Hf-Nb HTSMA system for industrial implementation.