Two challenges facing high temperature shape memory alloys (HTSMAs) involve extending the actuation fatigue life and maximizing the actuator response. Thus, the focus of this study was to determine alternative processing parameters to form the optimal microstructure for actuation applications involving NiTiHf HTSMA wires. In this study, the effect of rapid thermal annealing (RTA) on the thermo-mechanical properties of Ni-lean Ni49.8Ti40.2Hf10 and Ni-rich Ni50.3Ti29.7Hf20 HTSMA wires was investigated. Both alloys were produced from a large-scale ingot, homogenized, then extruded, and subsequently cold-drawn to wire. In the final pass, the wires were rapid thermal annealed in an Ar environment for short times at various temperatures. The samples were characterized using differential scanning calorimetry, scanning electron microscopy equipped with energy dispersive spectroscopy, transmission electron microscopy, and thermo-mechanical testing. RTA was shown to be a time efficient technique to control grain size and oxidation of small diameter Ni-lean and Ni-rich NiTiHf HTSMA wires. The resulting grain size proved to be critical to controlling the actuation fatigue life and actuation strain response; however, an inverse relationship was found between fatigue life and actuation strain, i.e., longer fatigue life also meant lower actuation strain.
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