ADVANCED MATERIALS & PROCESSES | OCTOBER 2024 43 10 grain size and texture analysis notoriously challenging. In addition, the thermomechanical processing of NiTiHf HTSMAs is much more difficult than NiTi; therefore, grain size control is challenging. To date, no published work has addressed the impact of grain size on the thermo- mechanical fatigue of any SMA. Texture. Texture effects have primarily been studied in uniaxial mechanical fatigue and crack growth experiments using cold-rolled NiTi plate[6]. These studies explore the consequences of different directions—rolling direction (RD), transverse direction (TD), and 45° from the rolling direction—on cold-rolled NiTi plates. However, how the effects of texture relate to actuation fatigue remain unexplored. Heating/Cooling Rate. While the impact of heating and cooling rates has been documented for unloaded differential scanning calorimetry (DSC) thermal cycles[7-9], their effect on actuation fatigue performance is unclear. Studies have shown that different heating rates can alter transformation strain and actuator force output in NiTi wires[10]. Although a study on NiTiHf found cooling rate to have no observable effect on recoverable strain values[11], the effect on actuation fatigue is yet to be investigated comprehensively. Stress State. The stress state of a material depends on its form and how it is loaded, such as uniaxial tension/ compression, torsion, or multiaxial tension. Most research on NiTiHf actuation fatigue has focused on uniaxial tension of wire or flat dog bone samples, with some studies on torque tubes[12-16]. However, the influence of different stress states on the same material under thermomechanical fatigue remains unexplored. NiTiHf ACTUATION FATIGUE: PROGRESS TO DATE Several key factors affecting thermomechanical cycling fatigue in NiTiHf alloys have been extensively studied, including composition, impurities, H-phase precipitation, applied load, and upper cycle temperature. Research into the degree of transformation has also been conducted, though ongoing studies are focused on uncovering the micromechanical mechanisms underlying these effects. Figure 2 provides a summary of how composition, H-phase precipitation, and impurities impact actuation fatigue in NiTiHf HTSMAs. This figure is derived from experiments on four batches of Ni-rich NiTiHf materials, each with the same target composition but differing Fig. 2 — Summary of the effects of composition, H-phase precipitation, and impurities on the actuation fatigue performance of NiTiHf HTSMAs. This study was conducted on four different batches of Ni-rich NiTiHf material with the same target composition but differing actual composition[17]. FEATURE
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