ADVANCED MATERIALS & PROCESSES | OCTOBER 2024 44 actual compositions due to the fabrication challenges for these alloys[17]. The following text provides additional context to these findings. Composition. Understanding the impact of composition on actuation fatigue is complex, as variations can significantly alter transformation characteristics. Studies have shown that lower Ni content leads to longer actuation fatigue life but at the cost of functional stability. However, there is a threshold beyond which increased Ni content reduces actuation lifetime without further improving functional properties[18]. Impurities. Impurities, particularly carbides and oxides, act as stress risers, promoting crack nucleation. In both NiTi and NiTiHf SMAs, higher impurity levels correlate with decreased mechanical and actuation fatigue life[18-21]. However, NiTiHf HTSMAs seem to be even more sensitive to impurities than NiTi alloys due to more pronounced intrinsic brittleness as compared to NiTi SMAs. H-phase Precipitation. Precipitation hardening in NiTiHf through H-phase precipitates enhances transformation reversibility and stability. Materials with homo- geneously distributed precipitates exhibit more stable responses than those with heterogeneous distributions. The optimal precipitate size for NiTiHf HTSMAs is ~20 nm[22]. Applied Load. The effect of various loads on the actuation fatigue performance in NiTiHf, NiTiZr, NiTiCu, and NiTi SMAs has been well studied[23-27]. Higher applied loads increase actuation strain but also accelerate irrecoverable strain accumulation, leading to reduced actuation lifetime and poorer functional fatigue. Upper Cycle Temperature (UCT). Increasing UCT, like higher applied loads, results in greater actuation strain and irrecoverable strain, reducing lifetime. However, higher UCT seems to reduce actuation degradation, unlike what is observed by increased loads. Additionally, increased UCT produces a greater actuation strain magnitude than can be obtained by increasing the applied load[28]. Degree of Transformation. The degree of transformation plays a crucial role in the actuation fatigue performance of NiTiHf HTSMAs, particularly in relation to UCT. By lowering the UCT below the austenite finish temperature (Af) in isobaric thermal cycling experiments, the fraction of transforming material can be limited, leading to partial transformation. This partial transformation can also be achieved by maintaining the lower cycle temperature (LCT) above the martensite finish temperature (Mf), or by adjusting both the LCT and UCT to fall between Mf and Af. Extensive partial cycle actuation fatigue experiments have been conducted on various NiTiHf HTSMA compositions under different cycling and loading conditions. One of the key findings is that when actuation cycling is limited by heating (as shown in Fig. 3a), the fatigue lifetime can be increased by an order of magnitude while still maintaining high actuation work output. This improvement cannot be replicated simply by reducing the applied load to extend fatigue life, as illustrated in Fig. 3b. The substantial enhancement in actuation performance through heating- limited partial cycling has been consistently observed across multiple NiTiHf compositions[29,30]. In addition to these findings, detailed microstructural characterization has been conducted on in-situ, interrupted, and completed thermomechanical fatigue tests to fully understand the failure mechanisms involved. While electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM) analyses are ongoing, preliminary data from digital image correlation (DIC) and x-ray computed Fig. 3 — (a) Schematic showing typical thermal hysteresis curves under an applied load for a SMA material. The dashed lines show a full thermal cycle between an upper cycle temperature (UCT) above Af and lower cycle temperature (LCT) below Mf. The solid lines represent a partially heated sample with a UCT below Af (PUCT) and a LCT below Mf. (b) Summary of actuation fatigue test results for full thermal cycles (diamond markers) under varying loads and partial heating thermal cycles (stars) under 300 MPa (with different levels of εPUCT)[29]. (a) (b) 1 1 FEATURE
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