ADVANCED MATERIALS & PROCESSES | OCTOBER 2024 42 NiTiHf ACTUATION FATIGUE: HOW MUCH DO WE REALLY KNOW? This synopsis of the current state of research on these unique high-temperature shape memory alloys highlights the additional investigation needed to expand their practical application. A. Demblon, J.H. Mabe, and I. Karaman, FASM* Texas A&M University, College Station Shape memory alloys (SMAs) are witnessing a surge in industrial applications, with NiTiHf emerging as a leading candidate for solid-state actuation. This alloy stands out due to its high transformation temperatures, substantial actuation work output, and exceptional functional stability. Its prominence is partly due to it being one of the most extensively studied high- temperature shape memory alloys (HTSMAs). Despite the significant research on NiTiHf, a major challenge remains: the extent of actuation fatigue and understanding the underlying mechanisms of actuation fatigue failure. Actuation fatigue is a combination of structural and functional fatigue that occurs when an SMA undergoes thermomechanical cycling (also known as actuation cycling)[1]. While structural fatigue in SMAs is akin to that in conventional materials—occurring through mechanical cycling between stress or strain amplitudes—it also arises due to repeated phase transformations. Functional fatigue, on the other hand, is marked by a decline in actuation strain upon cycling, potentially rendering the system inoperative before structural fatigue becomes a critical issue. To fully grasp these fundamental fatigue mechanisms, it is essential to investigate how factors like chemistry, microstructure, and testing conditions influence actuation fatigue. However, literature on the actuation fatigue behavior of NiTi-based SMAs is sparse, and even more so for NiTiHf HTSMAs. This article reviews the key factors affecting actuation fatigue performance in NiTiHf HTSMAs as well as the current state of research in this area, and attempts to identify remaining challenges. FACTORS AFFECTING ACTUATION FATIGUE Figure 1 illustrates the primary factors influencing actuation fatigue in SMAs, categorized by the stages of actuator development ecosystem they impact. While not exhaustive, this overview captures the major concepts influencing fatigue and failure mechanisms. Colored arrows denote well-studied factors with established effects on actuation fatigue and white arrows indicate topics that have been less explored, particularly in the context of NiTiHf HTSMAs. FACTORS REQUIRING FURTHER STUDY Several factors affecting actuation fatigue in NiTiHf HTSMAs remain under-researched. These include grain size, texture, heating/cooling rates, and stress state. Grain Size. Most studies on grain size focus on binary NiTi alloys, examining mechanical fatigue and crack growth[2-5]. Research on grain size effects in NiTiHf and other HTSMAs is limited, partly because these materials are in the martensite phase at ambient temperatures, making *Member of ASM International Fig. 1 — Primary factors affecting actuation fatigue in NiTiHf SMAs and the stages of actuator development ecosystem at which these factors are influenced. The white arrows indicate topics not yet studied for thermomechanical cycling fatigue in NiTiHf HTSMAs. 9 FEATURE
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