Voxel-Level Design of Functionally Graded NiTi and NiTiHf Shape Memory Alloys via Additive Manufacturing

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). 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 presentation 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.