Actuation Fatigue life of NiTiHf High Temperature Shape Memory Alloys

The NiTiHf system of high temperature shape memory alloys (HTSMA) is being investigated by numerous researchers as an improved alternative to the thoroughly studied binary NiTi system. In addition to the higher transformation temperatures the nickel rich NiTiHf compositions possess a significantly improved thermo-mechanical stability compared to the NiTi alloys, while maintaining a comparably high work output. The improved actuation performance of NiTiHf HTSMAs makes them preferable for an ever-growing array of applications, specifically in high temperature aerospace and automotive industries. The lack of a thorough understanding of the materials actuation fatigue limits is hindering implementation of these alloys in production applications. Adoption of SMA actuators in general is also hindered by the deterioration of actuation stroke during a devices lifetime, which can be common in binary NiTi devices. This deterioration prevents precise and repeatable actuator control over a devices lifetime. Optimizing the microstructure through careful control of composition, processing, and heat treatments is one way to improve performance. However, current research has shown that consistent performance of actuators cross multiple large-scale production melts is difficult to achieve. For instance even small deviations in composition (~0.1 at.%) can result in large changes to the actuation performance and shape memory effect response. To address these melt to melt variations this investigation studied the careful control of thermal cycling parameters to obtain reliable and repeatable actuation strokes. Control of these same thermal parameters was also shown to extend life. In this presentation the impact of composition and processing on fatigue life will be discussed. The test results of actuation fatigue specimens from multiple melts of NiTiHf tested over a range of upper and lower cycle temperatures and at varying percentages of full actuation will be presented. Stable actuation and increased fatigue life using these techniques will be shown.