Extending the Fatigue Life of NiTiHf High Temperature Shape Memory Alloys through Partial Thermal Cycling

NiTiHf high temperature shape memory alloys (HTSMAs) are being used in an ever-growing array of applications, specifically in the aerospace and automotive industries. One of the difficulties facing further implementation is ensuring the actuation fatigue lifetime is sufficiently long as to prevent the HTSMA components from being a limiting factor to the mean time between failures of a system. Another potential problem for widespread use is the deterioration of actuation stroke during lifetime, which can be problematic when attempting to have a high-fidelity repeatable design. One way of solving these issues is to optimize the microstructure through careful control of composition, processing, and heat treatments. Current research shows composition of large-scale productions is incredibly difficult to control, and small deviations in composition (~0.1 at.% Ni) can result in changes in transformation temperature by 50?C or more. Four NiTiHf compositions were investigated. The initial goal to simply extend the actuation fatigue lifetime and provide a stable actuation response morphed into determining material factors that influence the actuation response of partially cycled samples.