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. The NiTiHf system is proving to be a better alternative to the thoroughly studied binary NiTi system for use as actuators. Not only due to higher transformation temperatures but also higher work output and thermomechanical stability. 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 by optimising 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 very small deviations in composition (~0.1 at.% Ni) can result in changes in transformation temperature by 50˚C or more. Thus, an investigation has been done to attempt to obtain a constant actuation stroke, whilst extending the actuation fatigue lifetime through actuation control by partial thermal cycling. Actuation fatigue samples were tested at four percentages of full actuation until failure and the differences in fatigue life and response were examined.