Compositional effects on strain-controlled actuation fatigue of NiTiHf high temperature shape memory alloys

Nickel-rich NiTiHf high-temperature shape memory alloys continue to garner significant interest for actuator applications. One issue impeding their widespread use is the question of actuation fatigue lifetime under high loads. This paper describes how these fatigue challenges can be overcome by partial thermomechanical cycling. By limiting actuation strain to less than the maximum value that is attainable with full transformation, actuation life cycles can be significantly extended. Strain-controlled cycling is achieved by limiting the upper cycle tem-perature during heating and stopping each cycle short of the austenite finish temperature. Four compositions of NiTiHf were tested to three levels of transformation. Strain-controlled cycling increased the actuation fatigue lifetimes of all compositions. However, specimens resistant to dislocation plasticity attained longer lives during full transformation cycling, but during partial cycling, they failed earlier than samples with higher resistance to plasticity. This highlights the importance of the competition between functional and structural fatigue.