Actuation fatigue performance of NiTiZr and comparison to NiTiHf high temperature shape memory alloys

The actuation fatigue performance of a Ni-rich Ni50.3Ti29.7Zr20 (at.%) high temperature shape memory alloy (HTSMA) was evaluated and compared to previous results for Ni50.3Ti29.7Hf20 HTSMA. The shape memory properties of these alloys are known to improve significantly after proper aging heat treatments. Yet, their widespread implementation in high temperature applications requires a thorough characterization and a detailed understanding of their actuation fatigue performance. Consequently, actuation fatigue tests were performed by thermal cycling the NiTiZr alloy under constant loads from 200 to 500 MPa until failure. The results revealed that NiTiZr exhibits a shorter fatigue life with lower recoverable strain levels than a comparable NiTiHf alloy at all applied stress levels and the difference in fatigue life between the two alloys became more distinct with increasing stress. Building upon the extensive research on the microstructure, mechanical and functional behaviors of NiTiHf and NiTiZr HTSMAs during the last decade, this work is the first fundamental study that directly compares actuation fatigue behaviors of Ni-rich NiTiZr and NiTiHf alloys.