Introduction of a normalized measure of functional fatigue for the benchmark of cyclic stability of pseudoelastic NiTi wires

A limited functional stability represents one of the the most difficult obstacles for the development of shape memory alloy (SMA) applications. It has been shown in the literature that an important factor which governs functional stability is the crystallographic compatibility (CC) between austenite and martensite. The CC is directly connected to the hysteresis width of the martensitic transformation. The better the CC, the narrower the hysteresis width and the higher the functional stability. As a consequence, the hysteresis width can be used to assess the functional stability of a SMA. In the present work, we study the functional performance during mechanical cycling of binary, pseudoelastic NiTi wires. Our results show that with increasing Ni content both hysteresis width and functional fatigue effects decrease. We present new results on both thermal and mechanical cycling, and introduce a normalized fatigue constant for SMA, which allows to compare functional stability of different alloys / material states. We also show benchmark fatigue data and invoke a crystallographic / microstructural scenario for interpretation.