The effect of temperature on superelastic fatigue of NiTi filaments

Near-equiatomic intermetallic NiTi alloy has already been utilized in wide range of medical, aerospace, civil engineering and automotive applications. Theoretically, superelastic cyclic loading shall be achievable in a wide temperature range between austenite finish and martensite desist temperatures. However, the stress induced martensitic transformation, which represents the deformation mechanism of superelasticity, is accompanied by an incremental plastic deformation which deteriorates a stability of cyclic superelastic response and superelastic fatigue performance of NiTi wires at higher temperature.

In this experimental work, we have attempted to find out how the fatigue performance of the superelastic NiTi alloy depends on a temperature. Cyclic superelastic loading and tensile tests were performed on a medical grade superelastic NiTi filament in the wide temperature range (‑90 °C to 190 °C) and changes in a microstructure of the tested filaments were observed by TEM. Cyclic stability of stress-strain superelastic response and fatigue life of the filaments were evaluated and analysed.

It was found that i) an unrecoverable strain appears only when forward and reverse martensitic transformations proceed under external stress and that they increase with increasing testing temperature and transformation stresses, ii) the number of superelastic cycles till failure decreases with increasing testing temperature and transformation stress, which correlates very well with accumulated unrecoverable strain. The accumulated unrecoverable strain during the superelastic cycling is proposed to be adopted as a quantifier for an accumulated material damage, which controls fatigue performance of the NiTi alloy (neglecting effects of inclusions, surface quality, corrosion etc.).