Deformation mechanisms of a NiTi alloy under high cycle fatigue

In this paper, high-cycle fatigue mechanisms of a pseudoelastic NiTi shape memory alloy (SMA) under different mean strains are investigated. The uni-axial cyclic tensile tests with small amplitude of 0.25% are operated at three different states, i.e. austenite/elastic region, austenite and martensite coexisted region and martensite/elastic region with a pre-strain. Mechanical results indicate that the cyclic deformation strain is mainly in the inhomo-geneous deformation band when the sample cycling in the B2 thorn B190 phase coexist status. Cyclic deformation in the B2 thorn B19' phase showed cyclic hardening effect. With increasing of the mean strain, the saturation stress is decreased, and saturation rate is reduced. The mean strain effect on the high-cycle fatigue life of NiTi is attributed to the inhomogeneous transformation of martensite. High resolution electron microscopy (HREM) observations and atomic simulations showed that partial dislocations with the Burgers vectors of 1/ 2 < 110 > are induced on (001)M twin planes, the interaction of dislocations and edge dislo-cations in martensite results in cyclic hardening in martensite. While cyclic softening in martensite at the upper plateau of stress-strain (S-S) curve is attributed to the grain reorientation/rotation phenomenon. It suggests that newly generated stress-induced martensite at the interface of martensite and austenite could increase the high-cycle fa-tigue of NiTi SMA.(c) 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

In this paper, the high-cycle fatigue mechanisms of a pseudoelastic NiTi shape memory alloy under different mean strains were investigated. The results show that the mean strain has an effect on the high-cycle fatigue life of NiTi, which is attributed to the inhomogeneous transformation of martensite and the phenomenon of grain reorientation/rotation.