Impact of material structure on the fatigue behaviour of NiTi leading to a modified Coffin-Manson equation

A complex influence of various structural parameters (including an increased dislocation density, size of Ni-rich (Ni4Ti3 and Ni3Ti2) particles, and volume fraction of Ti-rich (Ti4Ni2Ox) particles as well as content of Ni in the alloy) on the strain-controlled fatigue behaviour of NiTi alloy has been studied in this work. It was revealed that in low-cycle conditions (εa≥2%) strain-controlled fatigue resistance of NiTi alloy may be improved by the creation of a microstructure which increases the part of deformation that is realized by martensitic mechanism. This part is suggested to be estimated by measuring (εcr) parameter, defined as the maximal strain which sample can completely recover after unloading and heating to the temperature higher than Af. A modified Coffin-Manson equation is suggested to describe and predict the strain-controlled fatigue behaviour in the range of high strain amplitudes (εa≥2%), where εcr0.2 serves as one of the coefficients, while another coefficient may be deducted from the empirical relation based on the bending test measurements. No correlation between NiTi alloy strain-controlled fatigue resistance and εcr0.2 parameter was observed for the high-cycle conditions (εa≤1.5%). In this case NiTi alloy performance may be improved by nanosize Ni-rich particles precipitation, creation of increased dislocation density or decreasing of the Ti4Ni2Ox volume fraction.