Enhancement of fatigue life of Ni–Ti–Fe shape memory alloys by thermal cycling

The effect of thermal cycling on the load-controlled tension–tension fatigue behavior of a Ni–Ti–Fe shape memory alloy (SMA) at room temperature was studied. Considerable strain accumulation was observed to occur in this alloy under both quasi-static and cyclic loading conditions. Though, in all cases, steady-state is reached within the first 50–100 cycles, the accumulated steady-state strain, ɛp,ss, is much smaller in thermally cycled alloy. As a result, the fatigue performance of them was found to be significantly enhanced vis-a-vis the as-solutionized alloy. Furthermore, under load-controlled conditions, the fatigue life of Ni–Ti–Fe alloys was found to be exclusively dependent on ɛp,ss. Observations made by profilometry and differential scanning calorimetry (DSC) indicate that the 200–500% enhancement in fatigue life of thermally cycled alloy is due to the homogeneous distribution of the accumulated fatigue strain.

Research highlights▶ Strain accumulates due to load-controlled fatigue of pseudo-elastic SMA. ▶ Thermal cycling (TC) reduces the amount of accumulated strain. ▶ Fatigue performance of TC SMA is far superior to the as-solutionized alloy. ▶ Homogenization of the strain in TC SMA is the reason for its fatigue performance.