Fatigue life of Ti–50 at.% Ni and Ti–40Ni–10Cu (at.%) shape memory alloy wires

Fatigue lives of Ti–Ni and Ti–Ni–10Cu alloys were measured using a rotary bending fatigue tester. The fatigue tests were carried out at 308, 323, 338, 368, and 398 K, respectively, under a constant strain amplitude condition. The Af points of Ti–Ni and Ti–Ni–10Cu alloys, which were annealed at 673 K for 3.6 ks, were 351 and 331 K, respectively. Two types of strain amplitude vs fatigue life curves were observed, one composed of two straight lines with one turning point whereas the other composed of three straight lines with two turning points. The upper turning point coincided with the elastic limit strain and the lower one with the proportional limit strain. The single turning point was observed in a fatigue test condition under which both the limit strains are almost the same. The fatigue life decreased with increasing test temperature in general. However, it became less sensitive to test temperature both in higher and lower temperature regions. Deformation mode and applied stress during fatigue testing are factors affecting fatigue life. However, the fatigue life of the Ti–50.0 at.% Ni is always longer than that of the Ti–40Ni–10Cu (at.%) if the fatigue life is plotted as a function of temperature difference between test temperature and Ms.