Fatigue of superelastic NiTi wires with different plateau strain

Abstract

Microstructures and thermomechanical responses of Ni-rich superelastic wires can be tuned in relatively large extent by varying the degree of cold work and heat treatment. What frequently remains unknown is the superelastic fatigue performance of the wires – whether and/or how it depends on the heat treatment. In this study, five superelastic NiTi wires (d=0.051mm) having similar transformation stresses but different microstructures and transformation strains (3.1, 3.9, 4.7, 5.6, 6.7 %) were produced from one spool of a hot worked NiTi wire by applying different cold work/heat treatments to investigate the influence of the wire microstructure (transformation strain) on its structural and functional fatigue performance. The wires were cycled in tension beyond the end of superelastic plateau at constant temperature in strain rate controlled mode until failure. It is found that various cold work/heat treatments do affect the superelastic fatigue performance of NiTi wires in a defined manner. As concerns the transformation strain dependence, the wires exhibiting large transformation strain show decreasing fatigue performance with increasing transformation strain, the wires exhibiting low transformation strain show opposite trend. On average, the furnace treated NiTi wires (recovered and precipitation hardened microstructure) showed better fatigue performance than the electropulse treated wires (nanosized but partially recrystallized microstructure).