Exceptional fatigue resistant NiTi wire mediated by R-phase.

• Ni 50.2 Ti 49.8 wire with R-phase as the matrix was produced in the as-drawn state. • Cyclic phase transformation in Ni 50.9 Ti 49.1 initiates microcracks in phase boundaries. • The absence of R-phase to B19′ transformation in Ni 50.2 Ti 49.8 helps avoid microcracks. • The fatigue life of Ni 50.2 Ti 49.8 is approximately 12 times that of Ni 50.9 Ti 49.1. Ni 50.2 Ti 49.8 and Ni 50.9 Ti 49.1 wires in cold-drawn state were compared regarding tensile fatigue life and fatigue fracture mechanism. Through process control, Ni 50.2 Ti 49.8 with R-phase matrix at room temperature was achieved in as-drawn state without post heat treatment. The R-phase imparts exceptional fatigue performance to Ni 50.2 Ti 49.8 wire with fatigue life approximately twelve times of that for Ni 50.9 Ti 49.1 which comprises B2 phase and a small amount of B19′ under the same test conditions. The cyclic phase transformation under loading initiates microcracks in phase boundaries, leading to premature fracture of Ni 50.9 Ti 49.1. In contrast, the R-phase presents exceptional stability against fatigue failure upon cyclic loading. [ABSTRACT FROM AUTHOR]