Deformation microstructure dependence of low cycle tensile fatigue life of NiTi wires

Abstract

This study prepared near-equiatomic NiTi wires with excellent cyclic tensile fatigue life and proposed the mechanism by which deformation microstructure impacts fatigue life. The fatigue life of Ni50.2Ti49.8 was discovered to be 1.8–2.1 times greater than Ni50.9Ti49.1 by the low cycle fatigue test with various test conditions (loading stress is 700 MPa/900 MPa, frequency is 5 Hz/2.5 Hz, and stress ratio is 0.1/0.2) at room temperature. Ni50.2Ti49.8′s matrix phase is R + B19′ with fewer slip systems. It tends to coordinate deformation with twinning, stacking faults, and wave dislocations during cyclic tensile. Cross-grain boundary twinning strengthens grain boundaries. Also, twinning can expand available space for dislocation movement, thereby reducing stress concentration and extending the fatigue life of Ni50.2Ti49.8. Ni50.9Ti49.1′s matrix phase is B2 + R and easy to form planar dislocation. This reduces the space for dislocation movement and causes premature fatigue fracture. Moreover, the second phase Ni4Ti3 in Ni50.2Ti49.8 is distributed uniformly throughout the matrix and exerts a strong pinning influence on the dislocation movement. This also facilitates the formation of multiple deformation microstructures in Ni50.2Ti49.8.