Cyclic stability in NiTi and NiTiCu thin films: Role of precipitates in low- and high-cycle regimes

Abstract

This study examines the influence of precipitates on the cyclic behavior of submicron-thick NiTi and NiTiCu thin films by conducting tensile and fatigue experiments. X-ray diffraction confirmed that both materials contain austenitic phases at room temperature. NiTi has exhibited superior cyclic stability in the low-cycle regime when compared to NiTiCu, while the cyclic stability of NiTiCu surpasses that of NiTi in the high-cycle fatigue regime. The enhanced cyclic stability of NiTi films in the low-cycle regime is attributed to the homogeneous distribution of semi-coherent Ni4Ti3 precipitates within the grain interior. In contrast, the reduced cyclic stability of NiTiCu films can be directly correlated to the presence of incoherent (Ni,Cu)2Ti precipitates at grain boundaries. Conversely, NiTiCu exhibited enhanced cyclic stability under rising stress ratio, attributable to a notable reduction in hysteresis area.