(The) effect of Cu addition on the mechanical properties of NiTi shape memory alloy thin films

Abstract

Shape memory alloys (SMAs) are a class of material which can revert to their original shape upon removal of thermomechanical stimulus. Among them, Ni-Ti-based SMAs have received a great attention due to their large work output and displacement, good corrosion resistance, and biocompatibility. Owing to the compositional sensitivity of Ni-Ti and toxicity of Ni, the addition of a third element has been extensively studied. In particular, the addition of copper (Cu) improves fatigue stability, and reduce sensitivity to composition. In this work, Ni-Ti and Ni-Ti-Cu free standing thin films were fabricated via microelectromechanical system (MEMS) based process, and tensile and fatigue tests were carried out using a custom-built micro-mechanical tester. Ni-Ti thin film was deposited as Ni-rich films, which also contains Ni4Ti3 semi-coherent precipitates. Lenticular-shaped semi-coherent Ni4Ti3 precipitates in the grain interior can act similarly to grain boundaries, resulting in grain refinement effects that highly improves its cyclic stability. In addition, (Ni,Cu)-rich Ni-Ti-Cu thin films were fabricated with the presence of (Ni,Cu)2Ti precipitates. As the Cu content increases, the volume fraction of precipitates increased while the volume fraction of austenite decreased as confirmed by X-ray diffraction (XRD) patterns. The shape and size of precipitates were confirmed using scanning transmission electron microscopy (STEM) - High-angle annular dark-field (HAADF) images and energy dispersive X-ray spectroscopy (EDS). Moreover, as Cu content in Ni-Ti-Cu increases, the shape of (Ni,Cu)2Ti precipitate changes from plate-like to spherical, along with a decrease in its size, in which can lead to improvement of fatigue stability.