Phase transformation and structure of functionally graded Ni-Ti bi-layer thin films with two-way shape memory effect

Ni-Ti thin film shape memory alloys (SMAs) have become a promising material for micro-actuators because of their excellent mechanical properties and biocompatibility. Films with composition gradient have the added feature of an intrinsic two-way shape memory effect. In this study two kinds of the Ni-Ti bi-layer thin films (Ni50.8Ti/Ni45TiCu5 and Ni50.8Ti/Ni49Ti) were developed. The thin films were deposited onto Si (1 1 1) substrates by RF magnetron sputter deposition from separate alloy targets. The three targets with atomic compositions of Ni45Ti, Ni47Ti and Ni42TiCu5 were prepared by vacuum arc re-melting (VAR). For crystallization, the as-deposited amorphous thin films were annealed at 773 K for 1 h. The annealed bi-layer thin films formed compositional gradients at the interface of the films through diffusion. The effect of the composition on film structure, surface morphology, transformation temperatures and mechanical properties was studied using grazing incidence X-ray diffraction, transmission electron microscopy, atomic force microscopy, electrical resistivity and nano-indentation. The chemical and stress gradient in the bi-layers affects the structure, phase transformation and shape memory effect. Thin films exhibit multi-step phase transformation. Also, the bi-layer presented pseudo elastic and shape memory effect, simultaneously. The depth recovery ratio due to pseudo elastic in the Ni50.8Ti/Ni45TiCu5 compared with the Ni50.8Ti/Ni49Ti bi-layer is higher while the recovery ratio due to shape memory in the Ni50.8Ti/Ni49Ti bi-layer is higher. The combination of pseudo elastic with shape memory effect produces a two-way shape memory effect with a reduced hysteresis in the bi-layers.