An ion implantation processing technique used to develop shape memory TiNi thin film micro-actuator devices

Abstract

Since shape memory alloys (SMA) thin films can recover large deformations and generate high recovery forces, there has been great interest to use them as the active elements in microactuator systems. We have studied a novel planar processing technique using ion irradiation to develop a thin film micro-actuator. In this technique, SMA thin film are first pre-strained to values ∼ 4%, and then ion irradiated with 5 MeV Ni ions. Theoretically, the irradiation-induced damage can suppress the martensitic transformation (MT) in a layer, whose thickness is chosen to be on the order of 1/2 to 1/3 the films thickness. When heated above the austenite finish temperature, large differential strains are created between the still transforming and damage layer, causing the film to bend out of plane. The partial energy stored in the damaged layer, from the prior transformation, is available to deform the martensite on subsequent cooling and MT, which causes an uncurling of the film. Thus, with thermal cycling, a reversible two-way motion occurs. We have studied the phase transformations in irradiated microstructure by TEM, thermomechanical tests, and X-ray diffraction. We have found that the irradiation damage mainly consists of amorphous matrix with nanocrystalline islands. The results will be discussed in terms of the effects of irradiation on the microstructure and its relationship with the film's motion.