Characterization of shape-memory alloy thin films made up from sputter-deposited Ni/Ti multilayers

A novel fabrication process for Ni-Ti shape-memory alloy thin films is presented. This process is based on the appropriate annealing of sputter-deposited Ni/Ti multilayers. X-ray diffraction shows that interdiffusion of the two constituents results either in the amorphization of the multilayer structure after annealing at 330 degreesC or in the recrystallization as a Ni-Ti intermetallic compound after annealing at temperatures above 400 degreesC. A single 30 min annealing step in the temperature range from 400 to 800 degreesC is sufficient to obtain Ni-Ti films showing martensitic phase transformations and the shape-memory effect. The influence of increasing annealing temperature on the transformation behavior is investigated by differential scanning calorimetry. The evolution of the transformation temperatures is found to be qualitatively similar to conventional sputter-deposited Ni-Ti films. The corresponding microstructure is studied by transmission electron microscopy. A very fine-grained structure is observed even after annealing at 800 degreesC. The film composition can be varied by adjusting the thickness ratio of the individual Ti and Ni layers. Transformation curves of films with nominal compositions of 49.5 and 54.0 at.% Ti are compared. It is demonstrated that Ni-Ti films made up from multilayers may possess an intrinsic "two-way" shape-memory effect, which is a very interesting feature in view of the development of thin film micro-actuators. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.