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 pres ented. This process is based on the appropriate annealing of sputter-deposi ted Ni/Ti multilayers. X-ray diffraction shows that interdiffusion of the t wo constituents results either in the amorphization of the multilayer struc ture 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 de greesC is sufficient to obtain Ni-Ti films showing martensitic phase transf ormations and the shape-memory effect. The influence of increasing annealin g temperature on the transformation behavior is investigated by differentia l 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 thick ness ratio of the individual Ti and Ni layers. Transformation curves of fil ms with nominal compositions of 49.5 and 54.0 at.% Ti are compared. It is d emonstrated that Ni-Ti films made up from multilayers may possess an intrin sic "two-way" shape-memory effect, which is a very interesting feature in v iew of the development of thin film micro-actuators. (C) 2000 Acta Metallur gica Inc. Published by Elsevier Science Ltd. All rights reserved.