Thermotracttve Titanium-Nickel thin Films Formicroelectromechanical Systems and Active Composites

Abstract

ABSTRACTThin films of thermoelastic titanium-nickel are of interest as a material basis for force-producing elements in microelectromechanical systems, and for active phases in mechanically-adaptive composite materials. The successful introduction of this material system into such application areas will depend on development of reliable thin film deposition protocols, together with the refinement of analytical models which successfully predict the response of active microstructures to a variety of dynamic thermal and mechanical stimuli. In the present paper we review some of our recent experimental and theoretical work which bear on these problems. With respect to thin film fabrication techniques we focus on problems of composition control and the manipulation of microstructure, with particular emphasis on opportunities afforded by amorphous precursor phases formed during low temperature processing, and the fine-grained, thermally stable crystalline microstructures obtainable using hot-substrate deposition. The films resulting from either approach retain the important thermomechanical response features of the well-known bulk-alloy system: shape memory and transformational superelasticity. The response can be modeled in terms of a continuum description augmented with internal variables that track fractional partitioning of the material between austenite and variants of the martensite.