Thermotracttve Titanium-Nickel thin Films Formicroelectromechanical Systems and Active Composites
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D. S. GRUMMON AND T. J. PENCE Department of Materials Science and Mechanics Michigan State University, East Lansing, MI 48824 ABSTRACT Thin 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. INTRODUCTION Equiatomic titanium-nickel forms the basis of an important class of shape-memory and superelastic alloys whose unique constitutive behavior may play a role in enabling materials technologies for smart material systems, microelectromechanical actuators, and mechanically active composite materials systems. They may also find application to problems in fatigue, damping of small systems, and functionally graded interfaces for joining dissimilar materials. The highly reversible and energetic displacive transformation exercised by titanium-nickel has several interesting technical implications: For example, ohmic electrical excitation can alter elastic compliance by a factor of four or more, of potential interest for control of impedance for sensors and transducers. Joule excitation can also alter internal friction and damping capacity, the latter being particularly high for the low-temperature martensite phase. Of particular interest to microelectromechanical systems, however, is the fact that the martensite-to-austenite transformation can generate large displacements and very high force output: the equivalent of one gram of force can, in principal, be generated from a 10 ptm wide film that is only 3 .tmthick. This corresponds to very high actuator energy-densities (exceeding 10 MJ-m-3 ) which is several orders of magnitude greater than can be achieved using other available microactuator materials [1]. The most serious disadvantage, apart from general thin film process sensitivities, is the relatively sl
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