SHAPE-MEMORY MATERIALS: CRYSTALLOGRAPHIC TEXTURE AND ITS CONSEQUENCES

Massachusetts Institute Of Technology, Cambridge MA

Investigators

Abstract

0002930 It is now well recognized that shape-memory materials derive their unusual and inherently nonlinear and anisotropic properties from the fine-scale rearrangements of phases, or ``microstructures,'' and that the strain produced in the pseudoelasticity effect, as well as that recovered during the shape-memory effect, depends on crystal orientations. Specially oriented single crystals of some shape memory materials can produce sizeable strains (~ 10%) due to phase transformations. Practical shape-memory materials are typically polycrystalline in nature, and because of their thermo-mechanical processing history, they are initially textured. It has recently been recognized that the initial crystallographic texture of sheets, wires and rods of shape-memory alloys is crucial in determining the overall properties of these materials. The pronounced anisotropies caused by the initial texture need to be properly accounted for in developing a robust computational capability for the improved design of geometrically-complex engineering components made from shape memory materials. We shall Experimentally study the effects of initial texture on the thermo-mechanical response of the shape-memory alloy Ti-Ni, in both bulk form, and thin films. This material is finding increased use as a functional/smart-material for a variety of applications. Ti-Ni alloys in thin-film form are promising materials for microactuators, because of the enhanced rate of heat transfer in thin films. Develop constitutive equations and computational procedures for modeling and simulation of the important pseudoelasticity and shape-memory effects observed in these materials. The mathematical models and procedures that we plan to develop should be useful in the design of components for enhanced thermo-mechanical performance.