Micromechanics study of thermomechanical characteristics of polycrystal shape-memory alloy films

In this paper a relaxed self-consistent model is developed to calculate the evolution of martensite plates in the constituent grains and the overall behavior of the polycrystalline shape-memory alloy (SMA) films. This relaxed model was developed based on the observation that, for a thin film whose thickness direction contains only a few grains, the geometrical constraint in this direction is lost and consequently the non-vanishing components of the internal stress exist only in the plane directions. Such an internal stress distribution is used to calculate the martensitic volume concentration of the individual grains at the considered temperature, and then by means of an averaging process the overall properties of the polycrystal SMA film. Several important characteristics are uncovered based on this relaxed constraint; they show that the properties of thin films can be distinctly different from those of the bulks. Most notable among these under a thermally-induced phase transformation is a narrower range of transformation temperatures for the films. Under a mechanical loading the stress-strain behavior of the films are found to exhibit a lower 'work-hardening' characteristic than the bulks due to the relaxed geometrical constraint. (C) 2000 Elsevier Science S.A. All rights reserved.