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 b ehavior of the polycrystalline shape-memory alloy (SMA) films. This relaxed model was developed based on the observation that, for a thin film whose t hickness direction contains only a few grains, the geometrical constraint i n this direction is lost and consequently the non-vanishing components of t he internal stress exist only in the plane directions. Such an internal str ess distribution is used to calculate the martensitic volume concentration of the individual grains at the considered temperature, and then by means o f an averaging process the overall properties of the polycrystal SMA film. Several important characteristics are uncovered based on this relaxed const raint; they show that the properties of thin films can be distinctly differ ent from those of the bulks. Most notable among these under a thermally-ind uced phase transformation is a narrower range of transformation temperature s for the films. Under a mechanical loading the stress-strain behavior of t he films are found to exhibit a lower 'work-hardening' characteristic than the bulks due to the relaxed geometrical constraint. (C) 2000 Elsevier Scie nce S.A. All rights reserved.