Multivariant model of martensitic microstructure in thin films

A novel multivariant model of martensite is developed through competing energetics to describe the coarsening, refinement and selection of the microstructure. In contrast with the conventional phase-field methods, a new set of field variables motivated by the hierarchical structure of multirank laminates is employed to represent each variant. As a result, the energy-well structure call be expressed explicitly in an elegant and unified fashion. The framework is applied to the investigation of pattern formation in martensitic thin films with trigonal symmetry. Various intriguing patterns are predicted and are found to be in good agreement with those observed in experiments. In addition, film orientations and patterns necessary to achieve large actuation strains are Suggested for dome-shaped and tunnel-shaped microactuators. It is found that the resulting morphologies evolve with coherent interfaces under various loading conditions. This Suggests that compatible walls provide a low-energy path during evolution, and the understanding of them leads to novel strategies of large strain actuation. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.