Development and Characterization of Shape Memory Alloys

Recent development of shape memory alloys is reviewed, emphasis being placed on the Ti-Ni, Cu-based and ferrous alloys which are considered as practical materials for applications among many shape memory alloys. Crystal structures of the parent and martensitic phases are described, and the crystallography of the martensitic transformations is also briefly explained. The origin of the shape memory effect and the shape memory mechanisms are discussed on the basis of the crystal structure and the crystallography of the martensitic transformations. Since an applied stress also induces the martensitic transformations, successive stages of the stress-induced martensitic transformations are reviewed briefly in Cu-based and Ti-Ni alloys, which show martensite-to-martensite transformations upon loading. Then, the transformation and mechanical characteristics of the shape memory alloys are reviewed in detail; i.e. phase diagrams, transformation temperatures, transformation process, stres-induced transformation, aging effects, cycling effects, fracture, fatigue, grain refinement, two-way shape memory effect, and so on. Recent develpment of sputter-deposited Ti-Ni thin films is also introduced.