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 marten
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S. Miyazaki University of Tsukuba, Tsukuba, Japan
Abstract: 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-tomartensite 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. Key words: Shape memory effect, Pseudoelasticity, Superelasticity, Martensite, Martensitic transformation, R-phase, Rhombohedral phase, Ti-Ni, Ni-Ti, Cu-AI-Ni, Cu-Zn-Al, Shape memory alloy, Ferrous shape memory alloy, Fatigue, Crack propagation, Fracture, Single crystal, Bi-crystal, Two-way shape memory effect, Thin film
M. Fremond et al., Shape Memory Alloys © Springer-Verlag Wien 1996
Development and Characterization of Shape Memory Alloys
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1. Introduction The shape memory effect (SME) appears in some special alloys which show crystallographically reversible martensitic transformations. The martensitic transformation is accompanied by a large shear-like deformation associated with a diffusionless structural change; the deformation generally amounts to about 20 times more than the elastic deformation. The martensite is deformable and it can also be induced from the parent phase by loading, both deformation modes being associated with no permanent strain in the shape memory alloys (SMAs). Thus, a large deformation induced in the SMAs can recover perfectly by heating to temperatures above the reverse-transformation finish temperature (Aj) after unloading (shape memory effect or SME) or simply by unloading at temperatures above Af (pseudoelasticity (PE) or superelasticity (SE)). The SME was first found in a Au-Cd alloy in 1951 [11, and then in In-Tl alloy in 1953 [2,31. However, the possibility for using the SME in actual applications was realized after the SME was found in a Ti-Ni alloy in 1963 [41. Since the Ti-Ni alloy has many complicated features and difficulty in making single crystals, the basic understanding of this alloy was not possible until the early 1980s. On the other hand, a Cu-Al-Ni alloy was also found to reveal the SME in 1964 [51 and in this alloy it was
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