Shape memory and related phenomena

• Published in: Progress in materials science 1992, Vol.36, p.203-224
• Main Author: Wayman, C.M.
• Format: Article
• Language: English
• Subjects: Applied sciences; Condensed matter: structure, mechanical and thermal properties; Deformation and plasticity (including yield, ductility, and superplasticity); Exact sciences and technology; Mechanical and acoustical properties of condensed matter; Mechanical properties of solids; Metals. Metallurgy; Physics
• ISSN: 0079-6425; 1873-2208
• DOI: 10.1016/0079-6425(92)90009-V

In the shape memory effect (SME), a material undergoes a martensitic transformation. After deformation in the martensitic condition, the apparently permanent strain is recovered when the specimen is heated to cause the reverse martensitic transformation. Upon cooling the specimen does not return to its deformed shape. In two-way SME a material normally exhibiting the SME is thermomechanically processed, after which, upon cooling through the martensite formation regime, it undergoes a spontaneous change in shape. Upon heating the inverse shape change occurs via the SME mechanism. When SME alloys are deformed isothermally in the temperature regime a little above that where martensite normally forms during cooling, a stress-induced martensite (SIM) is formed. This martensite disappears (reverses) when the stress is released, giving rise to a superelastic stress--strain loop with some stress hysteresis. This property applies to the parent phase undergoing a stress-induced martensitic transformation. Some SME alloys show rubber-like flexibility. When bars are bent, they spontaneously unbend upon release of the stress. In order to obtain this behaviour, the martensite after its initial formation usually must be aged for a period of time; unaged martensites show typical SME behavior. The rubber-like behavior, unlike superelasticity (also a rubber-like manifestation) is a characteristic of the martensite phase--not the parent phase. Pseudoelasticity is a more generic term which encompasses both superelastic and rubber-like behavior. As such, it is less descriptive. Using the terms superelastic and rubber-like behavior is more specific and tends to avoid ambiguity by emphasizing parent phase and martensite properties, respectively. Both are isothermal phenomena.