Shape memory and related phenomena

Abstract

More than half the papers presented at ICOMAT-89 (International Conference on Martensitic Transformations, Sydney, Australia, July, 1989) were concerned with various aspects of the shape memory effect (SME), a relatively new phenomenon associated with a martensitic transformation. Accordingly, a material undergoes a displacive, shear-like martensitic transformation when it is cooled below a certain temperature, Ms. The transformation is completed when a lower temperature, Mr, is reached, where the material is said to be in the martensitic state. When this martensite is deformed (below Mr) it undergoes a strain which is completely recoverable upon heating. The shape recovery (memory) begins at a temperature, As, and is completed at a higher temperature, Af. The Ms, Mr, As and Af temperatures depend upon the particular alloy system, and recovery strains typically range from 2-10%. The martensite in shape memory alloys (SMA's) may also be isothermally induced above the Ms temperature by the application of stress, known as stress-induced martensite (SIM). This martensite disappears (reverses) when the applied stress is removed, resulting in a mechanical type (as opposed to thermal type) shape memory. The formation of SIM and its reversion gives rise to superlastic behavior. The two-way SME combines aspects of the 'one way' SME and SIM formation. Following a variety of thermomechanical treatments a specimen will exhibit a spontaneous shape change upon cooling between Ms and Mr because of microstresses inbuilt during processing. The microstresses effectively function ('program') as the external stress in SIM formation, but since the internal stresses are not released, the martensite will only reverse upon heating, at which time the one way SME operates. The shape memory effect was initially reported in 1951 by Chang and Read who studied a Au-Cd alloy3 I)* Since then many other alloy systems have been found to exhibit SME behavior. A brief history of the shape memory effect can be found in Ref. (2), and an excellent review of the development of SMA's has been provided by Miyazaki and Otsuka. (3) The more practical and engineering aspects of SMA's, which are not covered here, will be found in the book, Engineering Aspects of Shape Memory Alloys, proceedings of an international workshop/conference held in 1988. (4) Additional information on the SME and SMA's can be found in various conference proceedings °-14) and monographs, os-ig) Table 1 presents a brief glossary of shape memory and related phenomena. Apparently recognizing that the rather exotic Au--Cd alloy of Chang and Read °) had little engineering significance, interest in the shape memory remained largely dormant until 1963, when similar