Introduction to Shape Memory Alloys

Abstract

Numerous metallic alloys are now known to exhibit a shape memory effect through which an article deformed at a lower temperature will regain its original undeformed shape when heated to a higher temperature. This behavior is basically a consequence of a martensitic phase transformation. When compared, the various shape memory materials are found to have common characteristics such as atomic ordering, a thermoelastic martensitic transformation that is crystallographically reversible, and a martensite phase that forms in a self-accommodating manner. The explanation of the shape memory phenomenon is now universal and well in hand. In addition to the familiar “one-way” memory, shape memory alloys also exhibit a “two-way” memory as well and a “mechanical” shape memory resulting from the formation and reversal of stressinduced martensite.Fundamental to the shape memory effect (SME) is the occurrence of a martensitic phase transformation and its subsequent reversal Basically, a shape memory alloy (SMA) is deformed in the martensitic condition (martensite), and the shape recovery occurs during heating when the specimen undergoes a reverse transformation of the martensite to its parent phase. This is the essence of the shape memory effect. Materials that exhibit shape memory behavior also show a two-way shape memory, as well as a phenomenon called superelasticity. These are also discussed. The shape memory response after deformation and thermal stimulation constitutes “smart” behavior, i.e., S timulated M artensite- A ustenite R everse T ransformation.