12: Review of Shape Memory Alloys (SMAs) as Smart Materials

Chapter 12 reviews shape memory alloys (SMAs). The shape memory effect (SME) is a property of materials that are capable of solid phase transformation from a body-centered tetragonal form called thermoelastic martensite to a face-centered cubic superelastic called austenite. These materials are named shape memory materials (SMM) and their thermal versions are called shape memory alloys (SMAs). This solid phase transformation from the body-centered tetragonal martensite crystalline structures to a thermoelastic face-centered cubic austenite crystalline phase by either temperature, stress or strain, is called the SME. These martensitic crystalline structures are capable of returning to their original shape in the austenite phase, after a large plastic deformation in the martensitic phase and return to their original shape when heated towards austenitic transformation. These novel effects are called thermal shape memory and superelasticity (elastic shape memory), respectively. SMAs belong to the large class of smart materials because of their ability to undergo large deformations and to regain their original shape, either during unloading (superelastic effect, SE) or via thermoelastic cycling (SME). As mentioned before this is due to a solid crystalline phase (FCC to BCC) transformation (solid-to-solid) (martensitic transformation). This transformation also enables the SMAs to transform from a higher state symmetry crystal lattice (austenite) to a phase with a less symmetric lattice (martensite).