Shape Memory Alloys - Technology and Industrial Applications

Shape memory alloys (SMAs) are metal alloys that undergo martensitic transformation under thermomechanical loading. These alloys can regain their initial shape on heating. This distinguishes them from conventional metal alloys. Effects such as superelasticity, constrained return, mechanical work generation and high damping capacity are the outstanding properties of AMFs.

First observed in 1932 on an Au-Cd alloy, the shape memory effect began to attract industrial interest in the early 1970s, with the development of nickel-titanium (Nitinol). There are a large number of shape memory alloys, but only three families have seen commercial development: Ni-Ti-based alloys, copper-based alloys and iron-based alloys. They are marketed in the form of wires, bars, plates, tubes and ribbons, in various cross-sections and diameters. They are also available in porous products and thin films.

As their performance is closely linked to their microstructural state, a basic knowledge of the production and processing conditions of the products used is essential. A precise definition of the application's specifications (number of cycles, level of stress or deformation imposed, temperature, etc.) is absolutely essential. Their main applications are in the biomedical, aeronautical and space sectors.

This article begins by describing the microstructural mechanisms behind the various properties observed in AMFs, as well as the limitations caused by fatigue and ageing phenomena (§ 1 ). It outlines the main characteristics of alloys used in industrial applications (§ 2 ), and reviews materials currently under development, such as high-temperature AMFs and magnetic AMFs (§ 3