Where are we at with shape-memory alloys in this ‘high-tech’ world?

Abstract

Some of the basic physics of shape-memory materials is outlined. Shape memory relies on a change in the crystallography of the material via a martensitic transformation. While many alloys show shape-memory properties, only certain materials have ‘made it’ to technological applications. The most notable of these is <i>Nitinol</i>, the commercial name for a nickel‒titanium alloy which was discovered in the US Navy Laboratories almost by accident. The most important current and future applications for <i>Nitinol</i> are, and will continue to be, as various medical devices. The material is most favourable for medical applications, firstly, because it exhibits shape-memory behaviour at very close to body temperature (37°C) and, perhaps equally importantly, only an extremely small percentage of human beings have any allergic reaction to either nickel or titanium. Not so important are magnetic shape-memory materials for which, particularly the material Ni<sub>2</sub>MnGa, there are increasing numbers of applications requiring the shape-memory behaviour to be controlled by an applied magnetic field. The properties of some shape-memory materials relevant to current applications are summarised and, consistent with the theme of sustainability, some comment is made on the likely future of shape-memory materials in the market place.