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 Nitinol, 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 Nitinol 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 Ni₂MnGa, 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.