Materials • Processes • Finishes
Fig. 1. Lee Products’ new miniature proportional valve uses an electrically heated shape memory wire to adjust the flow, resulting in a valve that is smaller, lighter and more energy-efficient.
Designers can take
advantage of either the shape memory or superelasticity characteristics of shape memory alloys to achieve dramatic improvements in product performance. Alistair Rae looks at some of the most recent developments and applications for these remarkable materials.
Les designers peuvent exploiter la mémoire de forme ou les propriétés super-élastiques des alliages à mémoire de forme pour obtenir des améliorations spectaculaires de la performance des produits. Alistair Rae nous parle des plus récents développements et applications pour ces remarquables matériaux.
Designer können entweder die Formgedächtnis- oder Superelastizitätseigen- schaften von
Formgedächtnislegierungen nutzen, um dramatische Verbesserungen der Produktleistung zu erzielen. Alistair Rae betrachtet die neuesten Entwicklungen und Anwendungen für diese erstaunlichen Werkstoffe.
Shape memory alloy boosts product performance
P 26
www.engineerlive.com
art of the design engineer’s job is to identify ways to deliver a step-change in product performance but without increasing costs or introducing undue project risk. Depending on the
type of product and the functions required, one possibility might be to introduce shape memory alloys – which are also known as memory metals or sometimes superelastic alloys, depending on how they are used.
A shape memory alloy (SMA) is a metallic
material that can be ‘programmed’ to assume a specific shape under certain conditions. This happens because the material undergoes a phase change between two crystalline forms – martensitic and austenitic. When a material is in its martensitic form, it can easily be deformed into a new shape using a deformation stress typically in the range 70 to 140 MPa (10,000 to 20,000 PSI). But when the alloy is heated through its ‘transformation temperature’ it reverts to its austenitic form and recovers its original shape. The austenitic form is much stronger, with yield strengths of up to 700 MPa (100,000 PSI), which is similar to that of high- strength alloy steel. A number of materials exhibit this type of behaviour, but Nitinol is perhaps the best-known example, being a series of alloys of nickel and titanium, plus small amounts of other elements. It is produced in many forms including wire, sheet and tube to suit a different applications.
Importantly, the transformation temperature can be controlled by adjusting the alloy composition and by heat treatment, with the shape recovery process taking place over a range of just a few degrees Celsius; material suppliers can typically control the start or finish to within a degree or two. This tunable characteristic is often employed in medical stents, which might be designed so that they morph slowly to their correct shape at body temperature. Indeed, some suppliers even advertise their products as ‘body temperature’ shape memory alloys for this reason. But the ability to change shape depending on
temperature makes shape memory alloys useful in a range of other applications such as actuation. Lee Products, for example, has launched a new type of miniature proportional valve that is claimed to be smaller, lighter and more energy-efficient than traditional solenoid-type valves (Fig. 1). At the heart of its design, it uses shape memory wire – rather than a spring – to vary flow and provide the actuation force. When a current is applied to the shape memory alloy wire inside the valve it heats up and changes shape; this deformation results in the valve opening in proportion to the applied current.
Eliminating the need for a magnet, coil wire or
steel armature makes the valve smaller and lighter. The product is therefore aimed at designers of portable, handheld, battery-powered instruments
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