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.
Part 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 such as gas detectors used in OEM, medical, laboratory and similar applications, as well as mass flow controllers, blood pressure cuff monitors, ventilators and oxygen concentrators. These valves are suitable for use on air or other gases that are clean or dry, non-corrosive and non-flammable.
Other actuation applications have been seen in the aerospace, automotive and other industries. The variable-geometry chevron, for example, is a shape memory alloy structure that reduces aircraft noise on take-off, then adopts a different shape in flight to cut noise and conserve fuel.
Superelasticity
As well as the shape memory effect, these alloys have another important characteristic: superelasticity, which is also referred to as pseudoelasticity. This occurs when an alloy is deformed slightly above its transformation temperature. The effect is caused by the stress-induced formation of some martensite above its normal temperature. For this reason, the martensite instantly reverts to undeformed austenite once the stress is removed, which manifests as a highly elastic effect in the bulk material, with elastic strains in the region of 10 per cent possible. Wires with this property will therefore extend well beyond the normal 'elastic limit,' springing back to their original shape very easily. Such materials have been used to make 'unbreakable' spectacle frames and were popular for antennas on early mobile telephones.
But the medical industry remains the key market for shape memory alloys because of the materials' biocompatibility. To this end, Nitinol Devices & Components (NDC) is to establish a new research and development and manufacturing centre in Costa Rica later in 2010. "Costa Rica presented the best combination of operating cost, workforce talent and tax advantages for a low-cost manufacturing facility," explained Jeff Lenigan, vice president of operations at NDC.
This new plant will produce medical guide wires, using NDC's core expertise in Nitinol processing, combined with extrusion, coating, and assembly operations. The plant is situated in a Free Trade Zone in San Jose, which also hosts other medical device companies. In its first year of operation, NDC plans to hire 30 employees - rising to 200 in the long-term - and invest around $3.5 million.
Mitchell Tatum, the plant's general manager, commented: "We plan to deliver product from the new plant by the first quarter of 2011."
NDC also offers a wealth of information about shape memory alloys on its website: the 'Nitinol University' section includes technical papers and design tips - including a 'stent calculator' worksheet to help designers calculate parameters such as strut widths, lengths and foreshortening, as well as providing simple estimates of strain.
Another company that is helping to make shape memory alloys more accessible to new users is Johnson Matthey, which has developed an interactive online inventory tool so customers can find popular, in-stock sizes and obtain pricing and delivery information for products including its Nitinol superelastic tubing, wire and sheet. The inventory tools can be accessed via www.jmmedical.com.
Johnson Matthey's range of standard Nitinol products includes superelastic alloys of various sizes, thicknesses and finishes. Each product type has its own inventory/ordering tool that can be accessed via the appropriate product line page. Nitinol tube, wire, sheet and machined parts are manufactured at the plant in San Jose, California, while micromachining takes place in San Diego.
In 2009 SAES Memry - which is part of SAES Getters, the Italian-owned shape memory alloy specialist - added Nitinol sheet to its portfolio of products for the medical device industry. The sheets, manufactured at the company's facility in Germany, are available in a variety of alloy compositions, including some in the superelastic state. Each can be supplied in different conditions such as cold worked, flat annealed and shape annealed. Surface finish options include standard dark oxide, oxide-free, and ground or polished sheet surfaces.
Biocompatibility means that many of the shape memory alloys will continue to be attractive for use in medical devices, but there are countless other applications for which these unusual materials can deliver significant benefits.
Designing components and assemblies to take advantage of the shape memory effect or superelasticity requires some additional knowledge, and processing can be difficult using conventional machining operations, but support is often available from the suppliers and the advantages in terms of product performance can make the additional effort very worthwhile.
