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SHAPE MEMORY ALLOYS Rethinking the future of alloys that think for themselves


Shape-shifting metallics they're the stuff of science fiction. Whether talking about "morphing" planes or modular robots that can break apart and regroup as new devices, the pace at which technology is advancing has put the sci-fi realm on a collision course with reality.

The "smart" materials come in many shapes and forms piezoelectric, altering shape under the influence of an electric charge; thermoresponsive, which shape-shift in response to ambient temperature; magnetic, obviously affected by magnetic fields; and polychromic, chromogenic and halochromic, which change color based on pH, temperature, light or electricity.

So what does this all mean? Get ready, because the day will come when you can alter the color of your car via some outside catalyst without having to buy a new vehicle or pay for a paint job.

Major applications of shape-memory alloys (SMAs) in automotive manufacturing are making inroads, but they aren't quite ready for the fast lane, at least not in North America, according to Joe Cain, product manager of Nitinol product manufacturing at Johnson Matthey Inc., San Jose, Calif. Nitinol is one type of SMA product.

"We tried for 10 years to develop that market, but exited the business about five years ago. The issue is reliability and understanding the new material," he explained.

"There are few industries that can afford the cost and research required to develop new markets, with the medical field being one," Cain said. "The price (of Nitinol) is extremely high and the content extremely low. People use it by the inch, not by the pound. Our suppliers want to sell us tons and we only need pounds for medical devices."

Cain has met with National Aeronautics and Space Administration officials who have an active program in SMA research, particularly for aircraft noise abatement, using shape-shifting alloys instead of hydraulic systems that require a lot of energy. However, no breakthroughs have been made yet.

"The problem for both aerospace and automotive is that reliability has to be perfect, in addition to the expense, so major applications in both these areas are years away.

So why do companies bother trying to perfect the technology?

"Because it's sexy and something new which will eventually pay off," Cain said. "We've been trying to convince the auto industry to get away from solenoid (electric switch) applications, which weigh a lot and require copper wire and batteries to operate under the hood, and replace them with something one-tenth the weight (Nitinol) that doesn't require electronics."

The potential in automotive is enormous, Cain said, with the Detroit automakers producing about 17 million cars a year that each require two turn signals, one of only several SMA applications, compared with just 200,000 items a year in medical products. "The auto industry is so well understood from a commercial standpoint that if any one of those applications does go commercial, there will be a huge lineup of people wanting to use it.

Dynalloy Inc., Costa Mesa, Calif., which manufactures SMAs used in electrically driven actuators, said there is a lot of research being done by various automotive companies, but it is still in the early stages. Of the 300 applications that Dynalloy has identified, only about 10 percent are on a path to actual use, Jeff Brown, the company's product manager, said. "This is due to the unusual and unique characteristics of the material, which the industry is generally unfamiliar with.

In Dynalloy's experience, SMAs aren't too cost prohibitive for wider commercial use and, in fact, cost savings is a positive factor. When used properly, they are generally less costly than motors or solenoids, he said. "There are a number of interesting new materials, some with unusual magnetic properties, some with unusual temperature properties, and in our experience they rarely overlap and in any given application one clearly performs better than the other."

SMAs come in many forms, including copper-aluminum-nickel, copper-zinc-aluminum, iron-manganese-silicon, nickel-titanium and polymer-based products. But metal-based materials, particularly the nickel-titanium group, have been found to be the most useful.

"From Dynalloy's perspective, as an electrically driven and heat-related material, the metal-based actuators are more appropriate. Generally they have a higher resistance than copper, so they heat easily with resistance heating, whereas polymers are generally non-conductive and difficult to heat electrically," Brown said. "Furthermore, the NiTi-based shape memory alloys are very strong for their size."

One thing helping the market, according to Brown, is a great deal of research into new alloys with higher transition temperatures and narrower temperature hysteresis, which makes the number of applications being explored today much higher than in the past.

There are several automotive applications already utilizing SMA materials in mass production, according to Giorgio Vergani, business development manager of SAES Getters SpA, a Milan, Italy, supplier of advanced materials for scientific and industrial applications. There are, in fact, specific applications where the technical and performance advantages of SMAs result in considerable cost savings.

From a cost perspective, the advantage could be even bigger in other commercial applications for SMAs vs. competitive technologies when comparing the ease of integration, smaller size, higher flexibility and lower weight of the new products.

Major competition at the moment is from established technologies such as direct-current motors, Vergani said, adding that it is always very difficult to switch application engineers to new technologies that they're not familiar with.

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