Builders and engineers must often choose between materials that are strong and those that are flexible—rarely will they find a substance with both properties in abundance. Researchers are trying to change this through the development of "shape metal" alloys that are strong enough to resist high levels of strain while also being flexible enough to recover their original shape when a certain amount of heat is applied.

Yuuki Tanaka and other researchers at Japan's Tohoku University report in Friday's issue of Science having discovered an iron-based shape metal alloy that shows an almost full recovery of shape change even when subjected to nearly twice the strain levels endured by shape metal alloys currently used in some cellular antennas, eyeglass frames and medical devices. The researchers report their alloy's strength is comparable with that of high-strength industry alloys.

Strong alloys that return to their original shape when heated take advantage of solid-to-solid "diffusionless" phase transitions, according to Texas A&M University mechanical engineering associate professor Ibrahim Karaman and Ji Ma, a Ph.D. candidate working in Karaman's Microstructural Engineering of Structural and Active Materials Research Group. In a commentary on the Tohoku University research (also in Friday's Science), Karaman and Ma write that this means that "atoms rearrange how they pack into crystals in an orderly fashion, and this process changes the material's macroscopic shape." The researchers also point out that the "superelastic" alloy created by Yuuki Tanaka and colleagues "almost doubles the useful range of deformation that can be induced in such alloys."

Superelasticity describes an important property exhibited by shape memory alloys. When strain is removed from the material (and a certain amount of heat is added), it recovers its original shape.

To this point, shape-memory alloys have mostly been used for implanted medical devices, such as stents that can expand to a wider diameter after being placed in an artery and warmed by the body. The iron alloy developed at Tohoku University may allow for smaller devices or greater ranges of motion, Karaman and Ma conclude.

The new alloy combines "high strength and superelastic strain, good ductility, high damping capacity, and mechanical-magnetic coupling," wrote the Texas A&M researchers, who were not involved in developing the material. This means that other uses for improved superelastic alloys might include blast protection, vibration isolation and noise reduction. In addition, measurements of strain on the alloy as it changes shape might someday be used to create a sensor that indicates when a stent is failing or a bridge or other structure is under too much stress.

Of course, all of these potential applications depend upon the researchers finding a way to repeatedly and economically reproduce such an alloy with consistent properties.

Image © Dan Moore