Superplastic metals stretch to a new low.

When heated to the right temperature, some metals can be drawn out like a string of taffy. Scientists have now found that this behavior, known as superplasticity, can occur at temperatures hundreds of degrees lower than expected.

This is good news for manufacturers, such as car makers, interested in making strong, lightweight parts from these stretchy metals. Superplastic metals can be molded almost like plastics themselves.

"You can make very complex shapes without machining," says Amiya K. Mukherjee of the University of California, Davis. He and his colleagues at Davis and the Ufa State Aviation Technical University in Russia report their findings in the April 22 NATURE.

The key to achieving superplasticity at low temperatures lies in a material's atomic structure. The researchers tested thumbnail-size samples of three metals: pure nickel, nickel aluminide, and a commercial aluminum alloy. They processed the metals by repeatedly twisting them under high pressure, breaking their internal structures into grains only a few nanometers in size.

The pure nickel ended up with a grain size of 20 nm, showing superplasticity between 280 [degrees] C and 350 [degrees] C, a little over one-third of its melting temperature. This result "contradicts the current opinion that a temperature of at least [half] of the absolute melting temperature is required for superplastic flow," says Terence G. Langdon of the University of Southern California in Los Angeles. The two alloys likewise became superplastic at lower than expected temperatures.

In a solid with a grain size of 20 nm, almost 30 percent of the atoms take on a noncrystalline structure, says Mukherjee. They lie along the edges of the grains, where "the chemical and physical properties are very different," he notes. "These atoms move and diffuse very fast" to fill in gaps opened by stretching. The process keeps the material from breaking, leading to superplasticity.

Only recently have materials scientists been able to make metals with such fine grains. Commercially available superplastic metals have grains at least 100 times as large.

Processing superplastic metals at lower temperatures means lower energy costs and less wear on tools. The time and expense of current commercial techniques for making superplastics have limited their use to special applications such as complex airplane parts and turbine engines.

"There is considerable interest in expanding the technology into a much wider industrial field--for example, the automotive industry," says Langdon. Automakers such as Lamborghini already make parts from superplastic metals, which is "OK for a $150,000 car, but not for a $20,000 one," he observes.

In addition to cost savings, such nanostructured metals provide exceptional strength at ambient temperature, says Mukherjee. These materials can be used to make strong but lightweight biomedical gear such as prosthetics or braces.

"This is opening a new, exciting window in the very active field of superplasticity research," says David C. Dunand of Northwestern University in Evanston, Ill.