Metal Manipulation: Technique yields hard but stretchy materials
Metalworking is an ancient craft, with time-tested practices that go back thousands of years. But now, working within the modern context of nanotechnology, researchers have found a way to make strong yet stretchy metals. Metallurgists might eventually incorporate such improved materials into countless applications, from micromechanical machines to biomedical implants.
In the Oct. 31 Nature, En (Evan) Ma and his colleagues at Johns Hopkins University in Baltimore report that they’ve combined a standard metalworking technology–rolling–with a programmed sequence of cooling and heating steps to process copper into a form that contains both nanoscale and microscale crystal grains. The resulting material has six times the strength of unprocessed copper yet retains most of the metal’s characteristic ductility, or stretchiness.
In the 1980s, materials scientists discovered that solid metals are stronger when their crystalline grains are on the order of 100 nanometers or so in diameter. Unfortunately, nanocrystalline materials’ superlative strength is usually accompanied by very low ductility.
Instead of accepting this apparent tradeoff, Ma and his colleagues sought a compromise.
About 75 percent of their material’s grains are just a couple of hundred nanometers wide, enhancing the metal’s strength. The rest of the copper’s crystals grow into larger, microscale grains. This combination gives the copper good ductility, says Ma.
The results are “really great,” comments Julia Weertman of Northwestern University in Evanston, Ill. “You have a good increase in strength without having to sacrifice the ductility.”
To give their material its unusual internal structure, the researchers first cooled a 1-inch cube of copper to nearly –200C and then passed the frigid cube between heavy rollers, flattening it to about 1 millimeter in thickness. Then they heated the resulting sheet to 200C for 3 minutes before returning it to room temperature.
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This procedure is much easier than many methods for making nanocrystalline materials, which rely on compacting powders of nanoscale particles, says Ma. He and his coworkers now plan to repeat their experiments with other pure metals and with alloys.
The new process ought to be applicable to these materials, comments Ruslan Valiev of the Institute of Physics of Advanced Materials at Ufa State Aviation Technical University in Russia. “It is important to check this experimentally, and I hope it will be done soon,” he says.
A strong, pure metal processed with the new technique might even replace alloys for some applications, suggests Ma. In biomedical implants, for instance, a pure metal might reduce the likelihood of corrosion or adverse reactions to a metal mixture, he says.
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