Frames from a dark-field TEM video of nanocrystalline nickel under strain show rapid aggregation of a group of grains.
A nanocrystalline metal is one whose average grain size is measured in billionths of a meter, much smaller than in most ordinary metals. As the grain size of a metal shrinks, it can become many times stronger, but it also usually loses ductility. To take advantage of increasing strength with decreasing grain size, researchers must first understand a fundamental problem: by what processes do nanosized crystals of metal stretch, bend, or otherwise deform under strain?
A team of researchers headed by Scott X. Mao of the Mechanical Engineering Department of the University of Pittsburgh, working at the National Center for Electron Microscopy (NCEM) at the Department of Energy’s Lawrence Berkeley National Laboratory, and using high-quality samples of nickel prepared at DOE’s Sandia National Laboratories, has now identified a prominent way in which nanocrystalline metals deform. The researchers report their findings in the July 30, 2004 issue of Science.
Ordinary coarse-grained metals deform when parts of a grain slip past one another as extra planes of atoms, called dislocations, move through the material. The process has been compared to moving a rug by flapping one end of it to create a wave, causing the rug to inch along bit by bit. But the trick won’t work if the rug is too short; likewise, if the dimensions of the crystal grains are too small, dislocations can’t be created or glide through the grain to allow deformation.
Paul Preuss | EurekAlert!
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