Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

New Light on How Metals Change Shape at the Nanoscale

03.08.2004


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.



Theorists have proposed that when grain sizes are too small for dislocations, a different mode of deformation comes into play: the grain boundaries themselves move, sliding past one another and allowing the grains to rotate to find new ways of fitting together.

"It’s a simple idea," says Zhiwei Shan of Mao’s laboratory at Pitt, "and many groups have researched aspects of it, but no one has reported direct evidence of a shift from dislocation-mediated deformation to grain-boundary-mediated deformation." Indeed, no one was sure where to look for the transition from one mode of deformation to the other. When the grains were reduced to 20 nanometers across? Ten? Perhaps as small as five?

To search for the effect, Shan used NCEM’s In-Situ Microscope, which he calls "the best in America" for this kind of research. NCEM’s Eric Stach explains that what makes the In-Situ’s otherwise standard transmission electron microscope unique is that it combines a stage area in which samples can be stressed or manipulated in other ways — and meanwhile videotaped — with a high voltage, 300-kilovolt electron beam that can penetrate thick samples and yield excellent 1.9-angstrom resolution.

The nanocrystalline nickel samples were mounted in a probe that placed them under load — stretched them, in fact — while images of small regions of the sample were captured on videotape at the standard rate of 30 frames per second.

But besides having an excellent instrument, says Stach, Zhiwei Shan made a crucial observation. An effect that was far from obvious in the most common TEM imaging method, called bright-field imaging, stood out clearly with the different technique of dark-field imaging.

"As the TEM’s electron beam passes through a sample, some of the electrons are diffracted," Stach explains. "Bright-field images are constructed using the direct electrons, while dark-field images use the diffracted electrons. In bright-field imaging, regions of the sample that scatter a lot of electrons, like defects such as dislocations, look darker. With dark-field images, strongly diffracting regions look brighter."

Shan agrees that "dark-field imaging was critical to the result." For when he viewed videotapes of the nickel sample under strain, he saw small regions rapidly brightening and growing larger — direct confirmation of grains sliding and rotating into positions of strong diffraction.

In a bright-field image these grain-boundary processes would have been impossible to distinguish from lattice dislocations, which in prior attempts is what other groups assumed they were seeing. It took dark-field observations to confirm that below a certain size, grain-boundary rotation indeed becomes prominent. The cut-off isn’t sharp, however.

"It’s continuous, not a sharp change," says Shan. "In describing grain-boundary deformations we chose the word ’prominent’ carefully, because even in nanocrystalline metal, dislocations still play a role." Trapped dislocations in the crystal lattice were observed even when the average grain size was as small as 10 nanometers.

Says Stach, "The material always chooses the easiest pathway to deform, and that can differ through a range of sizes." Although the In-Situ Microscope observations confirm the grain-boundary model of nanocrystalline deformation, whichever process predominates at a given grain size depends on a variety of conditions.

"Grain boundary-mediated plasticity in nanocrystalline nickel," by Zhiwei Shan, Eric A. Stach, Jörg M. K. Wiezorek, James A. Knapp, David M. Follstaedt, and Scott X. Mao, appears in the July 30, 2004 issue of Science.

The Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.

Paul Preuss | EurekAlert!
Further information:
http://www.lbl.gov

More articles from Process Engineering:

nachricht No compromises: Combining the benefits of 3D printing and casting
23.03.2018 | Fraunhofer-Institut für Produktionstechnik und Automatisierung IPA

nachricht Intelligent wheelchairs, predictive prostheses
20.12.2017 | Fraunhofer-Institut für Produktionstechnik und Automatisierung IPA

All articles from Process Engineering >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Spider silk key to new bone-fixing composite

University of Connecticut researchers have created a biodegradable composite made of silk fibers that can be used to repair broken load-bearing bones without the complications sometimes presented by other materials.

Repairing major load-bearing bones such as those in the leg can be a long and uncomfortable process.

Im Focus: Writing and deleting magnets with lasers

Study published in the journal ACS Applied Materials & Interfaces is the outcome of an international effort that included teams from Dresden and Berlin in Germany, and the US.

Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) together with colleagues from the Helmholtz-Zentrum Berlin (HZB) and the University of Virginia...

Im Focus: Gamma-ray flashes from plasma filaments

Novel highly efficient and brilliant gamma-ray source: Based on model calculations, physicists of the Max PIanck Institute for Nuclear Physics in Heidelberg propose a novel method for an efficient high-brilliance gamma-ray source. A giant collimated gamma-ray pulse is generated from the interaction of a dense ultra-relativistic electron beam with a thin solid conductor. Energetic gamma-rays are copiously produced as the electron beam splits into filaments while propagating across the conductor. The resulting gamma-ray energy and flux enable novel experiments in nuclear and fundamental physics.

The typical wavelength of light interacting with an object of the microcosm scales with the size of this object. For atoms, this ranges from visible light to...

Im Focus: Basel researchers succeed in cultivating cartilage from stem cells

Stable joint cartilage can be produced from adult stem cells originating from bone marrow. This is made possible by inducing specific molecular processes occurring during embryonic cartilage formation, as researchers from the University and University Hospital of Basel report in the scientific journal PNAS.

Certain mesenchymal stem/stromal cells from the bone marrow of adults are considered extremely promising for skeletal tissue regeneration. These adult stem...

Im Focus: Like a wedge in a hinge

Researchers lay groundwork to tailor drugs for new targets in cancer therapy

In the fight against cancer, scientists are developing new drugs to hit tumor cells at so far unused weak points. Such a “sore spot” is the protein complex...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Invitation to the upcoming "Current Topics in Bioinformatics: Big Data in Genomics and Medicine"

13.04.2018 | Event News

Unique scope of UV LED technologies and applications presented in Berlin: ICULTA-2018

12.04.2018 | Event News

IWOLIA: A conference bringing together German Industrie 4.0 and French Industrie du Futur

09.04.2018 | Event News

 
Latest News

Magnetic nano-imaging on a table top

20.04.2018 | Physics and Astronomy

Start of work for the world's largest electric truck

20.04.2018 | Interdisciplinary Research

Atoms may hum a tune from grand cosmic symphony

20.04.2018 | Physics and Astronomy

VideoLinks
Science & Research
Overview of more VideoLinks >>>