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?
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
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.
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.
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
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.
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
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.
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.