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A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 1 5
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SUCCESS ANALYSIS
SPECIMEN: TUNGSTEN NANOCRYSTALS
VITAL STATISTICS
Researchers at the University of Pittsburgh, Drexel Uni-
versity, and the Georgia Institute of Technology designed a
new way to study atomic-scale deformation mechanisms,
revealing an interesting phenomenon in tungsten. The group
is reportedly the first to observe atomic-level deformation
twinning in bcc tungsten nanocrystals. Twinning has been
observed in large-scale bcc metals and alloys during deforma-
tion. However, whether twinning occurs in bcc nanomaterials
was unknown until now.
SUCCESS FACTORS
Observation of atomic-scale twinning was made inside
a transmission electron microscope (TEM). This kind of study
was previously impossible because TEM imaging requires
samples less than 100 nm in size and it is difficult to make
bcc samples that small. Graduate student Jiangwei Wang and
Professor Scott Mao, both at the University of Pittsburgh, de-
signed a novel way of making bcc tungsten nanowires. Under
a TEM, two small pieces of individual nanoscale tungsten crys-
tals were welded together, creating a wire about 20 nm in di-
ameter. This wire was durable enough to stretch and compress
while the twinning phenomenon was observed in real-time
using a high-resolution TEM.
Next, Christopher Weinberger, an assistant professor at
Drexel University, developed computer models that show the
mechanical behavior of the tungsten nanostructure at the
atomic level. Along with Weinberger’s modeling, Ting Zhu,
an associate professor at Georgia Tech, conducted advanced
computer simulations, using molecular dynamics to study
deformation processes in 3D. Zhu’s simulation reveals that
tungsten’s strength behavior is not without problems. “If you
reduce the size to the nanometer scale, strength can be in-
creased by several orders or magnitude,” says Zhu. “But there
is a dramatic decrease in the ductility. We want to increase
strength without compromising ductility, so we need to under-
stand the controlling deformation mechanisms.”
ABOUT THE INNOVATORS
Scott Mao is a professor in the Swanson School of Engi-
neering at the University of Pittsburgh and Jiangwei Wang is
a graduate student. Christopher Weinberger is an assistant
professor in Drexel’s College of Engineering and Ting Zhu is an
associate professor in the Woodruff School of Mechanical En-
gineering at Georgia Tech.
WHAT’S NEXT
Results should lead to further investigation of deforma-
tion mechanisms in nanoscale metals and alloys, ultimately
enabling the design of nanostructured materials to fully real-
ize their latent mechanical strength, say researchers.
Contact Details
Scott Mao, University of Pittsburgh
Swanson School of Engineering
3700 O’Hara St.
Pittsburgh, PA 15261
412.624.9602,
Ting Zhu of Georgia Tech, worked with colleagues at the University
of Pittsburgh and Drexel University to develop a better under-
standing of a key deformation mechanism in nanoscale tungsten.
Courtesy of Candler Hobbs.
The computer model (left) and experimental image (right) reveal
atomic-level deformation twinning in a tungsten nanowire under
axial compression. The lattice of the deformation-induced twin
band (between yellow lines) is a mirror image of that of the parent
crystal. Courtesy of Ting Zhu.