Tracking real-time atomic movement between crystal grains in metals
Observations of grain sliding revealed unexpected movements that dictate mechanical behavior in metals

Date:
March 22, 2022
Source:
Georgia Institute of Technology
Summary:
Using advanced microscopy coupled with novel computer simulations to
track atomic movement, researchers conducted real-time atomic-level
observations of grain boundary deformation in poly-grained
metallic materials called polycrystalline materials. They observed
previously unrecognized processes that affect material properties,
such as atoms that hop from one plane to another across a grain
boundary. Their work pushes the limits of atomic-level probing,
and enables a deeper understanding of how polycrystalline materials
deform, and smarter design of new materials for extreme engineering
applications.



FULL STORY ========================================================================== Metallic materials used in engineering must be strong and ductile --
capable of carrying high mechanical loads while able to withstand
deformation without breaking. Whether a material is weak or strong,
ductile or brittle, however, is not determined simply by the crystal
grains that make up the material, but rather by what happens in the
space between them known as the grain boundary.

Despite decades of investigation, atomic-level deformation processes at
the grain boundary remain elusive, along with the secret to making new
and better materials.


========================================================================== Using advanced microscopy coupled with novel computer simulations
that track atomic movement, researchers at the Georgia Institute of
Technology conducted real-time atomic-level observations of grain boundary deformation in poly- grained metallic materials called polycrystalline materials. The team observed previously unrecognized processes that
affect material properties, such as atoms that hop from one plane to
another across a grain boundary. Their work, published in Science this
March, pushes the limits of atomic-level probing, and enables a deeper understanding of how polycrystalline materials deform. Their work
opens new avenues for the smarter design of new materials for extreme engineering applications.

"It is amazing to observe the step-by-step movements of atoms, and
then use this information to decipher the dynamic sliding process of
a grain boundary with complex structure," said Ting Zhu, professor in
the George W. Woodruff School of Mechanical Engineering and one of the
lead authors on the study, which included collaborators from Beijing
University of Technology.

To develop new and better polycrystalline materials, it is critical
to understand how they deform at an atomic level. The team sought to
achieve real- time observation of grain boundary sliding, a well-known
mode of deformation which plays an important role in governing the
strength and ductility of polycrystalline materials. They chose to work
with platinum because its crystal structure is the same as other widely
used polycrystalline materials like steel, copper, and aluminum. Using platinum, their results and insights would be generally applicable to
a wide range of materials.

A Combination of Novel Methods Several key innovations were required to
carry out the experiment. The team used a transmission electron microscope (TEM) to capture highly magnified images of atoms at grain boundaries. The
TEM sends an electron beam through a film-like platinum specimen,
processed by the team to be thin enough for electron transmission. They
also developed a small, millimeter-sized testing device that applies
mechanical force to a specimen and is affixed to the microscope. The
TEM and device work in tandem to create atomic-level images of grain
boundaries during deformation.



==========================================================================
To observe the atomic-scale grain boundary sliding more clearly than
through viewing the TEM images alone, the researchers developed an
automated atom tracking method. This method automatically labels each
atom in every TEM image and then correlates them between images,
enabling the tracking of all atoms and their movement during grain
boundary sliding. Finally, the team conducted computer simulations of
grain boundary sliding using atomic structures extracted from the TEM
images. The simulated sliding helped the team analyze and interpret
events that happened at the atomic scale. By combining those methods,
they were able to visualize how individual atoms move at a deforming
grain boundary in real time.

Results While it was known that grain boundaries slide during deformation
of polycrystalline materials, real-time imaging and analysis by Zhu
and his team revealed a rich variety of atomic processes, some of them previously unknown.

They noticed that, during deformation, two neighboring grains slid against
each other and caused atoms from one side of the grain boundary plane to transfer to the other. This process, known as atomic plane transfer, was previously unrecognized. They also observed that local atomic processes
can effectively accommodate transferred atoms by adjusting grain boundary structures, which can be beneficial for achieving higher ductility. Image analysis and computer simulations showed that mechanical loads were high
during the atomic processes, and that this facilitated the transfer of
atoms and atomic planes. Their findings suggest that engineering the
grain boundaries of fine-grained polycrystals is an important strategy
for making materials stronger and more ductile.

Looking Ahead Zhu and his team's demonstrated ability to observe, track,
and understand atomic-scale grain boundary deformation opens more research opportunities to further investigate interfaces and failure mechanisms
in polycrystalline materials. Greater understanding of atomic-level
deformation can inform how materials are evolved during grain boundary engineering, a necessity for creating exceptional strength and ductility combinations.



==========================================================================
"We are now extending our approach to visualize atomic-scale deformation
at higher temperatures and deformation rates, in pursuit of better
materials for extreme applications," said Xiaodong Han, another lead
author of the paper and a professor at the Beijing University of
Technology.

Zhu believes that the data-rich results from their real-time atomic-level observations and imaging could be integrated with machine learning for
deeper investigation of material deformations, and this could accelerate
the discovery and development of materials faster than previously
thought possible.

"Our work shows the importance of using very high-resolution microscopy
to understand atomic-level material behavior. This advancement will
enable researchers to tailor materials for optimal properties using
atomic design," said Zhu.

Funding: X.D.H. and L.W. acknowledge support by the Beijing Outstanding
Young Scientists Projects (grant BJJWZYJH01201910005018), the Basic
Science Center Program for Multiphase Evolution in Hypergravity of
the National Natural Science Foundation of China (grant 51988101),
the Beijing Natural Science Foundation (grant Z180014), and the Natural
Science Foundation of China (grants 51771004, 51988101, and 91860202).


========================================================================== Story Source: Materials provided by Georgia_Institute_of_Technology. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Lihua Wang, Yin Zhang, Zhi Zeng, Hao Zhou, Jian He, Pan Liu, Mingwei
Chen, Jian Han, David J. Srolovitz, Jiao Teng, Yizhong Guo,
Guo Yang, Deli Kong, En Ma, Yongli Hu, Baocai Yin, XiaoXu Huang,
Ze Zhang, Ting Zhu, Xiaodong Han. Tracking the sliding of grain
boundaries at the atomic scale. Science, 2022; 375 (6586): 1261
DOI: 10.1126/science.abm2612 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220322182659.htm

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