Observations of grain sliding have revealed unexpected motions that dictate the mechanical behavior of metals

Metallic materials used in engineering must be strong and ductile – able to withstand high mechanical loads while being able to resist deformation without breaking. However, whether a material is weak or strong, ductile or brittle is not simply determined by the crystal grains that make up the material, but rather by what happens in the space between them, called the grain boundary. . Despite decades of investigation, atomic-level deformation processes at the grain boundary remain elusive, as does the secret to making new and better materials.

Using advanced microscopy coupled with new computer simulations that track atomic motion, researchers at the Georgia Institute of Technology have made real-time, atomic-level observations of grain boundary deformation in metallic materials at multiple grains called polycrystalline materials. The team observed previously unrecognized processes that affect material properties, such as atoms jumping from one plane to another across a grain boundary. Their work, published in Science this March, pushes the boundaries of atomic-level probing and provides a better understanding of how polycrystalline materials deform. Their work opens new avenues for the smarter design of new materials for extreme engineering applications.

“It’s amazing to observe the stepping motions of atoms and then use that information to decipher the dynamic sliding process of a structurally complex grain boundary,” said Ting Zhu, professor at the George W. Woodruff School of Mechanical. Engineering and one of the lead authors of the study, which included collaborators from Beijing University of Technology.

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

A combination of new methods

Several key innovations were required to complete the experiment. The team used a transmission electron microscope (TEM) to capture highly magnified images of atoms at grain boundaries. The TEM sends a beam of electrons through a film-like specimen of platinum, which the team has treated to be thin enough for electron transmission. They also developed a small millimeter test device that applies mechanical force to a specimen and is attached to the microscope. The TEM and device work in tandem to create atomic-level images of grain boundaries during deformation.

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

Results

While grain boundaries have been known to slip during the deformation of polycrystalline materials, real-time imaging and analysis by Zhu and his team revealed a rich variety of atomic processes, some of which were hitherto unknown. unknown.

They noticed that, during deformation, two neighboring grains slide against each other and cause the transfer of atoms from one side of the grain boundary plane to the other. This process, known as atomic plane transfer, was previously unrecognized. They also observed that local atomic processes can efficiently adapt to transferred atoms by adjusting grain boundary structures, which can be beneficial in achieving higher ductility. Image analysis and computer simulations have shown that mechanical loads are high during atomic processes, and this facilitates 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.

Look forward

Zhu and his team’s demonstrated ability to observe, track, and understand grain boundary deformation at the atomic scale opens up more research opportunities to further investigate interfaces and failure mechanisms in polycrystalline materials. A better understanding of deformation at the atomic level can shed light on how materials evolve during grain boundary engineering, a necessity for creating combinations of exceptional strength and ductility.

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

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

“Our work shows the importance of using very high resolution microscopy to understand the behavior of materials at the atomic level. This breakthrough will allow researchers to tailor materials for optimal properties using atomic design,” Zhu said.

Funding: XDH and LW acknowledge the support of the Beijing Outstanding Young Scientists Projects (grant BJJWZYJH01201910005018), the Basic Science Center for Multiphase Evolution in Hypergravity program 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).

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