Engineers model nanoscale crystal dynamics in easy-to-observe systems

Grain boundaries move with the flow

In a Rice University study, a polycrystalline material spinning in a magnetic field is reconfigured as grain boundaries appear and disappear due to circulation at the vacancy interface. The different colors identify the crystal orientation. credit: Biswal Research Group/Rice University

Engineers at Rice University mimicking atomic-scale processes have made them large enough to see how shear affects grain boundaries in polycrystalline materials.

That boundaries can change so easily, it was not entirely surprising to the researchers, who used spinning arrays of magnetic particles to see what they suspected happens at the interface between miscible crystal domains. .

How interfacial shear at the crystal-zero limit can actually develop microstructures, according to Sibani Lisa Biswal, professor of chemical and biomolecular engineering at Rice’s George R. Brown School of Engineering, and graduate student and lead author Dana Lobmeyer.

reported in technology science advance Can help engineers design new and better materials.

To naked eyeCommon metals, ceramics and semiconductors appear uniform and solid. But on a molecular scale, these materials are polycrystalline, with defects known as grain boundaries, The organization of these polycrystalline aggregates controls properties such as conductivity and strength.

Under applied stress, grain boundaries can form, reconfigure, or disappear entirely to accommodate the new conditions. even though colloidal crystal Seeing boundaries as moving, has been used as a model system to control their phase transition has been challenging.

“What sets our study apart is that in most colloidal crystal studies, grain boundaries form and remain stable,” Lobmeyer said. “They’re essentially set in stone. But with our rotation Magnetic FieldGrain boundaries are dynamic and we can see their movement.”

In the experiments, the researchers induced colloids of paramagnetic particles to form 2D polycrystalline structures by spinning them with magnetic fields. as Recently a previous study showedThis type of system is suitable for visualizing the characteristic phase transitions of atomic systems.

Here, they observed that the gas and solid phases can coexist, resulting in polycrystalline structures that include particle-free regions. They showed that these vacancies act as sources and sinks for the movement of grain boundaries.

New study also shows how long their system has been Read-Shockley Principle Hard condensed material that predicts incorrect orientation angles and energies of low-angle grain boundaries, characterized by a small misalignment between adjacent crystals.

by applying Magnetic Field Feather colloidal particlesLobmeyer inspired iron oxideSaw to collect -embedded polystyrene particles and form grain boundaries in the form of crystals.

“We usually started with several relatively small crystals,” she said. “After some time, the grain boundaries began to disappear, so we thought it might lead to a single, complete crystal.”

Instead, new grain boundaries are formed due to shearing at the zero interface. Similar to polycrystalline materials, these followed the incorrect direction angle and energy predictions made by Reed and Shockley more than 70 years ago.

“Grain boundaries have a significant impact on the properties of materials, so understanding how voids can be used to control crystalline materials gives us new ways to design them,” Biswal said. “Our next step is to use this tunable colloidal system to study annealing, a process that involves multiple heating and cooling cycles to remove defects within the crystalline material.”

The National Science Foundation (1705703) supported the research. Biswal is the William M. McCardell Professor of Chemical Engineering, Professor of Chemical and Biomolecular Engineering and Materials Science and Nanoengineering.

Using electron microscopy and automated atom-tracking to learn more about grain boundaries in metals during deformation

more information:
Dana M. Lobmeyer et al, Grain boundary dynamics driven by magnetically induced circulation at the void interface of 2D colloidal crystals, science advance (2022). DOI: 10.1126/sciadv.abn5715

provided by
Rice University

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