Engineers model nanoscale crystal dynamics in an easy-to-understand system

Engineers model nanoscale crystal dynamics in an easy-to-understand system

Grain boundaries go with the flow

A Rice University study reconfigures a polycrystalline material that spins in a magnetic field when grain boundaries appear and disappear due to circulation at the interface between the voids. The different colors identify the crystal orientation. Credit: Biswal Research Group / Rice University

Rice University engineers who mimic atomic-scale processes to make them large enough to see have modeled how shear affects the grain boundaries of polycrystalline materials.

That it borders can change so easily was not entirely a surprise to researchers, who used spinning arrays of magnetic particles to see what they suspect is happening at the interface between misaligned crystal domains.

According to Sibani Lisa Biswal, professor of chemical and biomolecular technology at Rice’s George R. Brown School of Engineering, and PhD student and lead author Dana Lobmeyer, interface shear at the crystal-void boundary can really drive the development of microstructures.

The technology reported in The progress of science can help engineers design new and improved materials.

To the naked eye, common metals, ceramics and semiconductors seem uniform and solid. But on a molecular scale, these materials are polycrystalline, separated by defects called grain boundaries. The organization of these polycrystalline aggregates controls such properties as conductivity and strength.

During applied stress, grain boundaries can be formed, reconfigured or even disappear completely to meet new conditions. Although colloidal crystals have been used as model systems to see boundaries move, control their phase transitions has been challenging.

“What sets our study apart is that in the majority of colloidal crystal studies, the grain boundaries are formed and remain stationary,” Lobmeyer said. “They are mainly carved in stone. But with our rotation magnetic fieldthe grain boundaries are dynamic and we can see how they move. “

In experiments, the researchers induced colloids of paramagnetic particles to form 2D polycrystalline structures by spinning them with magnetic fields. As recently shown in a previous studyThis type of system is well suited for visualizing phase transitions that are characteristic of nuclear systems.

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

The new study also shows how their systems follow the long-term Read-Shockley theory of hard condensed matter that predicts the misorientation angles and energies of low-angle grain boundaries, those characterized by a slight skew between adjacent crystals.

By applying one magnetic field on colloidal particlesurged Lobmeyer iron oxide-Embed polystyrene particles to assemble and saw when the crystals formed grain boundaries.

“We usually started with many relatively small crystals,” she said. “After a while, the grain boundaries began to disappear, so we thought it could lead to a single, perfect crystal.”

Instead, new grain boundaries were formed due to shear at the cavity interface. Like polycrystalline materials, these followed the misorientation angle and energy predictions made by Read 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 offers us new ways to design them,” said Biswal. “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 in crystalline materials.”

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

Use electron microscopy and automatic atomic 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, The progress of science (2022). DOI: 10.1126 / sciadv.abn5715

Provided by
Rice University

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