| Date | 7th, Jun 2022 |
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Rice University engineers who mimic atom-scale processes to make them big enough to see have modeled how shear influences grain boundaries in polycrystalline materials.
That the boundaries can change so readily was not entirely a surprise to the researchers, who used spinning arrays of magnetic particles to view what they suspect happens at the interface between misaligned crystal domains.
According to Sibani Lisa Biswal, a professor of chemical and biomolecular engineering at Rice’s George R. Brown School of Engineering, and graduate student and lead author Dana Lobmeyer, interfacial shear at the crystal-void boundary can indeed drive how microstructures evolve.
Graduate student Dana Lobmeyer at the custom rig she used to create macro-scale models of shear-induced grain boundary movement and formation. Image credit: Rice University
The technique reported in Science Advances could help engineers design new and improved materials.
To the naked eye, common metals, ceramics, and semiconductors appear uniform and solid. But at the molecular scale, these materials are polycrystalline, separated by defects known as grain boundaries. The organization of these polycrystalline aggregates governs such properties as conductivity and strength.
Under applied stress, grain boundaries can form, reconfigure, or even disappear entirely to accommodate new conditions. Even though colloidal crystals have been used as model systems to see boundaries move, controlling their phase transitions has been challenging.
“What sets our study apart is that in the majority of colloidal crystal studies, the grain boundaries form and remain stationary,” Lobmeyer said. “They’re essentially set in stone. But with our rotating magnetic field, the grain boundaries are dynamic and we can watch their motion.”
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 study, this type of system is well suited for visualizing phase transitions characteristic of atomic systems.
Here, they saw that gas and solid phases can coexist, resulting in polycrystalline structures that include particle-free regions. They showed these voids act as sources and sinks for the movement of grain boundaries.
