Model system puts grain boundaries in a spin

Engineers at Rice University who mimic atom-scale processes to make them big enough to see have modeled how shear influences grain boundaries in polycrystalline materials. Their finding that these boundaries can change 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 Dana Lobmeyer, interfacial shear at the crystal-void boundary can drive how microstructures evolve. They report their findings and the new model system, which could help engineers design new and improved materials, in a paper in Science Advances.

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 a material’s 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 these 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.”

The researchers induced colloids of paramagnetic particles to form 2D polycrystalline structures by spinning them with magnetic fields. As shown in a previous study, this type of system is well suited for visualizing the phase transitions characteristic of atomic systems.

In this study, the researchers saw that gas and solid phases can coexist, producing polycrystalline structures that include particle-free regions, or voids. They showed that these voids act as sources and sinks for the movement of grain boundaries.

The new study also demonstrated how their model system follows the long-standing Read-Shockley theory of hard condensed matter. This theory predicts the misorientation angles and energies of low-angle grain boundaries, which are characterized by a small misalignment between adjacent crystals.

By applying a magnetic field to the colloidal particles, Lobmeyer prompted the iron-oxide-embedded polystyrene particles to assemble and then watched as the crystals formed grain boundaries.

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

Instead, new grain boundaries formed due to shear at the void interface. Similar to polycrystalline materials, these grain boundaries 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,” 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 crystalline materials.”

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.