Grain Boundary Motion in Two-Dimensional Hexagonal Boron Nitride

An in-depth understanding and precise controlling of grain boundary (GB) motion at the atomic scale are crucial for grain growth and recrystallization in polycrystalline materials. So far, the reported studies mainly focus on the GB motion in the ideal bicrystal system, while the atomic mechanisms of GB motion in polycrystals remain poorly understood. Herein, taking two-dimensional (2D) hexagonal boron nitride (h-BN) as a model system, we experimentally investigated the atomic-scale mechanisms of the GB motion in 2D polycrystals. Since GB motion is directly related to the GB structures, this article is organized following the configurations of GBs, which can be divided into straight (including symmetric and asymmetric GBs) and curved GBs. We revealed that (I) for symmetric GBs, the shear-coupled motion alone is insufficient to drive the continuous GB motion in polycrystalline materials, and GB sliding is also needed. (II) For asymmetric GBs, GB motion follows a defaceting-faceting process, in which dislocation reactions are crucial. (III) For curved GBs, shear-coupled GB motion (during grain shrinking) leads to grain rotation, and the rotation direction highly depends on the misorientation angles. (IV) Finally, we will discuss the characteristics of binary lattice h-BN and find that partial dislocations participate in the GB motion at high misorientation angles (>38°). Our results build up the framework of the atomic-scale mechanisms of the GB motion in 2D polycrystalline materials and will be instructive for technological applications such as grain growth and GB engineering.