Probing Grain Boundary Dynamics in Polycrystalline Materials by In-SituTEM

Understanding grain boundary (GB) dynamics in polycrystalline materials is crucial for predicting their macroscopic properties, such as mechanical strength, ductility, and conductivity. The migration and interaction of GBs are key factors affecting the thermal and mechanical stability of these materials. In situ transmission electron microscopy (TEM) plays an important role in investigating GB dynamics at the atomic scale. However, the lack of statistics for atomic scale study presents challenges in uncovering the predominance of multiple mechanisms in certain dynamic processes. This study employs cutting-edge four-dimensional transmission electron microscopy (4D-STEM) techniques for <i>in situ</i> experiments to shed light on the mechanisms of grain boundary migration at the microstructural and statistical level. <br/>In the first part of the study, an <i>in situ</i> 4D-STEM experiment was conducted on a Pt polycrystal thin film sample at an elevated temperature. The datasets were cyclically collected in the same region of the sample after predetermined annealing times. Crystallographic orientation information was derived from the 4D-STEM datasets through cross-correlation between the experimental and simulated diffraction patterns of the Pt sample. By utilizing grain segmentation and inter-frame association, the evolution of grain orientation and GB dynamics was traced. The observations revealed ubiquitous grain rotation and GB migration during the annealing process, as well as a strong correlation between GB migration and grain rotation. Subsequently, atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observations of grain boundary migration were conducted on the Pt sample. These observations at the atomic scale demonstrated that GB migration occurs through disconnection propagation, leading to the induction of local shear strain. This shear strain accumulates near the GB and needs to be released before further migration can occur. The findings from this observation indicate that grain rotation and annealing twin formation serve as effective pathways for releasing the strain generated by shear-coupled GB migration. This comprehensive study provides valuable insights into the complex mechanisms governing grain boundary migration in polycrystalline materials, contributing to our understanding of their macroscopic properties and potential applications in material science and engineering.