LAUSR

Phase transition is a physical phenomenon that attracts great interest of researchers. Although the theory of second‐order phase transitions is well‐established, their atomic‐scale dynamics in polycrystalline materials remains elusive. In this work, second‐order phase transitions in polycrystalline Cu2Se at the transition temperature are directly observed by in situ aberration‐corrected transmission electron microscopy. Phase transitions in microcrystalline Cu2Se start at the grain boundaries and extend inside the grains. This phenomenon is more pronounced in nanosized grains. Analysis of phase transitions in nanocrystalline Cu2Se with different grain boundaries demonstrates that grain boundary energy dominates unsynchronized phase transition behavior. This suggests that the energy of grain boundaries is the key factor influencing the energetic barrier for initiation of phase transition. The findings advance atomic‐scale understanding of second‐order phase transitions, which is crucial for the control of this process in polycrystalline materials.