Two-step grain boundary diffusion mechanism of a dopant accompanied by structural transformation

Dopant diffusion along grain boundaries (GBs) plays a critical role in modulating the GB chemistry, which further governs the microstructures and properties of polycrystalline materials. Here, we have investigated atomistic GB dopant diffusion behaviors by directly tracing GB structures and chemistries in a Ti-diffused Al ₂ O ₃ GB, using atomic resolution electron microscopy, spectroscopy and theoretical calculations. Our observations unveiled that the dopant diffusion has induced a GB structural transformation. Furthermore, such structural transformation led to an unexpected dramatic variation of GB diffusion coefficients for Ti diffusion, which differ by two orders of magnitude in the two different GB structures. These findings provide mechanistic insights into the dopant diffusion and segregation phenomena in GBs, which renews our fundamental understanding towards the intricate nature of GB diffusion processes.