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

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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 investigate atomistic GB dopant diffusion behaviors by directly tracing GB structures and chemistries in a Ti-diffused Al2O3 GB, using atomic resolution electron microscopy, spectroscopy and theoretical calculations. Our observations unveil that dopant diffusion introduces a GB structural transformation. Furthermore, such structural transformation leads to an unexpected dramatic variation of GB diffusion coefficients for Ti diffusion, which differ by one order of magnitude between the two different GB structures. These findings provide mechanistic insights into the dopant diffusion and segregation phenomena in GBs, providing fundamental understanding towards the intricate nature of GB diffusion processes.