01
The same grain boundary carries two distinct diffusion branches — a fast one and a slow one — rather than a single effective rate.
02
The split follows the boundary's facet structure: geometry and local defect content, not composition, set the transport rate.
03
Cr segregation and Cr transport are coupled, so the boundary chemistry evolves as diffusion proceeds.
04
Bicrystal rates bracket the scatter reported for Ni polycrystals, explaining that scatter as boundary-to-boundary variation.
This study used a high-purity Ni bicrystal as a model system to understand how chromium atoms move through a defined Σ11 grain boundary. By combining radiotracer diffusion, EBSD/OIM, SIMS comparison, DFT-supported segregation analysis and atomistic interpretation, the work showed that even one grain boundary can contain different structural segments that act as distinct diffusion pathways.
Why a model bicrystal
In a polycrystal, every measured diffusion coefficient is an average over thousands of boundaries with different characters. A bicrystal removes that averaging: one boundary, one known misorientation, one measurable transport path. That is what makes it possible to attribute a transport signature to a specific boundary structure rather than to a statistical population.
What we found
The Σ11{110} boundary is not structurally uniform along its length. It facets, and those facets — the 'defects of defects' — carry measurably different chromium mobilities. Grain-boundary segregation, supported by DFT, explains why: the local atomic environment at each facet sets both the site energy for Cr and the available migration path.
