Atomic-scale observations of distinct segregation behaviors driven by site anisotropy in a near-Σ3 grain boundary

The segregation of dopants at grain boundaries (GB) affects the bulk mechanical and electrical properties, as well as the microstructure evolution in polycrystalline materials. The structure and atomic arrangements at GBs play a crucial role in dictating the segregation behaviors. However, the effect of different GB arrangements within a single GB on the segregation behavior is not well understood yet. For this reason, a near-Σ3 twist boundary was fabricated to introduce GB-site anisotropy using magnesium aluminate spinel single crystals doped with yttrium (Y). The small deviation angle from the exact Σ3 configuration introduced a network of secondary GB dislocations identified as Shockley partials (1/6<112>). Detailed atomic-resolution imaging on conventional cross-sectional projection, as well as a pseudo-plan-view projection, was carried out to directly observe the Y segregation. Three distinct Y segregation behaviors were identified within a single GB: ordered (between dislocation network), disorder (near dislocations), and negligible Y segregation (remaining regions between the dislocation network). Atomic-resolution imaging and atomic configuration models of the GB revealed that the distinct segregation behaviors were correlated to changes in the local atomic configurations, especially the arrangement of O atoms. The ordered Y segregation was spatially correlated to coherent and symmetric segments of the boundary with larger excess free volumes. The regions with no Y segregation presented an O atom arrangement similar to that in the bulk lattice, where the limited free volume hindered the Y segregation. These results provided experimental evidence of how different atomic arrangements led to distinct segregation behaviors within a single GB.