Atomic-scale structure and chemistry of a twin boundary complexion in a ternary Mg alloy

Grain-boundary complexions critically influence interfacial migration and the macroscopic properties of polycrystalline materials. A spinal-like periodic segregation structure has been reported at coherent twin boundaries in Mg-RE-Ag alloys, but its identity remained unresolved due to indistinguishable RE and Ag atomic columns in Z-contrast imaging. Using atomic-resolution chemical mapping in an aberration-corrected transmission electron microscope, we directly resolve its atomic configuration. The structure matches the equilibrium δ phase (NdAgMg 11 ) but lacks a complete unit cell, and is therefore identified as a δ-phase-like complexion. The complexion contains a (110) δ twin plane that coincides with the { 10 1 ¯ 2 } α twin plane of the matrix, with Ag segregating to the (110) δ twin plane and both interfacial layers. First-principles calculations reveal the energetic origin of this segregation.