Periodic Solute Segregation Weakens Chemically Stabilized Grain Boundaries

Solute segregation can chemically stabilize grain boundaries (GBs), but whether such interfaces remain mechanically robust under load remains unclear. Here, atomic-resolution HAADF-STEM and atomistic modeling identify a periodically Nd-decorated asymmetric [12̅10] tilt GB in a deformed Mg-Nd alloy. Voronoi analysis and segregation-energy calculations show that Nd occupies distorted boundary-core sites and reduces local interfacial strain energy. Contrary to the common expectation that rare-earth segregation strengthens magnesium interfaces, shear simulations show that periodic Nd segregation substantially reduces the mechanical resistance of this boundary. Under loading, the segregated regions undergo amorphization-like structural rearrangement, redirecting stress relaxation from the intrinsic direction-dependent modes of the pure boundary─interfacial twinning or localized interfacial dislocation activity─to premature grain-boundary sliding. Thus, an energetically favorable segregated interface can become a mechanically weak pathway, demonstrating that chemical stabilization does not necessarily translate into mechanical strengthening and that load-induced deformation pathways must be considered when designing solute-engineered interfaces.