Periodic Solute Segregation Weakens Chemically Stabilized Grain Boundaries

Abstract

Abstract 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.