Nanoscale control of coherency stress in Ni-Pd interfaces through grading and ternary buffer layers

Pd-based hydrogen separation membranes often develop residual coherency stresses at interfaces because of lattice mismatch with adjacent metallic layers, which can compromise structural stability. This study uses molecular dynamics simulations to investigate how interface architecture controls stress accommodation and defect-mediated relaxation in three representative systems: a sharp Ni|Pd interface, a compositionally graded Ni-Pd interface, and a ternary Ni-Co-Pd interface incorporating a Co buffer layer. All architectures were evaluated under identical crystallographic, thermodynamic, and interatomic-potential conditions at 300 K to isolate geometric effects. The sharp Ni|Pd architecture confines the ∼10.5% lattice mismatch within an atomically narrow region and produces highly localized residual stresses, reaching -9.1 GPa compressive and 2.8 GPa tensile. Compositional grading redistributes the mismatch over a broader transition region and lowers the peak tensile stress by 43%, while the Co-buffered ternary architecture further reduces the peak compressive stress by 53% through strain partitioning. Defect screening using centrosymmetry parameter (CSP), polyhedral template matching (PTM), and dislocation extraction analysis (DXA) shows that relaxation occurs through defect-mediated accommodation rather than ideal coherent behavior. The ternary architecture exhibits the strongest suppression of defect-like signatures, reducing the mean HCP-classified fraction from 14.3% to 5.0% and decreasing DXA-detectable line length from 427.8 to 199.1 nm. These results demonstrate that nanoscale grading and intermediate buffer layers effectively suppress residual coherency-stress localization and provide a mechanical route for improving structural stability in lattice-mismatched metallic interfaces.