Thermal diffusion dominates Cu enrichment in optically dark areas of Al-Cu-Mg alloys: mechanistic insights for fatigue resistance enhancement

Abstract Optically dark areas (ODAs) on fatigue fracture surfaces of Al-Cu-Mg alloys, linked to premature failure, exhibit Cu-rich nanoparticles and surface oxidation. The mechanism driving Cu enrichment in ODAs (between mechanical friction and thermal diffusion) remains debated. To clarify, mechanical fragmentation (ball milling) and high-temperature annealing (500°C, air/nitrogen) were employed. Ball milling induced surface furrows and Al oxidation but no significant Cu segregation, indicating friction alone is insufficient. Conversely, thermal exposure promoted marked Cu diffusion and surface enrichment, with nitrogen yielding higher Cu-rich phase coverage (via XRD/DSC confirmed) and stronger light absorption (metallic Cu dominance) compared to air (CuO formation). Results demonstrate thermal diffusion, not mechanical friction, dominates Cu enrichment in ODAs. This mechanistic insight provides a pathway to enhance fatigue resistance in Al-Cu-Mg alloys by controlling Cu segregation during service.