Effect of long-term aging and Cu addition on clustering, strength and strain hardening in Al-Mg-Zn-(Cu) crossover alloys

This study investigates clustering and precipitation in Al-Mg-Zn-(Cu) crossover alloys, focusing on Cu’s role and the long-term aging (LTA) process. LTA at 60 °C for 42 days promotes the formation of a dense cluster/precipitate structure, significantly enhancing strain hardening while maintaining high elongation. This treatment achieves an optimized balance of yield strength (∼400 MPa) and elongation (∼17 %), outperforming conventional aging methods such as pre-aging, and paint baking. The addition of Cu plays a critical role by promoting higher cluster number densities, refining spatial distribution, and influencing chemical compositions. Cu effectively hinders dislocation motion, thereby increasing yield strength, and alters strain-hardening behavior by impeding dynamic recovery and reducing dislocation annihilation. Unlike in 6xxx-series alloys, no strain-induced clustering occurs in LTA state, with partial redissolution observed for some elements. Detailed analysis of strengthening mechanisms reveals the need for precise evaluation of individual cluster types and compositions to fully understand relationships between cluster volume fraction, size, spacing, and mechanical properties. While clustering behavior is well-studied in 2xxx, 6xxx and 7xxx-series alloys, research in Al-Mg-Zn-Cu crossover alloys remains comparatively underdeveloped. This work provides new insight into clustering in crossover alloys and demonstrates LTA as a promising route to unlock their full mechanical potential.