Strength and ductility enhancement in Al-Cu-Zn-Mg alloy via multimodal heterogeneous structures and multiscale precipitates

This study reported a strategy of Sc-Hf microalloying and thermomechanical processing to synergistically regulate the layered heterogeneous grain structure and multiscale precipitates in Al-Cu-Zn-Mg alloys. The results indicated that the combined addition of Sc and Hf achieved optimal grain refinement in Al-Cu-Zn-Mg alloys (reducing grain size by 64.3%). Through thermodynamic calculations and microstructural characterization, the evolution of precipitates during solidification and the influence of Sc and Hf atoms on the competitive behavior of multi-phase co-precipitation during aging were elucidated. Following the thermomechanical processing, the coarse second phase particles in the Sc-containing alloy promoted recrystallization through particle-stimulated nucleation; meanwhile the submicron second-phase particles pinned the grain boundaries to inhibit the growth of the recrystallization grains, thereby forming a layered heterogeneous grain structure. The yield strength, ultimate tensile strength and elongation of the Sc-Hf-alloyed alloy reached 457 MPa, 558 MPa and 12.5%, respectively, representing the improvements of 16.0%, 14.6%, and 30.2% over the base alloy, respectively. Strength gains stemmed from the multiple precipitates with a higher number density and a smaller size, the reduced deformed grains and the increased dislocation density. Furthermore, the heterogeneous grain structure simultaneously retarded the crack propagation and mitigated the stress concentration near coarse/fine grain interfaces, resulting in more uniform plastic deformation and improving ductility. The present work offered a novel approach for synergistically enhancing strength and ductility in Al-Cu-Zn-Mg alloys.