Simultaneously enhanced ageing kinetics, strength and ductility of Al-Mg-Zn alloys by coupling Cu addition and combined pre-treatments

Abstract

The development of high-performance aluminum alloys remains a critical challenge in industrial applications, among which crossover Al-Mg-Zn alloys have recently attracted much attention due to their excellent mechanical performance. However, the slow ageing response limits their wider application as compared to 7xxx aluminum alloys. To overcome the limitation, this study proposes a synergistic strategy incorporating Cu microalloying and combined pre-treatments (pre-ageing and pre-deformation). A high number density of T-phase precursors and dislocations are formed during combined pre-treatments, which prompts the formation of strengthening precipitates during the final ageing and results in uniform precipitate distribution. The precipitate free zone (PFZ) is narrowed and precipitate-sparse zones (PSZ) are eliminated as well. Therefore, a simultaneous enhancement of ageing kinetic and mechanical properties is achieved. Such improvement is further amplified by Cu addition, which assists the nucleation of GPI zones and enhances the dislocation density during pre-treatments. The Cu-modified alloy with optimized heat treatments achieves an exceptional strength-ductility synergy (520 MPa yield strength with 11% elongation) and rapid ageing response (32 h to peak ageing). These results represent a 44% strength improvement and 94% ageing acceleration compared to the direct-aged Cu-free counterpart (338 MPa yield strength with 10% elongation, and 536 h to peak ageing). This work provides a guideline for designing high-performance crossover aluminium alloys through thermomechanical treatments.

Keywords

Al-Mg-Zn(-Cu) alloys / ageing behavior / microstructure evolution / strength-ductility synergy

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