Abstract: Precipitation via thermal treatments is among the most effective approaches to strengthening and is widely applied in the Al industry. Thermal treatments combined with deformation are capable of finely regulating the process of precipitation and distribution of precipitates. Deformation-induced defects exert significant impacts on the precipitation and already present precipitates, which however is often overlooked. In this study, the interactions between deformation and precipitation/precipitates, and their impacts on mechanical properties were systematically investigated in the solution-treated (ST) Al-0.61Mg-1.17Si-0.5Cu (wt.%), processed by multi-pass equal channel angular pressing (ECAP) and thermal treatments. Novel deformation-mediated cyclic evolution of precipitates is discovered: ST→ (1,2 passes: deformation induced precipitation) Guinier Preston (GP) zones→ (An250/30) Q' and L phases→ (3-pass: deformation induced fragmentation/resolution) spherical precipitates→ (4-pass: deformation induced further fragmentation/resolution) GP zones. On this basis, we extend the quasi-binary phase diagram of Al-Mg₂Si along deformation as the third dimension and construct an innovative defect phase diagram for the Al-Mg-Si-based system. To testify to the effect of deformation-mediated cyclic evolution of precipitation/precipitates on the optimum mechanical properties, peak-aging treatments were performed in samples of ST and 3-pass states. Based on the microscopic characterizations, a distinctive mechanism of peak-aging strengthening is proposed. Notably in the 3-pass ECAPed and peak-aged sample the dominant strengthening phases become the L precipitates that thrived from the segmented and spherical L phases, rather than β'' precipitates in the solely peak-aged ST sample. Our work provides a feasible example for exploring the combined processing technique of multi-step deformation and thermal treatments, to optimize the mechanical properties.

Key words: Al-Mg-Si-Cu alloys, Precipitations, Equal channel angular pressing, Defect phase diagram, Microstructure