The interactive effect of alloy composition and pre-straining on the precipitation behavior of Al–Mg–Si–Cu alloys

The effects of pre-straining on the precipitation behavior and mechanical properties of a series of Al–Mg–Si–Cu alloys with different Mg/Si ratios and Cu addition were systematically investigated by hardness measurement, tensile testing, scanning electron microscopy (SEM) imaging and atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging. Regardless of alloy composition, pre-straining can effectively accelerate the precipitation kinetics of the alloys. However, the effect on the age hardening response is highly dependent on alloy composition. Pre-straining can effectively promote the age hardening response of the Si-rich alloys, with Cu addition further enhancing the effect. For the Mg-rich low Cu alloy, the age hardening is detrimentally affected by pre-straining, and only compensated to some extent by Cu addition. The branched effect of pre-straining may be linked to the different precipitation reactions of the alloys. Due to the accelerated precipitation kinetics induced by dislocations, the presence of clusters with different sizes or densities, linked to a changed alloy composition, can evolve into notably different precipitate microstructures. In addition, heterogeneous precipitation at the dislocations can result in the formation of various string-like post-β″ precipitates (such as B′, Q′, β′, QP2 phases), which produce distinct strengthening potentials. This work suggests that the alloy composition, especially the Mg/Si ratio and Cu content, would have to be fully taken account when designing a pre-straining process.