The effect of pre-deformation on the precipitation behavior of AlCuMg(Si) alloys with low Cu/Mg ratios

The addition of a trace amount of Si significantly affects the precipitation sequence of the Al–Cu–Mg alloy with a low Cu/Mg ratio, such as forming Si-modified Guinier-Preston-Bagaryatsky (GPB) zones to suppress the precipitation of the S phase. The introduction of pre-deformation also affects the precipitation behavior during the subsequent aging process. In this paper, the atomic-resolution high-angle-annular-dark-field (HAADF) imaging technique and first-principles calculation are used to study the effect of pre-deformation on the precipitation behavior of an Al-3.0Cu-1.8Mg-0.5Si alloy during aging at 180 °C. In order to learn the effect of minor Si-addition, a Si-free alloy with similar chemical composition is also studied. It is found that for Si-free alloy, the 6% pre-deformation promotes the precipitation of the S phase dispersedly during the subsequent aging, leading to an increase of the peak hardness. For Si-containing alloy, the 6% pre-deformation prior to aging suppresses the precipitation of Si-modified GPB zone acting as the original main strengthening precipitates and promotes the formation of successive composite precipitates including the S phase, various GPB zones, C phase and sub-units of C/Qʹ phase. The successive composite precipitates are easy to grow up and coarsen, which weakens the contribution of precipitation strengthening compared to the small Si-modified GPB zone. As such, the 6% pre-deformation prior to aging at 180 °C provides a positive strengthening effect for Si-free alloy, but a negative strengthening effect for the Si-containing alloy. [Display omitted] •The 6% pre-deformation is beneficial to the aged Si-free AlCuMg alloys, while detrimental to the aged Si-containing alloys.•The detrimental effect is due to the precipitation of the large-sized zig-zag composite precipitates.•A new GPB zone-variant was found and identified in the aged Si-containing alloys with pre-deformation.