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For enhancing the corrosion resistance of the T6-aged high-strength Al alloys with higher strength, retrogression and reaging (RRA) treatments were used to optimize the morphologies, sizes, distribution of precipitates, especially grain boundary precipitates (GBPs). The effects of different retrogression treatments on the microstructures and mechanical properties were studied so as to gain suitable RRA process for 7B50 Al alloy plates. It is found that increasing the retrogression temperature or time will promote the coarsening of transgranular and intergranular precipitates in the center and surface layers of 7B50 Al alloy plates as well as the precipitation of stable η-MgZn2 phase, which will decrease the strength and raise the conductivity. The retrogression temperature will greatly affect the strength and conductivity. The continuously distributed GBPs induced by T6 aging become discontinuous after RRA treatment, accompanying with slightly increasing sizes of transgranular precipitates. Based on the strength and conductivity of the center and surface layers, 165℃/6 h is the suitable retrogression process for 7B50 Al alloy plates. However, the severe deformation of the surface grains compared to that of the central grains caused by hot rolling leads to a higher content of subgrains or substructures in the surficial grains, which promotes the surface layer to quickly reach the peak aging, and the subsequent retrogression treatment results in much more stable η phase in the surface layer. The formation of stable η phase as well as the coarsening or growth of transgranular precipitates could be mainly responsible for the strength difference between the surface and center layers. Although there are some differences about the grain structures between the surface and center layers after quenching/RRA treatments with some local subgrain growth, the positive impact of RRA treatment to the strength is apparently unable to compare with the obvious strength reduction caused by early precipitation of stable η phase in the surface layer. Thus, the RRA treatment cannot relieve the property difference between the center and surface layers of 7B50 Al alloy plates, but it can make the strength and conductivity of the center and surface layers to concurrently meet some working requirements.

Alloying elements, present in the aluminum solid solution or the precipitates, influence the corrosion resistance of Al-Mg-Mn-Zn alloys. In this study, sensitizing treatment was applied to an Al-Mg-Mn-Zn alloy to modify the precipitation at the grain boundaries or in the grains. Transmission electron microscopy (TEM) and scanning electron microscope (SEM) were used to characterize various second-phase particles and determine their orientation relationship with the Al matrix. After sensitizing treatment, τ-phase (Mg(Al, Zn)) is observed to precipitate along the grain boundaries in a coarser size, producing a discontinuous grain boundary precipitate structure. In addition, Mn-rich particles are found to form with various shapes, such as global, plate and rhombus. [ABSTRACT FROM AUTHOR]