Effect of PFZ and Grain Boundary Precipitate on Mechanical Properties and Fracture Morphologies in Al-Zn-Mg(Ag) Alloys

The effect of PFZ and grain boundary precipitates formed in aging processes on the macroscopic mechanical properties in Al-Zn-Mg(-Ag) alloys were evaluated using TEM, SEM, tensile test and nanoindentation. Decreases in width of PFZ and smaller size of grain boundary precipitates aging at lower temperatures and/or the addition of Ag processes improved the tensile properties, and the presence of PFZ was found to be harmful to the fracture. Nanoindentation hardness results clarified that the hardness within PFZ is smaller than that in grain interiors, indicating that, in the alloy with large width of PFZ, preferential deformation occurs within PFZ in the initial stage of deformation, and this causes lower elongation regardless of the same level of proof stress and the same sizes of grain boundary precipitates. From the quantitative correlation between precipitate microstructures in the vicinity of grain boundaries, mechanical properties and fracture morphologies, the deformation process of the alloys is considered to divided by three types; i.e. in case of the alloys with the small width of PFZ and the small size of grain boundary precipitates, in case of the alloys with the large width of PFZ and the small size of grain boundary precipitates and in case of the alloys with the large width of PFZ and the large size of grain boundary precipitates.