Suppression of Intergranular Fracture in 7000 Series Aluminum Alloys

The 7000 series aluminum alloys suffer from intergranular fracture (IGF) that limits the use of the alloys, although they have highest strength among aluminum alloys. The types of IGF can be classified into two categories: (i) with smooth fracture surface showing practically no plastic deformation that takes place in hydrogen embrittlement and stress corrosion cracking, and (ii) with shallow and fine dimples on the fracture surface showing localized plastic deformation inside precipitate free zones. In this study, attempts have been made to suppress the IGF of both types by (a) controlling precipitate microstructure on grain boundaries by quench control and (b) controlling grain boundary morphology by strain induced boundary migration. The IGF of type (i) (hydrogen embrittlement) was successfully suppressed both by the two controlling processes.

Effect of Zr Addition on Overaging and Tensile Behavior of 2618 Aluminum Alloy

The effect of Zr addition on overaging and tensile behavior in a 2618 Al–Cu–Mg–Ni–Fe alloy has been investigated in this study. The chemical composition of the base 2618 alloy, containing ~0.1 wt % of Zr, was modified by adding Zr to reach the target content of 0.25 wt %. Cast bars were T6 heat-treated according to industrial parameters, involving soaking at 525 °C for 8 h, quenching in hot water (50 °C), and artificial aging at 200 °C for 20 h. Both the T6 2618 and 2618 + Zr alloys were overaged at 250 and 300 °C for up to 192 h, to evaluate the decrease in hardness with high temperature exposure time. The tensile behavior of the alloys was investigated in the overaged condition, both at room temperature and at 250 °C. The microstructure of the as-cast and solution-treated samples was investigated by optical and scanning electron microscopy, while the precipitate microstructure at the nanoscale was analyzed by transmission electron microscopy in overaged condition. Experimental data revealed that the presence of 0.25 wt % Zr does not induce modifications at the macroscale on the microstructure of 2618 alloy while, at the nanoscale, the presence of Zr-based precipitates was observed. The overaged Zr-enriched alloy showed increased yield and ultimate tensile strength in comparison to the base alloy, at equal heat treatment condition, both at room temperature and 250 °C.