Influence of Zn Content on Grain Boundary Precipitates on Stress Corrosion Cracking of Al–Zn–Mg Alloys under Environments Containing Chloride Solutions

Abstract

A commercial AA7075 (Zn: 5.4 mass%) and experimental Al–Zn–Mg alloys with 8.5 or 10.5 mass% of Zn were used to investigate the influence of Zn in the alloys on the composition of grain boundary precipitates and the susceptibility to the stress corrosion cracking (SCC) of the alloys in NaCl solutions. The stress-strain curves of the alloys under the slow strain rate test (SSRT) demonstrated that the susceptibility of the Al–Zn–Mg alloys to SCC increased as the Zn content increased from 5.4 to 8.5 mass% in the alloys. The susceptibility of the Al–10.5Zn–Mg alloy to SCC was not evaluated because of its brittleness. A coarse mainly Zn-containing precipitate was observed at the grain boundary only in the Al–8.5Zn–Mg alloy by STEM/EDS analysis. Calculated Al–Zn–Mg–Cu–Cr phase diagram predicts that MgZn₂ is appeared in the Al–8.5Zn–Mg alloy but not in the Al–5.4Zn–Mg alloy. Anodic polarization measurements of the alloys demonstrated that the dissolution current density increased and the pitting corrosion potential decreased as the Zn content in the alloys increased. The generation of active grain boundary precipitates, such as the mainly Zn-containing precipitates, appears to lead to a higher susceptibility to SCC in Al–Zn–Mg alloys.

Fig. 10 (a) HAADF STEM image of grain boundary precipitates in Al–8.5Zn–Mg alloy. Enlarged (b) HAADF image and (c–e) EDS maps marked by Area 4 in Fig. 10(a).