Stress Corrosion Cracking in an Extruded Cu-Free Al-Zn-Mg Alloy

Adrian Lervik,Randi Holmestad,Calin D Marioara,Roy Johnsen,Lars Lodgaard,Otto Lunder,John C Walmsley

doi:10.3390/met10091194

Abstract

Stress corrosion cracking (SCC) in Cu-free Al-Zn-Mg (7xxx) aluminium alloys limits its use in many applications. In this work, we study in detail the microstructure of a peak and slightly overaged condition in an AA7003 alloy using transmission- and scanning electron microscopy in order to provide a comprehensive understanding of the microstructural features related to SCC. The SCC properties have been assessed using the double cantilever beam method and slow strain rate tensile tests. Grain boundary particles, precipitate free zones, and matrix precipitates have been studied. A difference in the SCC properties is established between the two ageing conditions. The dominating difference is the size and orientation of the hardening phases. Possible explanations correlating the microstructure and SCC properties are discussed.

Highlights

Wrought Al-Zn-Mg-(Cu) (7xxx) aluminium alloys are of great interest in automotive and aerospace applications due to their high strength-to-weight ratio and formability [1,2]
The low-Si alloy appears similar to the high-Si alloy, but is expected to contain a lower fraction of β-Mg2 Si particles in the air-cooled conditions
We demonstrated that the T6 and T7 tempers consisted of bulk precipitates that differed both in orientation relationship and size, independent of the prior cooling rate from

Summary

Introduction

Wrought Al-Zn-Mg-(Cu) (7xxx) aluminium alloys are of great interest in automotive and aerospace applications due to their high strength-to-weight ratio and formability [1,2]. The 7xxx alloy series is known for its susceptibility to stress corrosion cracking (SCC), which limits usage in many applications [3]. Both the underlying understanding and the suggested ways to mitigate SCC have been reviewed several times [3,4,5,6,7,8]. SCC in aluminium usually occurs along grain boundaries when a susceptible material is stressed above some critical value in a corrosive environment. The propagation mechanism is a subject of controversy as the fundamental mechanism(s) is not fully understood

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Discussion

Conclusion