Effect of microstructural features on stress corrosion cracking behaviour of 7017 and 7150 aluminium alloy

Abstract

The commercial 7xxx series aluminum alloys are based on medium strength Al-Zn-Mg and high strength Al-Zn-Mg-Cu systems. This series aluminium alloys are widely used as structural materials in aircraft, space vehicles, lightweight- armoured carriers, and automotive applications and in aerospace industry due to their high strength to weight ratio. However, this series of aluminum alloys are susceptible to stress corrosion cracking (SCC), particularly when aged to the peak-aged state of T6 temper in aqueous environment containing chloride ions. In this present paper, a comparative study of stress corrosion cracking (SCC) beahviour of 7017 and 7150 aluminum alloy of T6 temper were investigated. SCC behaviour of these alloy tempers has been studied by slow strain rate test (SSRT) in air and in 3.5 wt.% NaCl solution at free corrosion potential (FCP) and at applied anodic potential (AAP). The SSRT results showed that the 7017 alloy of T6 temper is not susceptible to SCC in 3.5 wt.% NaCl solution at FCP, but the 7150 alloy of T6 temper is susceptible to SCC in 3.5 wt.% NaCl solution at FCP. Further, the results showed that, for both the alloy SCC was severely damaging at applied anodic potential. The observed SCC behaviour of the alloys of T6 temper has been explained with the TEM micrographs, SEM fractographs and optical micrographs of secondary crack morphology. Finally, attempts have been made to understand the structure-properties relationship of the alloys and to establish proper SCC mechanism.