Influence of Multistep Aging on the Stress Corrosion Cracking Behavior of Aluminum Alloy 7010 Available to Purchase

The role of multistep aging on the stress corrosion cracking (SCC) behavior of aluminum alloy 7010 has been investigated. The material in the form of 5-mm-thick sheets was heat-treated to two different tempers. The heat treatments are peak-aged, obtained by a two-step aging (i.e., solution treated at 465°C, water quenched at room temperature, and aged at 100°C/8 h and 120°C/8 h) and over-aged, obtained by a three-step aging (i.e., solution treated at 465°C, water quenched at room temperature, and aged at 100°C/8 h followed by 120°C/8 h and 170°C/8 h). SCC behavior was investigated using slow strain rate testing (SSRT) in air and 3.5% sodium chloride (NaCl) solution (environment). U-bend testing was carried out as a qualitative test. SSRT results show a remarkable improvement in the SCC resistance in three-step aged samples. For example, the percent reduction in area dropped from 28.1% (in air) to just 24.4% (in environment), and there was no drop in the percent elongation (~10%) when tested at a strain rate of 10^–6/s. Two-step aged samples were found to be inferior compared with the three-step aged sample—the percent reduction in area of two-step aged samples dropped from 9.9% (in air) to 3.3% (in environment) and percent of elongation dropped from 10% (in air) to 3% (in environment). U-bend test results are also in conformity with the SSRT results, showing high resistance to SCC in the three-step aged alloy, though no direct correlation can be made. Anodic dissolution along the grain boundaries leading to intergranular cracking was evident in the SSRT and U-bend fractured specimens of the two-step aged alloy, whereas, such an attack was less prominent in the three-step aged alloy leading to a relatively more ductile fracture. The breaking of grain boundary precipitate network in the three-step aged condition could be a possible reason for better SCC resistance of the three-step aged alloy.