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.

In the present study, we investigated the hydrogen embrittlement susceptibility of Al-4%Cu-1.5%Mg alloys subjected to several heat treatments by means of SSRT tensile test and humid gas stress corrosion cracking(HG-SCC) test. For SSRT tensile test, the tensile test pieces were cut from cold-rolled sheets of 1mm thickness. The test pieces were solution-treated at 500 °C for 1h, water-quenched and aged at 140oC for 72h or 360h. SSRT tensile test was performed in two environments, humid air (HA) and dry nitrogen gas (DNG) at a strain rate of 1.39×10-6s-1. Fracture surfaces were observed with a scanning electron microscopy(SEM). For HG-SCC test, compact tension(CT) test pieces were cut from hot-rolled plate of 6mm thickness. The CT test pieces were solution-treated, water-quenched and aged at 190 °C for 9h, 50°C for 96h or 140°C for 72h. HG-SCC test was carried out based on High Pressure Institute of Japan standards; HPIS E103:2018. The pre-cracked CT specimens with stress loading were kept for 90 days in two environments, HA and DNG. After 90 days, in order to observe whether cracks propagated due to HG-SCC, the specimens were loaded up to fracture rapidly, followed by SEM observation. Tensile properties obtained by SSRT tensile tests were almost the same in two environments. Also, fracture surfaces were not affected by test environments. Moreover, in HG-SCC tests, crack propagation was not observed at each test conditions. Therefore, Al-4%Cu-1.5%Mg alloy had high-resistance to hydrogen embrittlement.

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