Hydrogen-environment Embrittlement in High-purity Al-Zn-Mg(-Cu) Alloys

From the viewpoint of a both practical and basic understanding on hydrogen-environment embrittlement (HEE) of high-strength aluminum alloys, high-purity Al-Zn-Mg(-Cu) alloys having contents of primary alloying elements comparable to 7075 alloy are subjected to slow strain rate tensile (SSRT) tests in a high-pressure hydrogen gas and atmospheric air with a controlled relative humidity. The characteristics of HEE have been studied as a function of aging stage, strain rate and test temperature. From the test results on the Al-Zn-Mg ternary alloy in the underaged (UA) with a high sensitivity to HEE, it is confirmed that humid air is so severer than high-pressure gaseous hydrogen to cause embrittlement in aluminum alloys. An increase in each of humidity and temperature of atomospheric air increases the susceptibility to HEE, leading to a remarkable intergranular cracking. The copper-bearing alloy having a higher tensile strength in the peak-aging (PA) state than the ternary alloy exhibits a superiority in the HEE resistance too. For the over-aged (OA) cupper-bearing alloy, the higher humidity of air produces the higher elongation and a transgranular dimple fracture, which suggest that the void formation becomes easier by hydrogen trapping at coarsened precipitate particles within grains.