Hydrogen embrittlement studies of aged and retrogressed-reaged Al-Zn-Mg alloys

The hydrogen embrittlement (HE) of Al-4.35Zn-1.4Mg-0.059Zr alloys in different artificial aging tempers and after retrogression and reaging (RRA) treatments has been investigated by tensile testing hydrogen precharged specimens. The influence of RRA and hydrogen charging on the dislocation structure was studied by TEM. The under-aged temper was the most susceptible while the over-aged temper was the most resistant to HE. The RRA treatment improved the HE resistance of all the tempers. This has been attributed to the reduction in dislocation density upon retrogression and reaging. Flat fractography features near the surface of the hydrogen charged specimen have been correlated to the depth of hydrogen penetration. The hydrogen dislocation interaction and hydride cracking mechanism of HE have been addressed.