Hydrogen embrittlement properties of medium-strength 7000 series aluminum alloys in high-humidity air

For medium-strength aluminum alloys 7003 and 7N01 having average (symbolized with S) or upper limit (symbolized with H) in the contents specified in JIS, SSRT tests were carried out in humid air with 90% relative humidity by using a smooth tensile specimen machined from the alloy plates with temper T5, T6 or T7. The susceptibility to hydrogen embrittlement (HE) was evaluated and the relation between the crack extension and microstructural factors was discussed. The following four materials were found to satisfy the useful combination in properties between a low HE susceptibility and yield strength σYS≥320 [MPa] ; 7003H·T5, 7N01S-T5, ·T6 and ·T7. It is supported from microstructural observations that the relatively high HE susceptibility in 7003S-T6 or 7N01H-T6 results from a heterogeneous distribution or a smaller size in grain boundary precipitates to favor intergranular cracking (IGC), while the low susceptibility in 7003S-T7 or 7N01S-T6 is attributed to the grain boundary hydrogen accumulation reduced by trapping effects of matrix precipitates, dispersoids or second phase inclusions, leading to retardation of IGC extension.