A Comparison in Hydrogen-Environment Embrittlement Response Between T6 and T73 Tempers of 7075 Aluminum Alloy

The hydrogen-environment embrittlement (HEE) response of a high-tensile strength 7075 aluminum alloy was studied in a comparison between T6(peak-aged) and T73(overaged) tempers subjected to slow-strain rate tensile (SSRT) tests in humid air. The SSRT test (strain rate range: 1.39x10(-4)similar to 6.95x10(-7)[1/s]) was carried out in 65% relative humidity air at temperatures between 10 C and 70 C in reference to an inert environment of dry nitrogen gas. With both increasing test temperature and decreasing strain rate, the alloy of T6 temper shows a more significant reduction in elongation and produces intergranular cracks in the fracture surface, resulting in a high susceptibility to HEE. In contrast, the alloy in T73 temper reveals rather an increase in elongation and transgranular dimple fracture, showing little trend of HEE. It is however found for the T73 alloy that instead of the more increase in total elongation under the lower strain rate, the uniform elongation (plastic strain until necking begins) decreases inversely. This indicates that an initiation of plastic instability is promoted by the effect of hydrogen: namely the void nucleation in grains is facilitated by hydrogen trapping at the sites of second-phase particle such coarsened MgZn2 precipitates within grains. The stop of intergranular cracking and the high resistance to HEE in the overage alloy are attributed to less diffusion of hydrogen to grain boundaries retarded by the trapping effect.