With respect to hydrogen embrittlement process in a 7075 alloy in humid air, it has been claimed that cracks at second-phase particles result in intergranular cracking. In this study, slow strain rate tensile tests in two environments and microscopic surface observation were carried out to investigate the hydrogen embrittlement process of a 7075 alloy. It was confirmed that elongation in humid air was smaller than in dry nitrogen gas due to hydrogen embrittlement. In both environments, cracks were observed in second-phase particles on the specimen surface during tensile deformation. The amount of cracks in second-phase particles was not larger in humid air than in dry nitrogen gas. Moreover, they did not grow into the aluminum matrix in both environments. Some intergranular cracks were observed on the specimen surface without a cracking of second-phase particles in humid air. Therefore, hydrogen embrittlement in a 7075 alloy based on intergranular cracking in humid air is not caused by a cracking of secondphase particles on the specimen surface.

Hydrogen embrittlement sensitivity of a high-Zn containing Al-Zn-Mg-Cu series alloy in T6 temper was evaluated by tensile testing at various strain rates in humid air (HA) and dry nitrogen gas (DNG). The elongation of Al-10Zn-2.6Mg-1.6Cu-0.2Cr alloy in HA was smaller than that in DNG due to hydrogen embrittlement. Quasicleavage fracture caused a loss of elongation at medium strain rates (10 3-10 4 s 1), and smooth intergranular cracking degraded ductility largely at slow strain rates (10 5-10 6 s 1). These results indicate that the higher content of Zn in Al-Zn-Mg-Cu series alloy increase hydrogen embrittlement sensitivity, and exhibits hydrogen embrittlement even when the strain rate is 10 3 s 1 .

Hydrogen in aluminum has been known to be the cause of blister and pore. Some aluminum alloy is susceptible to stress corrosion cracking, which is based on intergranular cracking arisen from hydrogen embrittlement. The behavior of hydrogen in aluminum has not been fully understood yet. Hydrogen gas plasma enables to introduce high hydrogen concentrations into specimen without Al (OH)3 layer on the surface of specimen. In this paper, we have investigated the behavior of hydrogen in a plasma charged aluminum by means of thermal desorption spectroscopy, a method to evaluate the amount and trap states of hydrogen. Cold-rolled pure aluminum were annealed, electro-polished and charged with hydrogen gas plasma. Immediately after hydrogen gas plasma charging, TDS tests were performed under ultra-high vacuum. The hydrogen desorption spectrums obtained by TDS tests had three peaks corresponding to the co-diffusion of hydrogen-vacancy pair, dislocation and pore. Compared to a sample without charging, in a plasma charged sample, the amount of hydrogen trapped in vacancies especially increased.

Behavior of hydrogen in tensile-deformed Al-9mass%Mg and Al-5.8mass%Zn-2.4mass%Mg alloys was investigated by means of hydrogen microprint technique, HMPT, a method to visualize the microscopic location of hydrogen evolution from specimen surface as silver particles. Both in the two alloys, surface relief was formed at most grain boundaries by the stretching, while hydrogen evolution was observed at some grain boundaries. The evolution of hydrogen was discussed with parameters such as the angle between grain boundary on the specimen surface and tensile direction, the angle between grain boundary on the surface and slip line inside the grain, the height of the surface relief, and maximum gradient of the surface relief. The results indicated that the shear deformation along grain boundary caused transportation of hydrogen atoms with gliding dislocations to the surface, breakage of surface oxide film. In the Al-Zn-Mg alloy, it was suggested that the preferential deformation in the precipitate free zone was attributed to hydrogen evolution.

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