Relationship Between Fatigue Crack Growth Behavior and Susceptibility to Hydrogen Embrittlement in 7000 Series Aluminum Alloys

The 7000 series alloys have the highest strength in the aluminum alloys, but lower fatigue properties than 2000 series alloys. Thus, 7000 series alloys are not applied to a large proportion of the aircraft components. However, the mechanism for this has not been elucidated yet. In humid air, hydrogen embrittlement based on intergranular cracking has been known to occur in 7000 series alloys. In order to explain the difference in the fatigue crack growth behavior in the two series alloys, the effect of the test environment on the fatigue crack growth of the two series alloys has been investigated in this study. Alloys of 7075- and 2024- types with relatively coarse equi-axed grains were T6- and T4- tempered, respectively, and subjected to fatigue crack growth test in humid and dry environments. Threshold value of ΔK(K_th) was larger in the 7075-type alloy than 2024-type alloys, which was presumed to reflect the higher strength of 7075-type alloy. Beyond ΔK_th, crack growth rate at low ΔK level was clearly larger in 7075-type alloy than 2024-type alloy, in the humid air. The rate of the 7075-type alloy was decreased by changing the environment to the flow of nitrogen gas. These results were in consistent with the fractognaphic observations: intergranular and quasi-cleavage fracture surfaces were found in the 7075-type alloy, and the fraction of the intergranular fracture surfaces of the 7075-type alloy was larger in humid air than dry nitrogen gas. In order to clarify these results, the two alloys were subjected to slow strain rate tensile test in the two environments to evaluate their sensitivity to hydrogen embrittlement. The sensitivity of 7075-type alloy was higher than 2024-type alloy. Thus, the large fatigue crack growth rate of the 7075-type alloy is attributable to hydrogen embrittlement. Besides, a mechanism was proposed for hydrogen intrusion near grain boundaries of the two alloys with different microstructures.