Fatigue Crack Growth Behavior and Susceptibility to Hydrogen Embrittlement in 2000 and 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. Crack growth rate at low Delta K levels was clearly larger in 7075-type alloy than 2024-type alloy, in the humid air. In order to investigate this result more in detail, 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. From this result together with fractography study, the larger fatigue crack growth rate of the 7075-type alloy is attributable to hydrogen embrittlement.