Published: Jan 1983
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The recently developed powder metallurgy (P/M) alloy X7091 (formerly CT91) has strength characteristics similar to ingot metallurgy (I/M) 7000 series alloys. However, its stress corrosion cracking resistance has been reported to be far superior to the conventional ingot-processed alloys. This suggests that P/M alloy X7091 may have far better corrosion fatigue behavior than I/M 7075-T651. Fatigue crack growth rates between 2.5 × 10−8 and 2.5 × 10−6 m/cycle (10−6 and 10−4 in./cycle) were measured for specimens in both the S-T and T-L orientations. In order to determine the effect of environment on the fatigue crack growth rates, tests were conducted in both an innocuous and an aggressive environment (R = 0.5; f = 0.2 Hz). Since it is known that the fatigue crack growth rates of aluminum alloys are sensitive to small amounts of water vapor, gaseous nitrogen with a dew point of −60°C (−75°F) was chosen as the innocuous environment. An aerated 3½% NaCl solution was used as the corrosive environment.
The corrosion fatigue crack growth rates were, on the average, ten times slower for the P/M X7091 alloy than the I/M 7075 alloy tested in both the T-L and S-T orientation. The crack propagation rates for each alloy are similar for both the S-T and T-L orientations when tested in a given environment. While the I/M 7075 specimens had a ten times slower growth rate in the gaseous nitrogen environment than those measured in the salt water, there was no such dramatic change between the crack growth rates measured in the two environments for the P/M X7091 alloy.
The crack closure stress was measured for specimens tested in the corrosion environment to determine if it was greater than the applied minimum stress. It was found that while the I/M 7075 alloy had no measurable closure stress, the P/M X7091 alloy had sufficient closure to reduce the effective cyclic loads by up to 40% of the applied load amplitude. The presence of a measurable closure stress in the 3½% salt water environment is correlated with the P/M X7091 alloy, which has a lower corrosion-fatigue crack growth rate than the I/M 7075 alloy.
Fractographic examination of the corrosion-fatigue fracture surfaces suggests that copious amounts of corrosion debris correspond to where the crack closure was greatest. This debris is absent when the crack closure stress was less than or equal to the applied minimum stress. This observation suggests that in addition to the plastic deformation zone in the wake of the fatigue crack, the presence of corrosion products between the fracture surfaces can increase the closure stress if the general corrosion rate of the fatigue fracture surface is small.
aluminum, fatigue crack propagation, corrosion fatigue, powder metallurgy (P/M), effective stress intensity, crack closure
Project Engineer, U.S. Army R&D Command, Dover, N.J.