The Effect of Corrosion Morphology on SCC and Fatigue Life of 3rd Generation Al-Li Alloys

Third generation Al-Li alloys like 2099, 2199, and 2060 are of interest for aerospace applications because of their high strength and modulus of elasticity, low density, and enhanced corrosion resistance when compared with incumbent non-Li 2xxx and 7xxx alloys. The corrosion morphology present in 3^rd generation Al-Li alloys is dependent upon temper/aging with T3 and substantially underaged alloys exhibiting intergranular and exfoliation corrosion while the commercial T8X tempers are immune to intergranular and exfoliation corrosion.  When corrosion is observed in the T8X tempers, the predominant corrosion morphology is pitting which is intragranular in nature and elongated with respect to pits typically observed in non-Li heat treatable Al alloys. These more-elongated pits have been observed in certain non-Li 2xxx alloys but are not the predominant morphology.  Stress corrosion cracking (SCC) testing on 2x99 alloys shows that the elongated pit morphology does not negatively impact SCC as demonstrated by the elongated pits not serving as crack initiation sites after long term exposures.  In fact, the SCC threshold for 2x99 alloys is 50 ksi in the ST direction, higher than any non-Li alloy.  Additionally, SCC testing for short times shows that elongated features are neither cracks nor intergranular corrosion which broadens over time, but rather the result of localized corrosion giving way to the intragranular elongated pitting typically observed in these alloys.  Finally, fatigue testing on 2099-T86 shows that the elongated pit morphology does not degrade fatigue life beyond that expected for 7050-T76.