EXFOLIATION AND RELATED MICROSTRUCTURES IN 2024-ALUMINUM BODY SKINS ON AGING AIRCRAFT

Exfoliation, a directional attack along elongated grain boundaries, ha s been examined in some detail in rolled 2024 aluminum sheet and plate for KC-135 aging aircraft body skin samples utilizing optical (light) metallography, scanning electron microscopy, and transmission electro n microscopy. A detailed analysis and comparison of precipitates withi n the grains and in the grain boundaries were performed, as well as an examination of elemental depletion profiles across grain boundaries. These observations suggest that corrosion-related anodic sites play a far less significant role in the propagation of exfoliation than do th e hard corrosion products creating wedging stresses within the elongat ed grain boundaries, which seem to demonstrate unique and unusual stru ctural or energetic features or both. (C) Elsevier Science Inc., 1997.