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The technique of transmission electron microscopy has been used for direct observation of the microstructures at fatigue crack tips in 1100-O, 2024-T4, and 7075-T6 aluminum alloys. The dislocation morphology and density, and the metallurgical structures which are formed in the plastic zone were studied. In 1100-O, the dislocations are distributed in a subgrain structure, while in 2024-T4 they are distributed uniformly throughout the grain, and many dislocation loops are found. In 7075-T6, dislocations are tightly packed and show little sign of motion or interaction. No correlation was found between the dislocation structure just beneath the fracture surface and the regularly spaced crack growth striations found on the surface. Dislocation densities 6 μ from the crack range from about 1010 lines/cm2 in 1100-O to about 1011 lines/cm2 in 7075-T6. No localized reversion or overaging was observed to occur in the plastic zones of these alloys. The possibility of a general change in metallurgical structure is discussed, and it is concluded that reversion of very small Guinier-Preston zones or a limited, general overaging would not have been detected by our techniques.
fatigue (materials), crack propagation, microstructure, aluminum alloys, dislocations, plastic flow
Grosskreutz, J. C.
Senior advisor for physicsPersonal member ASTM, Midwest Research Inst., Kansas City, Mo.
Shaw, G. G.
Senior electron microscopist, Midwest Research Inst., Kansas City, Mo.