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The structures formed around fatigue cracks of aluminium were examined using a 500 kV electron microscope in order to confirm the previous results obtained by a 100 kV electron microscope and to find a clue to the elucidation of the mechanism of fatigue fracture involved. There was a well-defined substructure around a fatigue crack. A large number of dislocation loops and dislocation dipoles were found near sub-boundaries. A strain-concentrating region where many voids existed was observed in the vicinity of the fatigue crack. These results are in good agreement with those obtained in previous work by use of the 100 kV electron microscope and are also consistent with the previous X-ray observations. There was direct evidence that propagation of fatigue cracks occurs through growing-up and joining-up processes of the voids formed in the strain-concentrating regions. The mechanism of void formation due to the coalescence of vacancies is proposed on the basis of the present observations. The short-circuit diffusion along the sub-boundaries with the driving force caused by the stress gradient near the crack is suggested to explain the vacancy migration through the strain-concentrating regions.

Studies on the Structure Developed around Fatigue Cracks of Aluminium Using a High Voltage Electron Microscope