Technique for Visualizing Subgrain Structure at Fatigue Crack Tips by Ion Etching and Scanning Electron Microscopy

The subgrain structure in ram size areas adjacent to fatigue cracks has been observed by ion etching followed by scanning electron microscopy. The etch patterns have been definitely identified as subgrain structure resulting from the fatigue strains by direct comparison with channelling electron microscopy. The technique is described as it was applied to a single crystal of aluminum which had been fatigued in Stage I by reversed bending.A 5 mm long section of crystal containing the crack was cut from the fatigue sample by spark machining. This section was sliced into pieces 2 to 3 mm thick, and a slice selected with the crack tip close to its center. The selected slice was placed in a shallow aluminum cup about 2 cm in diameter.