Dislocation Structures Around the Crack Tips in the Early Stage in Fatigue of Iron

Dislocation structures around the early stage fatigue cracks up to 170 µm in length formed in polycrystalline iron specimens were directly observed through an ultrahigh-voltage electron microscope operating at 2 MV. For this purpose, thin foils perpendicular to the surface were prepared from bulk specimens cycled up to 60 percent of the fatigue life of 2.5 × 10⁵ cycles, by means of electroplating and jet electropolishing technique.

There were found some small straight cracks penetrating the cells (size: 2 to 5 µm), formed beneath the specimen surface as well as in the interior. Some straight screw dislocation lines parallel to the direction of the crack growth, one of the projections of the 〈111〉 directions, were observed in the cells just ahead of the crack. No marked entanglement of dislocations was observed within these cells.

On the other hand, some tangled dislocations were observed in the cells formed just around the cracks growing along grain boundaries.

The results just described are compared with those of the early stage fatigue crack formed in two kinds of face-centered-cubic (fcc) metals with different stacking fault energy (copper and α-brass) obtained previously, and the mechanisms of fatigue crack initiation and growth in the early stages are discussed.