Dislocation arrangements in fatigued iron

Observations are reported on the dislocation arrangements in cyclically stressed high purity iron. For a stress amplitude causing failure in 2 × 10 6 cycles the dislocations were arranged in the form of patches of tiny loops. At higher stress amplitudes the patches became larger with heavily jogged screw dislocations being associated with the loops. A sub-grain structure was also observed in a few areas.