Fatigue damage at room temperature in aluminium single crystals - II. TEM

Dislocation structures in aluminium single crystals fatigued at room temperature have been studied by TEM and compared with those reported for copper crystals. The fatigue tests were performed in push-pull at a stress amplitude 4 MPa, R = -1 and frequency of 20 Hz. A band structure was observed in thin foils with planes parallel to the stress axis and the Burgers vector. Some parts of the bands were similar in appearance and dimensions to the ladder-like pattern formed in the same sample by several microcracks in a PSB on the surface containing b. This suggests that the microcracks are associated with dislocation walls in the PSBs. In the matrix, the dislocation arrangement was a cell structure, but areas of a high dislocation density which might be a vein structure were also seen. Dislocation walls in PSBs might be described as two sets of entangled edge dislocations, one set being the primaries and the other being large jogs in primary screw dislocations lying on the cross-slip planes. A large number of dislocation loops were observed, suggesting that their formation was a significant form of irreversible deformation in single crystal Al fatigue. Three types of dislocation loops were identified.